Tran­script of video below:

Hi there, my name is Paul, and this is Expo­sure Ther­a­py. In this video, I’ll teach you about one of the most fun­da­men­tal con­cepts in pho­tog­ra­phy — the pho­to­graph­ic stop. The stop is ubiq­ui­tous — it’s every­where — and under­stand­ing it will make you an effi­cient pho­tog­ra­ph­er. How­ev­er, to learn why the stop is so vital, we need to estab­lish a foun­da­tion of knowl­edge about the basics of expo­sure and reci­procity law. These will be our first top­ics, so let’s begin!  

What is exposure in photography?

Expo­sure is the total amount of light used by your camera’s image sen­sor to make a pho­to. It has a direct influ­ence on the bright­ness of your pic­tures. The total expo­sure, that is, the “vol­ume” of light received by the sen­sor is deter­mined by two fac­tors: the inten­si­ty of light pass­ing through the lens and the time dura­tion of that expo­sure. The fol­low­ing equa­tion shows this rela­tion­ship: 

Expo­sure = Inten­si­ty × Time

The aper­ture con­trols the inten­si­ty of light. It’s a vari­able-sized cir­cu­lar open­ing found inside most lens­es. Mean­while, the shut­ter con­trols dura­tion, being the accu­mu­la­tion of light over a peri­od of time. On your cam­era, the aper­ture and shut­ter are the only set­tings for con­trol­ling the total amount of light reach­ing the sen­sor.

A third ele­ment, called ISO, is an elec­tron­ic func­tion that sim­u­lates changes to expo­sure but with­out adding or sub­tract­ing light. In oth­er words, adjust­ing the ISO changes the bright­ness of your pic­ture with­out chang­ing the expo­sure. Togeth­er, the aper­ture, shut­ter, and ISO con­trol what I call the Effec­tive Expo­sure, that is, the bright­ness of your pic­ture. It’s expressed with the fol­low­ing equa­tion: 

Effec­tive Expo­sure = Inten­si­ty × Time × ISO

Reciprocity in Photography

These equa­tions reveal a com­mon bond between the expo­sure con­trols. Reci­procity rep­re­sents the rela­tion­ship both inten­si­ty and dura­tion have on the result­ing expo­sure. That’s because many com­bi­na­tions of inten­si­ty and dura­tion can pro­duce pho­tos with iden­ti­cal expo­sures. Even more com­bos of inten­si­ty, dura­tion, and ISO can make pho­tos with the same effec­tive expo­sures. 

To get a bet­ter sense of reci­procity, let’s take a moment to con­sid­er pure math. Con­sid­er the num­ber 100. It’s the prod­uct of 50×2. But it can also be the prod­uct of 25×4, 20×5, 10×10, or 2×5×10.  There are many dif­fer­ent equa­tions that equal 100.

The same prin­ci­ple applies to light and pho­tog­ra­phy. Sev­er­al com­bi­na­tions of aper­ture and shut­ter speed can pro­duce the same total expo­sure. For exam­ple, you can achieve the same total expo­sure using an aper­ture val­ue of ƒ/16 and shut­ter speed of 1/250 sec­ond, or ƒ/11 and 1/500 sec­ond, or even ƒ/5.6 and 1/2000 sec­onds; all three per­mu­ta­tions pro­duce equiv­a­lent expo­sures.

Stu­dents attend­ing my begin­ner cours­es get this con­cept quick­ly but ques­tion its usefulness—what’s the point of mak­ing these adjust­ments if the expo­sure remains unchanged? The point is artis­tic.

Although the pri­ma­ry pur­pose of the aper­ture and shut­ter is to reg­u­late expo­sure and pic­ture bright­ness, they have sec­ondary char­ac­ter­is­tics that can change the artis­tic appear­ance of your pho­to­graph, giv­ing it a dis­tinct char­ac­ter.

If you’re hap­py with your expo­sure in terms of bright­ness but not in terms of the depth of field—say you want more dra­mat­ic focus separation—you can increase the size of the aper­ture. This change increas­es the light inten­si­ty pass­ing through your lens, rais­ing the expo­sure and mak­ing your pic­ture brighter than orig­i­nal­ly intend­ed. To com­pen­sate, you’d sim­ply select a faster shut­ter dura­tion; this change decreas­es the expo­sure by an amount equal to the change you made to the aper­ture. 

It seems easy, right? You’ve made a change that added one quan­ti­ty of light, and  then sub­tract­ed an equal amount of light to bal­ance the expo­sure. 

How­ev­er, there’s a com­pli­ca­tion: dif­fer­ent units of mea­sure express your expo­sure set­tings. F‑numbers express the aper­ture, time units express the shut­ter speed, and ISO is a unit itself. So how do we rec­on­cile changes between f‑numbers, dura­tion, and ISOs? 

We do it with the pho­to­graph­ic stop, which uni­fies every­thing.

What is the photographic stop?

In pho­tog­ra­phy, a stop is a unit that describes the change or dif­fer­ence between expo­sure val­ues. Adding one stop dou­bles your expo­sure, but sub­tract­ing one stop halves your expo­sure. There­fore, a stop mul­ti­plies or divides your expo­sure by two depend­ing on whether you’re adding or sub­tract­ing light. (And remem­ber, mul­ti­ply­ing by half is the same as divid­ing by two.) 

You can add or sub­tract mul­ti­ple stops. For exam­ple, adding two stops dou­bles your expo­sure and dou­bles it again, which cre­ates an expo­sure four times brighter than the orig­i­nal (because 2 × 2 = 4). Con­verse­ly, sub­tract­ing three stops halves your expo­sure, then halves it again, and halves it a third time, which cre­ates an expo­sure that’s one-eighth as bright as the orig­i­nal (because ½ × ½ × ½ = ⅛). 

The pho­to­graph­ic stop rec­on­ciles how changes to the aper­ture, shut­ter speed, and ISO affect the bal­ance of expo­sure and pic­ture bright­ness. Vir­tu­al­ly every cam­era shows the degree of change applied to each set­ting using stops or frac­tions of stops. 

You can check this on your cam­era right now. Grab your cam­era, select Shut­ter Pri­or­i­ty mode, and rotate the com­mand dial to adjust the shut­ter speed. By default, most cam­eras will make a one-third stop change to the val­ue for every detent (or click) of the wheel’s rota­tion.  So, for exam­ple, if you start at 1/500 sec­onds and rotate the dial by three clicks towards the faster direc­tion, you’ll arrive at 1/1000 sec­onds. These two val­ues dif­fer by one stop—minus one stop if mov­ing from 1/500 to 1/1000 because that change halves the light, and adding one stop if mov­ing from 1/1000 to 1/500 because that change dou­bles the light. 

Now try it with f‑numbers. Set your cam­era to Aper­ture Pri­or­i­ty mode and count the clicks between ƒ/8 and ƒ/16.  It’s six clicks on most cam­eras, rep­re­sent­ing a change of two stops because each click of the wheel applies a one-third stop change. Whether the shift is plus or minus two stops depends entire­ly on whether you’re adding light by mov­ing towards low­er f‑numbers or sub­tract­ing light by mov­ing towards high­er f‑numbers. 

In prac­tice, these changes won’t impact your expo­sure because both pri­or­i­ty modes are auto­mat­i­cal­ly exposed. The cam­era bal­ances your inputs by auto­mat­i­cal­ly apply­ing an inverse trans­for­ma­tion to the set­ting it con­trols. How­ev­er, your cam­era can some­times mis­read the scene and pro­duce poor auto expo­sures. You can fix these errors with expo­sure com­pen­sa­tion, which lets you raise or low­er the stan­dard expo­sure set by the cam­era. A numer­ic scale express­es changes in expo­sure com­pen­sa­tion, and the change to expo­sure between each adja­cent num­ber is one stop. Most cam­eras allow you to adjust expo­sure com­pen­sa­tion from ±2 to ±5 stops in one-third stop incre­ments. Note that some cam­era mak­ers refer to the num­bers express­ing expo­sure com­pen­sa­tion as “EV.” EV stands for Expo­sure Val­ue, and in this sense, they’re syn­ony­mous with stops. 

Photographic stops and manual mode

Under­stand­ing the con­cept of pho­to­graph­ic stops is essen­tial when set­ting expo­sures man­u­al­ly and lets you quick­ly deter­mine the ide­al bal­ance of the aper­ture, shut­ter speed, and ISO. 

Let’s pre­tend we’re out­side on a sun­ny after­noon and want to cap­ture a por­trait. We can quick­ly obtain good expo­sure by using the Sun­ny 16 rule of thumb. It states we can get an accu­rate expo­sure in direct sun­light by set­ting our aper­ture to ƒ/16 and select­ing a shut­ter speed that inverse­ly match­es the ISO val­ue. There­fore, ISO 100 would cor­re­spond to a shut­ter speed of 1/100 sec­ond, and ISO 400 would match with a shut­ter speed of 1/400 sec­ond, and so on.

Let’s take this rule of thumb and apply it to that day­light por­trait. Our start­ing expo­sure val­ues are ƒ/16, 1/200 sec­ond, and ISO 200. How­ev­er, por­traits gen­er­al­ly ben­e­fit from a shal­low depth of field because it cre­ates a visu­al sep­a­ra­tion between the sub­ject and their back­ground, and an aper­ture of ƒ/16 isn’t ide­al for this goal. Let’s choose ƒ/5.6 instead because it accom­plish­es the effect and is achiev­able by most lens­es. It takes nine clicks of the con­trol dial to move the aper­ture from ƒ/16 to ƒ/5.6, and this trans­lates to a three-stop increase in light inten­si­ty. If we were to take a pic­ture now, our expo­sure would be three stops (that is, eight times) too bright. Since our change to the aper­ture adds light, we must sub­tract an equal amount of light from the remain­ing val­ues to bal­ance our expo­sure. To remove three stops of light from the shut­ter, we must turn the shut­ter con­trol dial nine clicks towards the faster direc­tion, which results in a shut­ter speed of 1/1600 sec­ond. Thus, we replaced our start­ing expo­sure val­ues of ƒ/16, 1/200 sec­ond, and ISO 200 with ƒ/5.6, 1/1600 sec­ond, and ISO 200. This change adds three stops on the aper­ture and sub­tracts three stops from the shut­ter speed, which makes the net dif­fer­ence zero. It impos­es a dra­mat­ic visu­al change with­out alter­ing the effec­tive expo­sure.

Now let’s see what hap­pens when we add ISO to the mix. Reset the cam­era to the orig­i­nal rule of thumb set­tings: ƒ/16, 1/200 sec­ond, and ISO 200. We’ll again choose ƒ/5.6  for the shal­low­er depth of field. How­ev­er, we’ll now split the bal­ance between the shut­ter and ISO. We’ll sub­tract one stop from the ISO by mov­ing from 200 to 100. This makes our pic­ture one stop dark­er. To remove the remain­ing two stops of light from the shut­ter, we’ll turn the shut­ter con­trol dial six clicks towards the faster direc­tion, result­ing in a shut­ter speed of 1/800 sec­ond. Thus, we’ve replaced our start­ing expo­sure val­ues with ƒ/5.6, 1/800 sec­ond, and ISO 100. This change adds three stops of light on the aper­ture, sub­tracts two stops of light from the shut­ter speed, and removes one stop of bright­ness from the ISO, cre­at­ing a net dif­fer­ence of zero stops.

Stops are ubiquitous

The under­ly­ing notion of stops—the act of mul­ti­ply­ing or divid­ing by two—is ubiq­ui­tous through­out all facets of pho­tog­ra­phy beyond adjust­ing expo­sure com­pen­sa­tion and bal­anc­ing man­u­al mode. For exam­ple, stops express the out­put pow­er of built-in and exter­nal flash units. The out­put on my ProPho­to D1 is adjustable in full stops or one-tenth stop incre­ments. Stops also indi­cate how much light is lost to colour, polar­iz­ing, and neu­tral den­si­ty lens fil­ters and light mod­i­fy­ing gels. Cam­era mak­ers use stops to spec­i­fy the effec­tive­ness of opti­cal and in-body image sta­bi­liza­tion sys­tems. And once the cam­era and lights are off and you’re in the dig­i­tal dark­room, edit­ing appli­ca­tions like Adobe Light­room and Cap­ture One Pro use stops to rep­re­sent the scale of their expo­sure adjust­ment slid­ers.

This shows that stops are inescapable. Rein­forc­ing your under­stand­ing and inter­nal­iz­ing that knowl­edge through prac­tice will help you become an effi­cient pho­tog­ra­ph­er. And all it takes is know­ing how to mul­ti­ply or divide by two. 

I hope you enjoyed this video and found it help­ful. If you have requests for future top­ics, let me know in the com­ments, and I’ll con­sid­er them for future videos. In the mean­time, you can learn more about pho­tog­ra­phy or join my group pho­tog­ra­phy cours­es in Toron­to by vis­it­ing ExposureTherapy.ca. See you next time.

The fol­low­ing arti­cle is a tran­script of the video above. 

Hi there, my name is Paul, and this is Expo­sure Ther­a­py. In this video, I’ll explain why your pho­tos of the moon are often over­ex­posed and how you can fix them. 

It’s sunny on the Moon

I teach group pho­tog­ra­phy work­shops, and occa­sion­al­ly, stu­dents will ask some vari­a­tion of the fol­low­ing ques­tion:

I tried tak­ing a pic­ture of the full moon last week, but it turned out too bright and fea­ture­less. How do I take pho­tos of the Moon, so it looks sim­i­lar to how my eyes see it?

My typ­i­cal (and slight­ly cheeky) response is to point out that it’s sun­ny on the moon. Then I wait for a beat or two to let it sink in. 

Is it sink­ing in for you?

Moonlight and sunlight

Let’s begin with the basics. The moon and the sun are the most promi­nent celes­tial bod­ies in the sky. An impor­tant dis­tinc­tion between the two is the nature of their light. The sun radi­ates its light—it glows. It’s sim­i­lar to the flames of a fire, neon signs, and the tung­sten fil­a­ments of light­bulbs. By con­trast, moon­light is sun­light that has bounced off its sur­face to end up on Earth, both in our eyes and our cam­eras. 

The problem with measuring light

Con­ven­tion­al­ly, the most reli­able way to get the “cor­rect” expo­sure of sub­jects that don’t glow is by using an inci­dent light meter, such as this Sekon­ic that I’ve had for 15 years. These devices mea­sure the amount of light falling on your sub­ject. I walk up to my sub­ject, point the white dome towards the cam­era, take a light read­ing, and the meter shows the cor­rect expo­sure set­tings for the scene.

How­ev­er, there’s an obvi­ous flaw with this method: with its aver­age orbital dis­tance of about 385,000 km, one does not sim­ply walk up to the moon. 

So what do we do? We remem­ber that it’s sun­ny on the moon. But let me qual­i­fy that state­ment.

Lunar phases

Except for lunar eclipses, sun­light illu­mi­nates half of the moon’s sur­face at any giv­en moment. How­ev­er, there’s a dai­ly change to the appar­ent shape of the sun­lit por­tion of the moon as seen from Earth. These dif­fer­ences in appear­ance, known as lunar phas­es, occur because as the moon orbits the earth, we see vary­ing amounts of its sun­lit half. 

New moon

The moon is not vis­i­ble dur­ing the new moon phase when it’s rough­ly between the earth and the sun. The new moon is invis­i­ble because it’s in the same part of the sky as the sun, and its “near side” — the hemi­sphere that always faces the earth, regard­less of phase — is in com­plete shad­ow. 

As the moon con­tin­ues its orbit, pro­gres­sive­ly more of its near side turns towards sun­light. First, it becomes a wax­ing cres­cent moon, then a first-quar­ter moon, fol­lowed by a wax­ing gib­bous moon, cul­mi­nat­ing in a full moon. 

Full moon

Dur­ing a full moon, Earth is rough­ly between the moon and sun. The full moon is com­plete­ly vis­i­ble because it’s oppo­site the sun in the sky, and the hemi­sphere of its near side is in full sun­light. 

The lunar phas­es con­tin­ue in reverse beyond the full moon as its near side grad­u­al­ly turns away from the sun. These phas­es are wan­ing gib­bous moon, last quar­ter moon, wan­ing cres­cent moon, and then a new moon. The peri­od from new moon to new moon marks an entire lunar month, which takes 29.53 days to com­plete, and is equiv­a­lent to a sin­gle day/night cycle on the moon.

Daylight is a surrogate for sunlight on the moon

We’ve estab­lished it’s sun­ny on the moon and that we see vary­ing amounts of the sun­lit hemi­sphere through­out a lunar phase cycle. But we still have the prob­lem of not get­ting close enough to the moon to get an expo­sure read­ing. The solu­tion, as earth­bound pho­tog­ra­phers have fig­ured out long ago, is tak­ing an inci­dent read­ing from some­where that’s with­in reach and has light iden­ti­cal to their subject’s posi­tion. 

It’s sun­ny on the moon, but it’s also sun­ny here on Earth. To take pic­tures, it’s entire­ly rea­son­able to assume that after­noon sun­light on Earth is iden­ti­cal in inten­si­ty to sun­light on the full moon. There­fore, expo­sure set­tings appro­pri­ate for direct after­noon sun­light on Earth will pro­duce cor­rect expo­sures of the full moon at night. 

This is why photos of the moon at night are overexposed

Any­one who’s tried tak­ing pho­tos of the moon at night using their camera’s auto­mat­ic set­tings has prob­a­bly found the results dis­ap­point­ing. Most auto­mat­ic pho­tos of the moon at night are irrepara­bly over­ex­posed, and there are two rea­sons why. First, the moon is hun­dreds of times brighter than the sur­round­ing night sky. And sec­ond, although this bright­ness gives it visu­al promi­nence, its scale with­in most pho­to­graph­ic com­po­si­tions is rel­a­tive­ly tiny at typ­i­cal focal lengths. Togeth­er, these fac­tors cause the cam­era to assume it’s tak­ing a pho­to of some­thing quite dark, and it com­pen­sates by let­ting in far more light than your intend­ed sub­ject requires, which wash­es out the moon. 

The best way to get “cor­rect” expo­sures of the moon at night is by tak­ing com­plete con­trol of your cam­era using man­u­al mode. But what qual­i­fies as a “cor­rect” expo­sure?

The moon is darker than you think

Even when it’s not over­ex­posed, many pho­tos of the night­time moon ren­der it brighter than its true light­ness. In astron­o­my, “albe­do” describes the aver­age sur­face reflectance of plan­ets, moons, and aster­oids. Albe­do mea­sures the frac­tion of inci­dent light the sur­face reflects in all direc­tions. The moon has an albe­do of 0.12, which means it reflects just 12% of the Sun’s light. This trans­lates to an aver­age sur­face light­ness described as slight­ly brighter than old asphalt. In com­par­i­son, Earth’s albe­do aver­ages to about 0.30. The pho­tos tak­en by the crews of NASA’s Apol­lo land­ings show just how dark the lunar sur­face appears in com­par­i­son to the astro­nauts’ white space­suits in direct sun­light. 

The pur­pose of the Apol­lo pho­tos was to cre­ate an accu­rate visu­al doc­u­ment of the lunar sur­face, its fea­tures, and of the astro­nauts and their equip­ment. 

 Since earth­bound pho­tog­ra­phers don’t have such mis­sion-crit­i­cal con­straints, we’re free to take cre­ative license in our depic­tions of the moon. Some pho­tog­ra­phers choose accu­rate depic­tions. Oth­ers pre­fer rep­re­sen­ta­tions that are brighter than true while ensur­ing sur­face details aren’t washed out. And a third group doesn’t care because their pri­ma­ry sub­ject is some­thing else, such as the moon­lit land­scape. Hence, every men­tion of “cor­rect” expo­sures fea­tures scare quotes. I believe that with­in the art of pho­tog­ra­phy, every expo­sure is cor­rect so long as a result is inten­tion­al. An expo­sure is only wrong when the effect is unde­sir­able. 

The lunar phase affects brightness

The moon’s phase affects how bright it appears on Earth. The illu­mi­nat­ed por­tion of the moon looks bright­est dur­ing the full moon and dark­est dur­ing the cres­cent moon. As our angle of view rel­a­tive to the sun decreas­es, the moon’s high­ly crat­ed and irreg­u­lar sur­face forms a greater amount of shad­ows as seen by observers from Earth. This low­ers the sur­face reflectance of the sun­lit por­tion vis­i­ble to us. 

Addi­tion­al­ly, the full moon appears brighter due to a phe­nom­e­non called oppo­si­tion surge. It occurs when a rough sur­face appears brighter when the light source is direct­ly behind the observ­er. The Apol­lo mis­sions pro­vide human-scale exam­ples of this effect in their pho­tos from the sur­face. The surge in bright­ness is quite sub­tle due to its gra­da­tion and the impact of colour con­stan­cy. The dif­fer­ence in bright­ness becomes rather stark when mak­ing a side-by-side com­par­i­son of two non-adja­cent patch­es of lunar soil. In some pho­tos, the effect is also notice­able on a small scale in the reflec­tions of astro­nauts’ hel­mets. In this famous exam­ple, the oppo­si­tion surge bright­ens the area around Buzz Aldrin’s shad­ow, as seen in his helmet’s reflec­tion. And here, we see it in the reflec­tion of David R. Scott’s shad­ow from Apol­lo 15. On a macro scale, the entire vis­i­ble sur­face of the moon expe­ri­ences an oppo­si­tion surge of bright­ness dur­ing the full moon phase. 

Lunar altitude affects its brightness and colour due to atmospheric light scattering

Regard­less of the lunar phase, the moon’s bright­ness and colour are also affect­ed by its alti­tude, which describes the appar­ent height of a celes­tial object above the hori­zon. It’s expressed in degrees, with the hori­zon at 0° and the zenith (direct­ly over­head) at 90°. 

The moon appears brighter at pro­gres­sive­ly high­er alti­tudes. The sun exhibits the same char­ac­ter­is­tics: sun­light is harsh­est at solar noon and faintest at sun­set. In both cas­es, the atmos­pher­ic scat­ter­ing of light caus­es the effect. 

Light scat­ter­ing occurs when pho­tons bounce off par­ti­cles in their paths, such as atoms and mol­e­cules. Par­ti­cles that are small­er than the wave­length of vis­i­ble light are more effec­tive at scat­ter­ing the short-wave­length pho­tons of blue light than the long-wave­length pho­tons of red light. 

Light scat­ter­ing occurs at all alti­tudes. When the moon or sun is near the hori­zon — either ris­ing or set­ting — the light reach­ing your eyes pass­es through a thick lay­er of the atmos­phere, which scat­ters a far more sig­nif­i­cant amount of blue light than red. Since a large por­tion of their light is scat­tered away from a straight-line path to your eyes when they’re near the hori­zon, they appear red­der. At high­er alti­tudes, the moon’s light pass­es through a com­par­a­tive­ly thin lay­er of the atmos­phere, scat­ter­ing just enough blue light to give the Moon its char­ac­ter­is­tic yel­low­ish colour, dis­tinct from the stark grey sur­face depict­ed in the Apol­lo pho­tos. 

As an inter­est­ing side note, if you’re an ear­ly bird, you’ve prob­a­bly noticed that sun­ris­es are less red than sun­sets. That’s because there’s a greater propen­si­ty for stronger winds dur­ing the day­time, which helps lift dust par­ti­cles into the atmos­phere and scat­ters even more blue light. The same effect doesn’t nec­es­sar­i­ly apply to the set­ting and ris­ing of the moon. I’ve per­son­al­ly wit­nessed many red­dish moon­ris­es; how­ev­er, they’ve all occurred close to sun­set, while dust per­me­at­ed the local atmos­phere.

All of this relates to tak­ing pho­tos of the moon. Expo­sure set­tings derived from mid-after­noon day­light are gen­er­al­ly cor­rect for pic­tures of a full-ish moon at an alti­tude of 45° or greater (that is, more than halfway up between the hori­zon and zenith). How­ev­er, these set­tings will like­ly be incor­rect for pho­tos of the moon while it’s near the hori­zon since Earth’s atmos­phere atten­u­ates much of its bright­ness. 

Crescent moons and earthlight

Have you ever gazed upon a cres­cent moon and real­ized that you could see details in its shad­ed por­tion?

Much as with the moon, some sun­light that strikes Earth’s sur­face and clouds reflects into space. This reflect­ed light is called earth­light. The sub­tle illu­mi­na­tion of the Moon’s dark side by earth­light is called earth­shine. The dis­tinc­tion between these two terms can be con­fus­ing at first, but it’s all quite sim­ple if illus­trat­ed with a dia­gram. Light from the sun is sun­light. Sun­light reflect­ed by the earth is earth­light. Earth­light reflect­ed off the moon’s dark side is earth­shine. [Use a vari­ant of this dia­gram: https://upload.wikimedia.org/wikipedia/commons/3/3f/Earthshine_diagram.png]

Earth­shine is most promi­nent­ly vis­i­ble dur­ing the moon’s cres­cent phase. An observ­er stand­ing on the moon’s near-side would see a very bright “gib­bous Earth” against the black sky. At this point, you should come to the grad­ual real­iza­tion that the moon expe­ri­ences Earth in phas­es, and these phas­es are com­ple­men­tary. Thus, a new moon on Earth coin­cides with a full earth seen from the Moon, and so on. Earth­shine peaks dur­ing the new moon but remains invis­i­ble because of the moon’s prox­im­i­ty to the sun in the day­time sky.  

Tak­ing pho­tos of earth­shine using your camera’s auto­mat­ic mode should give decent results because earth­shine is clos­er in bright­ness to the typ­i­cal night or twi­light sky. How­ev­er, a sin­gle expo­sure can’t cap­ture detail in both because the dif­fer­ence in bright­ness between earth­shine and the sun­lit por­tion of the moon is too sig­nif­i­cant. Against a dark sky, your cam­era will over­ex­pose the cres­cent.

How to take photos of the moon at night

The point of this video is to explain the futil­i­ty of a sin­gle solu­tion. That’s because the moon’s bright­ness varies with its phas­es and alti­tude. More­over, the accu­ra­cy of your expo­sure to the moon’s true light­ness is also an artis­tic deci­sion. The solu­tion requires inter­nal­iz­ing a fun­da­men­tal prin­ci­ple: it’s sun­ny on the moon. 

How­ev­er, for those of you inclined to pre­scrip­tive rec­om­men­da­tions, start with expo­sures appro­pri­ate for the full moon high in the sky and incre­men­tal­ly work your way down to dim­mer moons.

Select man­u­al shoot­ing mode, and choose appro­pri­ate expo­sure set­tings for a sub­ject in direct after­noon sun­light on earth. At ISO 200, this means select­ing ƒ/5.6 and 1/2000s, or ƒ/8 and 1/1000s, or ƒ/11 and 1/500; all of these dif­fer­ent set­tings pro­duce the same expo­sure. When the moon is low­er in the sky or dur­ing a minor phase, increase your expo­sure by select­ing a low­er f‑number or slow­er shut­ter speed, or both. Expe­ri­ence and prac­tice using your cam­era make the process faster and eas­i­er. How­ev­er, it would help if you start­ed from the prin­ci­ple that it’s sun­ny on the moon.

Conclusion

I hope you found this video inter­est­ing and help­ful. I enjoy talk­ing my stu­dents through these types of ques­tions instead of stat­ing the cor­rect set­tings with­out explain­ing why they’re right. If you have requests for top­ics, let me know in the com­ments, and I’ll con­sid­er them for future videos. In the mean­time, you can learn more about pho­tog­ra­phy or join my group work­shops in Toron­to by vis­it­ing ExposureTherapy.ca. See you next time.

In this post, I’ll show you what to look for when inspect­ing a used lens that you’re buy­ing in per­son. The fol­low­ing is a tran­script of the video linked above.

New pho­tog­ra­phers who are pas­sion­ate about their hob­by quick­ly devel­op an enthu­si­asm for lens­es and the cre­ative pos­si­bil­i­ties they open. This desire often leads them to the sec­ond­hand mar­ket, which offers a cost-effec­tive way to buy pho­to­graph­ic equip­ment. 

Unfor­tu­nate­ly, when buy­ing a pre-owned lens direct­ly from a sell­er met through online clas­si­fieds such as Craigslist, Face­book Mar­ket­place, and Kiji­ji, you’re giv­ing up the peace of mind offered by store refunds and man­u­fac­tur­er war­ranties in exchange for a low­er price. Such trades involve items sold as-is using cash-only (or cash-like) trans­ac­tions. The nature of these deals means it’s your respon­si­bil­i­ty to con­firm that the sell­er’s descrip­tion of the item is accu­rate because if you dis­cov­er a prob­lem after the sale is com­plete, you almost cer­tain­ly have no recourse.

For­tu­nate­ly, you can pro­tect your­self against a bad trade and con­firm that a lens is in good work­ing order by per­form­ing a thor­ough inspec­tion of the lens on the spot. The fol­low­ing is a detailed list of what you should do and check when buy­ing a used lens is per­son.

1. Bring your camera

Although I rec­og­nize that this point is obvi­ous, it bears men­tion­ing: Remem­ber to bring the cam­era for which you’re buy­ing the lens, and don’t for­get the bat­tery and mem­o­ry card. 

This point deserves a short sto­ry. A few years ago, I was sell­ing my Canon 85 mm lens and arranged to meet with a young woman at a Star­bucks near my home. She was about fif­teen min­utes late and, cru­cial­ly, had for­got­ten to bring her cam­era. Her real­iza­tion quick­ly turned into embar­rass­ment, which threw her off bal­ance. She per­formed the most rudi­men­ta­ry inspection—confirming that the front and rear glass ele­ments weren’t broken—gave me her cash, and quick­ly left. I’m hon­est, so she got a good lens; how­ev­er, she could’ve eas­i­ly been ripped off because she did­n’t bring a cam­era to con­firm that the lens was func­tion­al.

2. Examine the lens exterior for wear and tear, scratches, and dents

Unless you’re buy­ing a rare col­lectible that’s spent its entire exis­tence in a pro­tec­tive case or a lens adver­tised as “like new,” most cam­era lens­es will have devel­oped some wear and tear from reg­u­lar use. Your goal is to estab­lish that the used lens you’re inspect­ing match­es the adver­tise­ment. Sig­nif­i­cant dif­fer­ences from the adver­tised descrip­tion and images of the lens serve as a con­ve­nient warn­ing that the sell­er is not entire­ly trust­wor­thy. 

In most cas­es, the pre-owned lens will match its adver­tised descrip­tion, and you can con­tin­ue with your exam­i­na­tion. Wear and tear are inevitable on lens­es that see use, espe­cial­ly by pro­fes­sion­al pho­tog­ra­phers. For exam­ple, it’s nor­mal to find scuff marks and wear of the paint on the fil­ter ring because it’s the front-most part of the lens. The ridges on rub­ber­ized zoom and focus­ing rings wear down with years of use. Hair­line scratch­es and scuff marks on the paint­ed or plas­tic exte­ri­or are also expect­ed and large­ly unavoid­able. Such super­fi­cial wear is nor­mal and won’t impact the opti­cal per­for­mance and char­ac­ter­is­tics of the lens. 

Dents on the bar­rel of the lens deserve greater scruti­ny because they sug­gest a more force­ful impact or drop. Such force could eas­i­ly knock the pre­ci­sion optics out of align­ment and reduce opti­cal per­for­mance. Ask the sell­er about the nature of the dam­age, keep it in mind, and con­tin­ue. 

3. Examine the front and rear glass elements.

Remove the lens caps to check the front and rear glass ele­ments. Clean glass is eas­i­er to check, so if you find fin­ger­prints, smudges, or dust on the glass, ask the own­er to clean them off before pro­ceed­ing with the inspec­tion. 

Exam­ine the front and rear glass ele­ments. Observe how reflec­tions pass along the sur­face of the lens­es. Ide­al­ly, the glass should be smooth and free from scratch­es, abra­sions, or thin­ning of the anti-reflec­tive coat­ing. 

In prac­tice, tiny scuffs and hair­line scratch­es, espe­cial­ly to the coat­ing, won’t affect image qual­i­ty in any mea­sur­able way. The only down­side to buy­ing a lens with scratched glass is that it may affect your future resale val­ue. Addi­tion­al­ly, if such a scratch wasn’t part of the seller’s descrip­tion of the lens, you could use it to your advan­tage by sug­gest­ing a reduced price.

(For those of you won­der­ing how bad­ly dam­aged a lens must be before its evi­dent in the pho­tos, take a look at the fol­low­ing pic­ture. Try to imag­ine what sort of dam­age caused this degree of soft­ness and loss of con­trast. Is it a scratch, or sev­er­al? Is it a crack, or sev­er­al? Now take a look at the lens that took the pho­to. How did you do? As it turns out, it takes sig­nif­i­cant dam­age to the front of a lens for the effect of that dam­age to be read­i­ly appar­ent in prac­ti­cal pho­tog­ra­phy.) 

4. Check lens for internal dust and fungus

With the lens caps removed, shine your phone’s LED light through the back of the lens while look­ing at its inter­nal com­po­nents through the front. Avoid look­ing direct­ly at the mag­ni­fied LED, as it’s incred­i­bly bright. 

If you’re in a dim­ly lit envi­ron­ment, you’ll see the con­cen­trat­ed beam form through the lens ele­ments. You’ll also see a heap of dust and tiny imper­fec­tion that will make you regret ever try­ing this tech­nique. Lens­es get dusty, and zoom lens­es get dusti­er. That’s because every  time you zoom a lens, glass has to move back and forth, expand­ing or col­laps­ing the inte­ri­or vol­ume. This motion dis­places air, either push­ing it out or suck­ing it into the lens. (On some cam­eras, you can feel air “blow­ing back” into your eye through the viewfind­er.)

For­tu­nate­ly, the dust found inside lens­es is mean­ing­less to pho­tog­ra­phers because it’s too small to mat­ter and does­n’t resolve in your pic­tures. You want to look for fun­gus, which can show as soft fluffy dots or fuzzy fibres or webs sprin­kled through­out the inte­ri­or glass. Fun­gus spores find their way into a lens on dust and pro­lif­er­ate after extend­ed peri­ods of stor­age in warm and humid envi­ron­ments. The fun­gus can grow and per­ma­nent­ly dam­age the glass of your lens unless it’s pro­fes­sion­al­ly cleaned. Always store your lens­es in cool and dry envi­ron­ments.

5. Examine the electronic contacts for signs of wear and dirt (where available)

The elec­tron­ic con­tact points found on the back of mod­ern lens­es facil­i­tate com­mu­ni­ca­tion with the cam­era. Ensure that the con­tacts are clean and don’t exhib­it signs of cor­ro­sion. The pres­ence of dirt and oth­er deposits on the elec­tron­ic con­tacts of a lens can wear down the thin gold-plat­ing and cause data com­mu­ni­ca­tion errors, which can result in loss of aper­ture con­trol, aut­o­fo­cus, opti­cal image sta­bi­liza­tion, and lens-relat­ed meta­da­ta. You can clean dirty pins, but cor­rod­ed ones require repair.

6. Examine the lens mount for damage

A lens is attached to a cam­era using the mount, which pro­vides a secure point of attach­ment and ensures that the lens and cam­era are cor­rect­ly aligned. The vast major­i­ty of mod­ern lens­es have met­al mount­ing rings, but a few bud­get-ori­ent­ed lens­es fea­ture plas­tic mounts. 

When you’re exam­in­ing a lens with a met­al mount, visu­al­ly con­firm that there’s no defor­ma­tion of the met­al tabs at the base of the lens. This kind of dam­age could pre­vent the lens from secure­ly attach­ing to the cam­era, or worse, dam­age the cam­er­a’s mount­ing ring if forced. Addi­tion­al­ly, check to ensure the lens mount is firm­ly attached to the lens barrel—the attach­ment screws shouldn’t loose or miss­ing.

Plas­tic mounts are less like­ly to deform but more like­ly to crack, chip, or wear down. Exam­ine the plas­tic mount and tabs for signs of cracks, and con­firm the mount is firm­ly attached to the lens bar­rel.  

Now it’s time to attach the lens to your cam­era.

7. Make sure the lens attaches tightly and locks into place with a click 

Attach the lens to the cam­era body and make sure it locks into place with an audi­ble click. The lens should fit rel­a­tive­ly tight­ly, although a tiny amount of rota­tion­al give is nor­mal. With that said, there should­n’t be any tilt­ing or sag­ging; the lens axis must always remain per­pen­dic­u­lar to the image sen­sor.

8. Confirm that the focusing ring works

Inspect­ing the focus­ing ring requires some under­stand­ing of what you’re buy­ing. To help you, I’ll cov­er the three main cat­e­gories.

Manual focus lenses:

Many vin­tage and some third-par­ty or spe­cial-pur­pose lens­es are focused by man­u­al­ly rotat­ing the mechan­i­cal­ly cou­pled focus­ing ring. Since there’s no aut­o­fo­cus fall­back, it’s essen­tial to con­firm that the focus­ing ring works cor­rect­ly and focus­es the lens. With the cam­era switched on and your eye to the viewfind­er, rotate the focus­ing ring from one extreme to the oth­er. The scene in the viewfind­er should shift in and out of focus. Addi­tion­al­ly, the focus­ing ring should rotate smooth­ly across its entire range of motion with­out any grit or sense of slack. 

Autofocus lenses (with mechanically coupled focusing rings):

The major­i­ty of aut­o­fo­cus lens­es designed for SLR cam­eras fea­ture focus­ing rings that are mechan­i­cal­ly-cou­pled to the opti­cal sys­tem. These types of lens­es often have a focus mode switch on the lens bar­rel that let’s you select between man­u­al focus and aut­o­fo­cus shoot­ing. In most cas­es, the focus­ing ring will always work regard­less of the focus mode. (Keep in mind, there are some excep­tions to this, so know what you’re buy­ing!)

Switch the cam­era on, turn the focus mode to Man­u­al Focus (MF), look through the viewfind­er, and rotate the focus­ing ring from one extreme to the oth­er. Then, repeat those with the focus mode turned to Aut­o­fo­cus (AF). In either case, the scene should shift in and out of focus and the focus­ing ring should move smooth­ly across its range of motion.

Autofocus lenses (with “focus by wire”):

There’s a small but grow­ing class of aut­o­fo­cus lens­es with elec­tron­i­cal­ly cou­pled focus­ing rings. These types of lens­es are infor­mal­ly called “focus by wire” because there’s no direct mechan­i­cal con­nec­tion between the focus­ing ring and the inter­nal lens ele­ments. Instead, your inputs are trans­mit­ted elec­tron­i­cal­ly to the motors dri­ving the focus­ing sys­tem. 

Turn the cam­era on, set the focus mode to Man­u­al Focus, look through the viewfind­er, and rotate the focus­ing ring. Since there’s no phys­i­cal con­nec­tion, you’re most­ly con­firm­ing the elec­tron­ic con­nec­tion is intact, that the motors work, and that the focus­ing ring rotates smooth­ly across its range of motion.

9. Confirm that autofocus works

Unfor­tu­nate­ly, aut­o­fo­cus errors can occur on both DSLR and mir­ror­less cameras—even on brand new lens­es. For exam­ple, the zoom lens I use to make these videos is my sec­ond copy. The first one had an aut­o­fo­cus so faulty that every two out of five shots were mis­fo­cused. I was lucky to notice the prob­lem before my 14-day return peri­od end­ed. Sad­ly, there’s no return pol­i­cy when buy­ing a used lens from some­one you meet on Craigslist. So don’t be shy about car­ry­ing out a thor­ough inspec­tion when buy­ing an item that’s sold “as-is.”

To con­firm that the elec­tron­ic focus­ing sys­tem works and the lens can aut­o­fo­cus accu­rate­ly, set your cam­era to use a sin­gle aut­o­fo­cus point and take sev­er­al pic­tures of near and far objects, chang­ing between them with every shot. Review each pic­ture at full mag­ni­fi­ca­tion to ver­i­fy that the aut­o­fo­cus was con­sis­tent­ly accu­rate. 

Pro-tip: you can shift between pho­tos while review­ing them at full mag­ni­fi­ca­tion by rotat­ing the main com­mand dial on your cam­era. 

10. Check the zoom ring for function and smoothness

The major­i­ty of zoom lens­es have mechan­i­cal­ly cou­pled zoom rings. Switch on your cam­era, look through the viewfind­er, and rotate the zoom ring from one extreme to the oth­er and con­firm that your angle of view changes. The zoom ring should rotate smooth­ly with an even amount of resis­tance through­out the range of motion. You should­n’t sense any under­ly­ing grit, impinge­ment, or slack. 

Since some zoom lens­es extend out­wards at longer focal lengths, it’s a good idea to inspect the new­ly exposed part of the bar­rel for abra­sions, dam­age, and debris. Gen­er­al­ly speak­ing, there should­n’t be much give or wob­bling, even at its max­i­mum exten­sion. How­ev­er, some lens­es can slow­ly extend when point­ing down or slow­ly retract when point­ing up. 

11. Can the lens communicate with your camera?

When inspect­ing an elec­tron­ic lens, it’s nec­es­sary to con­firm that the lens can suc­cess­ful­ly com­mu­ni­cate with the cam­era. In a sense, you’ve already con­firmed this by engag­ing the aut­o­fo­cus. How­ev­er, since there are some man­u­al focus lens­es with elec­tron­i­cal­ly con­trolled aper­tures, it’s a good idea to be spe­cif­ic. 

You can con­firm that a cam­era rec­og­nized an elec­tron­ic lens when it dis­plays an aper­ture val­ue oth­er than 0. As anoth­er option, you can take a pic­ture and look at its meta­da­ta. When every­thing works cor­rect­ly, the cam­era should dis­play the zoom range, set focal length, and set aper­ture val­ue in the picture’s meta­da­ta.

12. Does the aperture work?

It’s impor­tant to make sure the aper­ture changes in size when adjust­ing the aper­ture val­ue. Don’t assume that putting the cam­era into Aper­ture Pri­or­i­ty mode, rotat­ing a dial, and watch­ing the f‑numbers change cor­re­sponds to a func­tion­ing iris diaphragm. Regard­less of the f‑number you set, a mod­ern lens will keep its aper­ture ful­ly open up to the point that you push the shut­ter but­ton to take a pic­ture. The aper­ture’s size is adjust­ed to your cho­sen f‑number only when you push the shut­ter but­ton. This behav­iour facil­i­tates more accu­rate aut­o­fo­cus­ing in dark­er envi­ron­ments and pro­vides a brighter view in the find­er. 

The fol­low­ing method should work for both DSLR and mir­ror­less cam­eras, even those with­out a depth of field pre­view but­ton. Put the cam­era into Man­u­al Expo­sure mode, select a large f‑number and a slow shut­ter speed (some­thing like 2 to 4 sec­onds), look into the lens from the front, and press the shut­ter down to take a pic­ture. Take note of the aperture’s size dur­ing expo­sure, and then take sev­er­al pic­tures more. The iris should close down to the same size con­sis­tent­ly. Any devi­a­tion in aper­ture size with­out a cor­re­spond­ing change to the f‑number could spell trou­ble for the con­sis­ten­cy of your expo­sures.

13. Does the optical image stabilization work?

If you’re check­ing a used lens that fea­tures opti­cal image sta­bi­liza­tion, ver­i­fy whether it oper­ates by turn­ing the switch on and off while look­ing through the viewfind­er and half-press­ing the shut­ter but­ton. And if you hap­pen to be inspect­ing a vari­able focal length lens, make sure you’re ful­ly zoomed in, because the sta­bi­liz­ing effect is more obvi­ous at longer focal lengths.

You can also place the cam­era into Shut­ter Pri­or­i­ty Mode, select a rel­a­tive­ly slow shut­ter speed, and take sev­er­al hand­held pho­tos with the sta­bi­liza­tion fea­tured enabled and then sev­er­al with it dis­abled.

When func­tion­ing cor­rect­ly, image sta­bi­liza­tion should reduce or elim­i­nate the motion blur asso­ci­at­ed with a shaky cam­era. 

Bonus tips

And now it’s time for the bonus round of quick tips. 

Many vin­tage lens­es have mechan­i­cal­ly-cou­pled aper­ture rings. When check­ing such lens­es, ensure the aper­ture opens and clos­es all the way and con­sis­tent­ly, and make sure the detents indi­cat­ing inter­me­di­ate steps are click­ing.

If you dis­cov­er that the lens comes with a UV or “pro­tec­tion” fil­ter already attached, ask the sell­er to remove it. Remov­ing the fil­ter accom­plish­es two things: it gives you a bet­ter look at the con­di­tion of the lens under­neath, and it demon­strates that the fil­ter thread­ing isn’t dam­aged. Imper­cep­ti­ble dents can dam­age the thread­ing and make it prac­ti­cal­ly impos­si­ble to remove or attach a fil­ter. 

If the lens fea­tures “weath­er resistance”—often des­ig­nat­ed by the char­ac­ters WR—check the con­di­tion of the rub­ber flange around the lens mount for cracks, tears, or notch­es. This type of dam­age will essen­tial­ly nul­li­fy the weath­er-resis­tance of both your lens and cam­era.

If the lens has a focus dis­tance window—which is a clear plas­tic win­dow with focus dis­tance mark­ings underneath—make sure that turn­ing the focus­ing ring or using the cam­er­a’s aut­o­fo­cus moves the under­ly­ing dis­play. 

Last­ly, it’s tremen­dous­ly impor­tant to under­stand what you’re seek­ing to buy. Before meet­ing with any­one, read reviews of the lens you’re con­sid­er­ing so that you can tell the dif­fer­ence between nor­mal quirks and flaws or faults. Such basic research could inform you, for exam­ple, that the focus­ing sys­tem of the Fuji­film 90 mm ƒ/2 lens can wob­ble about when there’s no pow­er to the lens—and that it’s a com­plete­ly nor­mal.

Con­clu­sion

Now you should know what to check for when buy­ing a used lens in per­son. If you have requests for top­ics, let me know in the com­ments, and I’ll con­sid­er them for future videos. In the mean­time, you can learn more about pho­tog­ra­phy by join­ing on of Expo­sure Ther­a­py’s group pho­tog­ra­phy lessons.

F‑numbers and the aperture have an inverse relationship

Hi there, my name is Paul, and this is Expo­sure Ther­a­py. In this video, I’ll explain the rea­son for the inverse numer­i­cal rela­tion­ship between f‑numbers and the aper­ture. This rela­tion­ship is a wide­spread point of con­fu­sion for many begin­ner pho­tog­ra­phers, who regard it as irra­tional or need­less­ly com­plex. My goal is to dis­pel the mys­tery around f‑numbers and demon­strate why they’re a per­fect­ly rea­son­able method for express­ing how the aper­ture affects expo­sure.

Under­stand­ing the rela­tion­ship between pic­ture bright­ness and both the shut­ter speed and ISO is straight­for­ward for stu­dents learn­ing the basics of pho­tog­ra­phy. Shut­ter speed is expressed numer­i­cal­ly in time units, with the most com­mon being frac­tions of a sec­ond; longer dura­tions result in brighter pic­tures, and short­er dura­tions result in dark­er pic­tures. ISO is also expressed numer­i­cal­ly; big­ger num­bers pro­duce brighter pho­tos, and small­er num­bers make dark­er pho­tos. 

In both cas­es, the rela­tion­ship between the set­ting and its effect on pic­ture bright­ness is easy to under­stand because there’s a pos­i­tive cor­re­la­tion, and they move in tan­dem. For exam­ple, when you dou­ble the expo­sure dura­tion, it dou­bles the bright­ness; when you halve the ISO, it halves the bright­ness. It’s a sim­ple rela­tion­ship that stu­dents in my pho­tog­ra­phy work­shops grasp with ease. 

The inverse relationship between f‑numbers and the aperture is confusing for many beginners

Unfor­tu­nate­ly, the rela­tion­ship between f‑numbers, aper­ture size, and pic­ture bright­ness is not as imme­di­ate­ly intu­itive. Begin­ners are con­fused by the neg­a­tive (or inverse) rela­tion­ship between f‑numbers and aper­ture size. In addi­tion, they have a hard time under­stand­ing why big­ger f‑numbers rep­re­sent small­er aper­tures that reduce bright­ness, and small­er f‑numbers define larg­er aper­tures that increase bright­ness. 

The best way to address this is by start­ing with the basics. Inside every inter­change­able lens is a ring of over­lap­ping blades col­lec­tive­ly known as an iris diaphragm or iris. Expand­ing or con­tract­ing the blades adjusts the open­ing in the cen­tre of the iris, called the aper­ture. 

Introducing the Entrance Pupil

When you hold a lens up and look at the aper­ture, what you’re see­ing is tech­ni­cal­ly called the “entrance pupil.” The entrance pupil is the opti­cal image of the phys­i­cal aper­ture as seen through the front of the lens. This dis­tinc­tion mat­ters because when you look at the front of a lens, you see the aper­ture through mul­ti­ple lay­ers of glass that affect its mag­ni­fi­ca­tion and per­ceived loca­tion in space com­pared to the phys­i­cal open­ing in the iris. For the sake of sim­plic­i­ty, I’ll use “aper­ture” when refer­ring to both the set­ting and the phys­i­cal open­ing and “entrance pupil” in ref­er­ence to dimen­sions.

Chang­ing the size of the aper­ture adjusts the inten­si­ty of light pass­ing through the lens. Increas­ing the aperture’s size allows more light to pass through the lens, increas­ing expo­sure and cre­at­ing a brighter pic­ture. Con­verse­ly, decreas­ing the aperture’s size reduces how much light pass­es through the lens, reduc­ing expo­sure and result­ing in a dark­er pho­to. 

Why are apertures expressed using f‑numbers?

We express aper­ture val­ues using f‑numbers and not as the mea­sured size of the entrance pupil, such as its diam­e­ter, radius, or area, because it neglects the essen­tial role of focal length. This can be demon­strat­ed with a thought exer­cise.

Let’s pre­tend we have two lens­es attached to iden­ti­cal cam­eras: one lens is 50 mm and the oth­er is 100 mm, and both have entrance pupils with 25 mm diam­e­ters. Since their entrance pupils are iden­ti­cal in size, an equal amount of light enters each lens. How­ev­er, because the focal length of the 100 mm lens is twice that of the 50 mm lens, the light pass­ing through it has to trav­el twice the dis­tance to reach its camera’s image sen­sor, which pro­duces a dark­er image. 

Reduc­tion in bright­ness occurs because light has the prop­er­ty of spread­ing out as it recedes from its source, and from the per­spec­tive of your camera’s image sen­sor, this source is the point inside the lens from which focal length is mea­sured. This trait of light to dif­fuse out­wards is described by the Inverse Square Law, which states that inten­si­ty is inverse­ly pro­por­tion­al to the square of the dis­tance. In this exam­ple, the inverse square law informs us that the 100 mm lens expos­es its camera’s image sen­sor to 1/4 the light com­pared to the 50 mm lens because it’s twice as long. This occurs because one over two squared equals one-quar­ter.

The 100 mm lens can pro­vide an expo­sure equal to its 50 mm coun­ter­part by open­ing its aper­ture to col­lect four times more light, assum­ing its aper­ture can open that much. Since aper­tures are rough­ly cir­cu­lar, we can deter­mine how big they should be by cal­cu­lat­ing the area of a cir­cle. An entrance pupil with a 25 mm diam­e­ter has an area of about 491 mm^2. The 100 mm lens would need an entrance pupil with an area of 1,964 mm^2, which is formed by a cir­cle with a 50 mm diam­e­ter. Sim­ple, right?

F‑numbers express ratios

For­tu­nate­ly, pho­tog­ra­phers don’t need to per­form such cal­cu­la­tions to take pic­tures! That’s because hid­den with­in these num­bers is a straight­for­ward rela­tion­ship. For exam­ple, notice how the expo­sure pro­duced by the 50 mm lens with a 25 mm entrance pupil is iden­ti­cal to the 100 mm lens with a 50 mm entrance pupil. This is because in both cas­es, the ratio of the focal length to the entrance pupil diam­e­ter is 2:1. 

This is pre­cise­ly why the f‑number is some­times called the f‑ratio. The f‑number express­es a ratio of the lens focal length to the diam­e­ter of the entrance pupil, and it’s defined by the equa­tion N=ƒ/D. Thus, the f‑number equals the focal length divid­ed by the entrance pupil diam­e­ter. It can also be mod­i­fied to solve for the entrance pupil diam­e­ter using the equa­tion D=ƒ/N. Thus, the entrance pupil diam­e­ter equals the focal length divid­ed by the f‑number. 

These equa­tions demon­strate that choos­ing the same f‑number on a lens of any focal length will result in the same amount of light pass­ing through the lens. They also explain the inverse rela­tion­ship between f‑numbers and expo­sure. For a giv­en focal length, as the aperture’s size increas­es, the ratio decreas­es, and vice ver­sa. 

A 50 mm lens set to ƒ/4 will have an entrance pupil diam­e­ter of 12.5 mm—because 50 divid­ed by 12.5 equals 4. A 24 mm lens set to ƒ/8 will have an entrance pupil diam­e­ter of 3 mm. Some lens­es can open to ƒ1.0, in which case the entrance pupil diam­e­ter and focal length are equal. 

The f‑number scale

The stan­dard f‑number scale is: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, and so on. The dif­fer­ence in expo­sure between adja­cent num­bers is one stop, which means that it either dou­bles or halves the amount of light pass­ing through the lens depend­ing on whether you’re open­ing or clos­ing the aper­ture. How­ev­er, the numer­ic sequence grows by a fac­tor of about 1.4 or shrinks by a fac­tor of about 0.7. 

Most pho­tog­ra­phers sim­ply com­mit the stan­dard f‑number scale to mem­o­ry. How­ev­er, if you’re hav­ing trou­ble, a more straight­for­ward method is to remem­ber just the first two numbers—1 and 1.4—because the rest of the scale is an iter­a­tion of dou­bling each in alter­nat­ing order. The next f‑number is always dou­ble the pre­vi­ous one. So the num­ber after ƒ/1.4 is dou­ble of ƒ/1, which is ƒ2. Like­wise, the num­ber after ƒ/2 is dou­ble of ƒ/1.4, which is ƒ/2.8.  And on and on it goes.

Last­ly, dou­bling the f‑number, such as chang­ing it from ƒ/2.8 to ƒ/5.6, reduces pic­ture bright­ness by one-quar­ter. And con­verse­ly, halv­ing the f‑number, such as adjust­ing from ƒ/8 to ƒ/4, increas­es pic­ture bright­ness four times. 

I hope this helped you under­stand the inverse numer­i­cal rela­tion­ship between f‑numbers and their effect on the aper­ture. If you have requests for future top­ics, let me know in the com­ments, and I’ll address them in future videos. In the mean­time, you can learn more about pho­tog­ra­phy on ExposureTherapy.ca. See you next time.

Portrait lenses and focal length

One of the most dis­cussed and mis­un­der­stood prop­er­ties of por­trait lens­es is focal length. If you ask your pre­ferred online com­mu­ni­ty for por­trait lens sug­ges­tions, chances are, many users will respond by rec­om­mend­ing spe­cif­ic focal lengths. 

Per­haps the most com­mon­ly rec­om­mend­ed focal length for por­trai­ture on full-frame cam­eras is 85 mm; oth­er pop­u­lar focal lengths include 50 mm, 105 mm, 135 mm, and 70–200 mm zooms. If you’re at all famil­iar with the con­cept of focal length, you should notice that most of these sug­ges­tions are in the short to medi­um tele­pho­to range.

When explain­ing their rec­om­men­da­tions, pho­tog­ra­phers claim that wide-angle lens­es make faces look bad or that such-and-such focal length is too wide for por­traits. When this sen­ti­ment is left as-is, the unfor­tu­nate impli­ca­tion is that some inher­ent and mys­te­ri­ous qual­i­ty of wide-angle lens­es caus­es ugli­ness and that longer focal lengths pro­vide the solu­tion. To help you under­stand these warn­ings and sug­ges­tions, I’ll briefly explain the con­cepts of scale and per­spec­tive dis­tor­tion.

Scale, focal length, and perspective

In my Com­po­si­tion for Begin­ners video, I touched upon the con­cept of scale, which I use to describe the appar­ent size of your sub­ject with­in the pho­to­graph­ic frame. Your subject’s scale is deter­mined by two fac­tors when you’re tak­ing a pic­ture: focal length and per­spec­tive. 

The focal length of a lens deter­mines its mag­ni­fy­ing pow­er. This is the appar­ent size of your sub­ject as pro­ject­ed onto the focal plane where your image sen­sor resides. A longer focal length cor­re­sponds to greater mag­ni­fi­ca­tion and a larg­er ren­di­tion of your sub­ject, and a short­er focal length results in less mag­ni­fi­ca­tion and a small­er ren­di­tion of your sub­ject.

The appar­ent size of a sub­ject at a fixed dis­tance from the cam­era is direct­ly pro­por­tion­al to the lens’s focal length. So, for exam­ple, if you pho­to­graph a kid hold­ing a beach­ball and then switch to a lens that is twice the focal length of the first, the ren­dered size of every ele­ment in your image, from the kid to the beach­ball, will be dou­bled in size along their lin­ear dimensions—meaning in height and width. That’s how focal length affects scale.

In pho­tog­ra­phy, per­spec­tive is your camera’s point of view and is deter­mined exclu­sive­ly by the posi­tion from which a pho­to is tak­en. For sim­plic­i­ty, con­sid­er this the cam­era-to-sub­ject dis­tance. Changes in the sub­jec­t’s dis­tance have an obvi­ous effect on their per­ceived scale in a pho­to­graph. Ask your sub­ject to come half as close, and they’ll appear twice as large; ask them to move twice as far back, and they’ll appear half as small. That’s how per­spec­tive affects scale.

To main­tain an equal sub­ject scale in the frame, the focal length and sub­ject dis­tance must change lin­ear­ly, togeth­er and in the same direc­tion. If your sub­ject dou­bles their dis­tance for a giv­en scale, you will have to dou­ble your focal length to main­tain the orig­i­nal scale; if your sub­ject halves their dis­tance, you’ll have to halve your focal length. For exam­ple, if you like the scale of your sub­ject at 50 mm, but cir­cum­stances force the pho­to to be tak­en from half the ini­tial dis­tance, you’ll need to use 25 mm to obtain the orig­i­nal scale. Unfor­tu­nate­ly, short­en­ing the sub­ject dis­tance can result in per­spec­tive dis­tor­tion.

Perspective distortion and telephoto compression

In pho­tog­ra­phy, per­spec­tive dis­tor­tion is an inevitable con­se­quence of how sub­ject dis­tance affects scale. Objects that are close to the cam­era appear much big­ger rel­a­tive to objects that are far­ther away. So, for exam­ple, Gin­ger looks three times larg­er than Vio­let because Vio­let is three times far­ther from the cam­era. This rela­tion­ship will hold whether their dis­tances from the cam­era are 1 and 3 m, 5 and 15 m, or 20 and 60 m, respec­tive­ly, because in each case, Vio­let is three times far­ther than Gin­ger. 

This rela­tion­ship stops being true when the cam­era starts to change its dis­tance for the sub­jects whose dis­tances are fixed rel­a­tive to one anoth­er. The dis­par­i­ty in their appar­ent size will decrease as the cam­era moves fur­ther back until these dif­fer­ences become imper­cep­ti­ble. This effect is known as “tele­pho­to com­pres­sion”; how­ev­er, despite its name, it occurs in pho­tos tak­en with all focal lengths when a dis­tant sub­ject is vis­i­ble. Tele­pho­to lens­es make it more obvi­ous because the “tele-com­pressed” sub­jects are shown at a larg­er scale in the frame.

Perspective in portraiture

Gin­ger and Violet’s rela­tion­ship plays out on a small­er scale with­in the fea­tures of a sin­gle sub­ject. Peo­ple aren’t flat, and we’re not card­board cutouts; our faces, heads, and bod­ies have depth and dimen­sion. In a stan­dard por­trait, your subject’s nose is clos­er to the cam­era than their eyes, which, in turn, are clos­er than their ears. These dif­fer­ences are rel­a­tive­ly insignif­i­cant at long work­ing dis­tances. How­ev­er, they become sig­nif­i­cant at the very close sub­ject dis­tances required to achieve a “stan­dard” por­trait com­po­si­tion using a wide-angle lens. This leads to per­spec­tive dis­tor­tion, char­ac­ter­ized by a nose that looks too large rel­a­tive to the face, a nar­row­er head, and ears that appear pinned back. From extreme­ly close dis­tances, the cheeks can occlude the ears alto­geth­er. 

The char­ac­ter­is­tics attrib­uted to per­spec­tive dis­tor­tion are entire­ly a con­se­quence of the cam­era-to-sub­ject dis­tance. The focal length of a lens doesn’t direct­ly influ­ence per­spec­tive. This bears repeat­ing: focal length does not affect per­spec­tive. Despite this, it’s often blamed for the effect because dif­fer­ent focal lengths are used for dif­fer­ent pur­pos­es and vary­ing sub­ject dis­tances. Wide-angle lens­es are typ­i­cal­ly used from short­er dis­tances, lest the sub­ject appears too small in the pic­ture, while long focal lengths are gen­er­al­ly used from far­ther away, lest the sub­ject appears too large. 

The subject distance is the only factor in perspective distortion

Let’s dive a bit deep­er. You’ve prob­a­bly seen this or sim­i­lar effects before. Here’s a famous vari­a­tion, known as a “dol­ly zoom,” from the movie Jaws. This effect is cre­at­ed by tak­ing your first shot from a close dis­tance and using a short focal length. Take the sec­ond shot from slight­ly far­ther back and with a pro­por­tion­ate­ly longer focal length. And on and on. Such ani­ma­tions com­mon­ly illus­trate how dif­fer­ent focal lengths affect our per­cep­tion of appar­ent facial geom­e­try. Most exam­ples, such as this one by Dan V., label each frame’s focal length but omit the sub­ject dis­tance, which is arguably more impor­tant since you can’t have per­spec­tive dis­tor­tion with­out chang­ing your per­spec­tive. Since focal length doesn’t affect per­spec­tive, we can illus­trate the same effect by vary­ing the cam­era-to-sub­ject dis­tance with­out adjust­ing the focal length. Ini­tial­ly, the dis­tor­tion is dif­fi­cult to see because the sub­ject becomes small­er. How­ev­er, the per­spec­tive dis­tor­tion becomes obvi­ous when crop­ping each pho­to to equal­ize the subject’s scale through­out the sequence. Adding dis­tance labels instead of focal lengths cre­ates a much more prac­ti­cal point of ref­er­ence.

How to choose the correct focal length for portrait photography

When some­one sug­gests that a par­tic­u­lar focal length is ide­al for por­trai­ture, they’re real­ly express­ing two pref­er­ences: one for the rel­a­tive appear­ance of facial pro­por­tions from a giv­en dis­tance and anoth­er for the subject’s scale with­in a com­po­si­tion. Only you can deter­mine whether you share the same pref­er­ences for both. 

Although there’s no ide­al uni­ver­sal dis­tance for por­trait pho­tog­ra­phy, we can find sev­er­al clues in prox­emics, which is the study of how peo­ple uncon­scious­ly struc­ture the space between them­selves and oth­ers. For exam­ple, con­sid­er the idea of inter­per­son­al dis­tance zones pro­posed by Edward T. Hall in 1966. These are divid­ed into the inti­mate dis­tance (from 0–45 cm), per­son­al dis­tance (from 45 cm to 1.2 m), social dis­tance (from 1.2 to 3.7 m), and pub­lic dis­tance (from 3.7 m and greater). Although these spe­cif­ic ranges are biased towards white Amer­i­can males and may not apply to you or your cul­ture, you like­ly have an approx­i­mate notion of what you con­sid­er com­fort­able inter­per­son­al dis­tances. 

Under­stand­ing this makes choos­ing the right focal length for por­trait pho­tog­ra­phy straight­for­ward. First, decide the approx­i­mate dis­tance from which you feel peo­ple look their best, and sec­ond, select a focal length that pro­duces the com­po­si­tion you want at your pre­ferred dis­tance. There­fore, if you pre­fer how peo­ple appear from longer dis­tances and favour tight­ly framed pho­tos that bor­der on head-n-shoul­ders, your style calls for a medi­um or longer tele­pho­to lens. Pho­tog­ra­phers who are par­tial to envi­ron­men­tal por­trai­ture, which show­cas­es peo­ple in their usu­al envi­ron­ment, can com­bine a long sub­ject dis­tance with a wide-angle lens. The per­mu­ta­tions are prac­ti­cal­ly end­less, so do what makes you hap­py.

Keep in mind: if you dis­cov­er a fond­ness for wide-angle close-scale por­traits, it’s impor­tant to know the ulti­mate pur­pose and audi­ence for your pho­tos. Researchers pho­tographed sub­jects simul­ta­ne­ous­ly from two cam­era dis­tances, 45 cm and 135 cm, in an exper­i­ment about the effect of per­spec­tive dis­tor­tion on social judg­ment. Exper­i­menters found that study par­tic­i­pants pre­ferred faces pho­tographed from out­side of the per­son­al dis­tance zone more than those pho­tographed from with­in it and rat­ed them high­er for attrac­tive­ness, com­pe­tence, and trust­wor­thi­ness. When you get an oppor­tu­ni­ty to pho­to­graph your favourite evil politi­cian, take a note from Pla­ton: get close and shoot wide. 

Jokes aside, this research was pub­lished in 2012, and mobile social media plat­forms have had years of explo­sive growth ever since. That means arms-length portraits—otherwise known as selfies—of celebri­ties, pub­lic fig­ures, and your secret crush are ubiq­ui­tous and acces­si­ble and pro­vide the gen­er­al pub­lic with con­stant expo­sure to exam­ples of per­spec­tive dis­tor­tion on con­ven­tion­al­ly attrac­tive faces, which is some­thing we weren’t privy to more than a decade ago. 

A final note on wide-angle vs telephoto for portraiture

One more thing: tele­pho­to lens­es have a unique ben­e­fit over wide-angle lens­es because their rel­a­tive­ly nar­row­er angle of view allows minute shifts in per­spec­tive to alter the photo’s back­ground dra­mat­i­cal­ly. It’s use­ful for remov­ing parts of the back­ground from the com­po­si­tion that you feel are dis­tract­ing. Since short focal length lens­es cap­ture a wider angle of view, equal­ly small move­ments will not accom­plish the same goal. 

Conclusion

And there you have it, a guide for choos­ing your next por­trait lens. Per­son­al­ly, my favourite lens for por­trai­ture is the Fuji­non 56 mm ƒ/1.2. It offers an angle of view equiv­a­lent to an 85 mm lens on full-frame cameras—so it’s a short tele­pho­to lens—features a huge ƒ/1.2 aper­ture at which it’s quite sharp, and has love­ly bokeh. With that in mind, I’ve used this lens for many oth­er sub­jects, rang­ing from still-life, street scenes, and land­scapes. This brings me to my final point: no mat­ter what lens you buy, no mat­ter what cat­e­go­ry of pho­tog­ra­phy it’s mar­ket­ed towards, I encour­age you to exper­i­ment using it on dif­fer­ent sub­jects and in a vari­ety of set­tings. Always explore and dis­cov­er, and don’t put your­self in a box. 

 If you have requests for future top­ics, let me know in the com­ments, and I’ll address them in future videos. In the mean­time, you can learn more about pho­tog­ra­phy on ExposureTherapy.ca. See you next time.

How do you select a focal length for your first prime lens?

Hi every­one, my name is Paul, this is Expo­sure Ther­a­py, and in this video, I’ll demon­strate how Adobe Light­room Clas­sic can help you select your next prime lens.

Most of the stu­dents that attend my pho­tog­ra­phy work­shops bring gear pur­chased as part of a bun­dle or kit mar­ket­ed towards begin­ners. The kits typ­i­cal­ly include a basic DSLR or mir­ror­less cam­era, and a zoom lens with an 18–55 or 16–50 mm focal length, which varies depend­ing on the cam­era make. Some kits include a 75–300 mm lens for greater reach, but these are rar­er. 

When the work­shops tran­si­tion to the top­ic of the aper­ture and depth of field, some stu­dents real­ize that their basic zoom lens­es can’t achieve the shal­low depth of field aes­thet­ic they desire. This is fol­lowed by requests for me to rec­om­mend a large-aper­ture prime lens, which inevitably leads to a dis­cus­sion about how to choose a use­ful focal length. And so I ask prob­ing ques­tions about their pre­ferred sub­ject mat­ter, style, work­ing dis­tance, bud­get, etc., all in an attempt to glean the ide­al focal length for each stu­dent. 

This line of inquiry is com­mon, but it’s also prob­lem­at­ic because it assumes begin­ners can pro­vide accu­rate answers to ques­tions and con­cepts they’ve like­ly nev­er care­ful­ly con­sid­ered up to this point. 

Is there a bet­ter way? There is, but I’ll need access to your com­put­er.

Adobe Lightroom Classic can help you determine your next prime lens.

If you take every pic­ture a pho­tog­ra­ph­er has shot on a zoom lens and sort the results by the focal lengths used, you’ll find an uneven dis­tri­b­u­tion of images among them: some will have a greater share of the total num­ber of pic­tures than oth­ers. Bar­ring a few excep­tions, I pro­pose that the focal length with the great­est share of the total—the plurality—is the ide­al focal length for that photographer’s next prime lens. 

How do you do this?

Your cam­era embeds infor­ma­tion about itself into every pho­to it saves. This is known as meta­da­ta. Exam­ples of this info include the time and date of cap­ture, the camera’s make and mod­el, and, cru­cial­ly, the set focal length of a zoom lens. This is true for vir­tu­al­ly every mod­ern DSLR, mir­ror­less, and point-and-shoot cam­era.

Adobe Light­room Classic—emphasis on the Clas­sic, as this can’t be done in their sim­pli­fied version—has a func­tion that lets you fil­ter your entire entire cat­a­logue, or a selec­tion of pho­tos, by a vari­ety of meta­da­ta attrib­ut­es, includ­ing by set focal length. When you acti­vate the focal length attribute, the appli­ca­tion dis­plays a list of every focal length you’ve used to take the select­ed images, along with the total num­ber of pho­tos shot using those focal lengths. My the­o­ry is that focal lengths with a com­par­a­tive­ly larg­er share of pho­tos are evi­dence of a pref­er­ence and can serve as a great start­ing point for pick­ing your next no-regrets prime lens. 

Now I’ll demon­strate the process.

[Demon­stra­tion in video]

Analysis and limitation

The first and most obvi­ous lim­i­ta­tion of this method is that it requires Adobe Light­room Clas­sic. The so-called mod­ern­ized ver­sion of Adobe Light­room, the one avail­able on both desk­top and mobile plat­forms, can’t fil­ter meta­da­ta by lens type or focal length. (On a side note: I firm­ly rec­om­mend Light­room Clas­sic over Light­room not-clas­sic.) I’ve also con­firmed that both Apple Pho­tos and Google Pho­tos don’t allow fil­ter­ing pic­tures by set focal length, despite their abil­i­ty to read and dis­play the data in ques­tion. I can’t com­ment about per­form­ing this type of analy­sis using oth­er apps, such as Cap­ture One Pro, Pho­to Mechan­ic, etc., sim­ply because I nei­ther own nor use them. So sor­ry.

Sec­ond­ly, it’s impor­tant to under­stand that both the upper and low­er lim­its of your zoom’s focal length range can own a greater share of the dis­tri­b­u­tion. This isn’t nec­es­sar­i­ly because you pre­fer these focal lengths, but more so because they’re the hard lim­it of the lens. For exam­ple, if my lens tops out at 55 mm, but I want a big­ger ren­di­tion of my sub­ject, I’m going to set­tle on 55 mm despite want­i­ng more. 

Last­ly, this analy­sis is lim­it­ed to the focal length range of your exist­ing zoom lens­es. How­ev­er, since the point of this method is to guide you towards a pre­ferred focal length from among those that you use, this lim­i­ta­tion is large­ly moot. I firm­ly believe that it’s more prac­ti­cal for begin­ners to expand their col­lec­tion of zoom lens­es before com­mit­ting to fixed-focal length prime lens­es. The ulti­mate point of this exer­cise is to engage in due dili­gence and pho­to­graph­ic intro­spec­tion so that you can avoid buyer’s remorse. 

Conclusion

And there you have it, an easy way to use Adobe Light­room Clas­sic to help you choose your next prime lens based on the focal lengths you use most often. If you have requests for future top­ics, let me know in the com­ments, and I’ll address them in future videos. In the mean­time, you can learn more about pho­tog­ra­phy on ExposureTherapy.ca. See you next time.