What is the Stop in Photography?

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.

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