We were tak­ing pho­tos of the Trin­i­ty Col­lege facade ear­li­er today towards the end of my cam­era basics pho­tog­ra­phy work­shop. Real­iz­ing that one of the stu­dents was off by him­self, I approached him. He point­ed down Tow­er Rd and asked how I would take a pho­to of the Sol­diers’ Tow­er and the CN Tow­er togeth­er from our van­tage point.

I con­sid­ered the scene and pho­tographed the shot fea­tured below. The pic­ture was tak­en with my Fuji­film X‑H2s using the Fuji­non 16–55mm ƒ/2.8 R LM WR lens. Using Pro­gram AE mode, my expo­sure set­tings were ƒ/5.6, 1/220 s, and ISO 320. Adjust­ing the expo­sure com­pen­sa­tion by –0.7 EV was my only input.

My com­po­si­tion­al goal was to find a per­spec­tive from which the CN Tow­er’s out­line was unin­ter­rupt­ed by oth­er build­ings. Addi­tion­al­ly, I angled left to include more of the red bricks of Wycliffe Col­lege.

I’m pret­ty hap­py with the pic­ture as shot, but I felt it need­ed some work in the dig­i­tal dark­room. In Adobe Light­room, the Upright tool auto-cor­rect­ed the key­stone effect, and I selec­tive­ly increased the sat­u­ra­tion of the reds and made some mod­est changes to the sky. Use the slid­er below the com­pare the in-cam­era jpeg against my take.

Exposure Therapy’s third photowalk in Toronto.

As always, I’d like to thank every­one who attend­ed. It was our biggest turnout to-date, and I strong­ly sus­pect the mild spring weath­er was an influ­enc­ing fac­tor. The group had a good rep­re­sen­ta­tion of skill lev­els and back­grounds, and we had our youngest par­tic­i­pant ever — a baby!

Photowalk details:

Date: March 26, 2023

Loca­tion: We met by Bri­an Jun­gen’s Couch Mon­ster: Sadzěʔ yaaghęhch’ill, made a slow pro­ces­sion toward Kens­ing­ton Ave., and explored the mar­ket. The group stopped at Wan­da’s Pie in the Sky for drinks and snacks.

Theme: Urban Mar­kets. The orig­i­nal theme asked the group to focus on the peo­ple, colours, and tex­tures of Kens­ing­ton Mar­ket. As the walk pro­gressed, it became obvi­ous that most of us could­n’t resist urban anthro­pol­o­gy, and it became a con­ver­sa­tion about street pho­tog­ra­phy and street por­trai­ture.

Some photos from the photowalk.

Unfor­tu­nate­ly, I did­n’t get a chance to take many pho­tos. It’s becom­ing obvi­ous that I can’t resist the oppor­tu­ni­ty to tutor and encour­age and talk dur­ing these events. And if you know me per­son­al­ly, you know it’s unusu­al because I’m a very intro­vert­ed per­son. I do most of the talk­ing dur­ing the for­mal work­shops because that’s my job, and I have to meet expec­ta­tions, but I’m pleas­ant­ly sur­prised by myself dur­ing these pho­towalks.

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.