The idea of a stop

Learning goal

You will be able to define a stop as light doubling or halving. You will show that each full step in the aperture, shutter speed and ISO series is one stop, and that this is why the three settings speak the same unit; you will read one-third-stop in-between steps, interpret the EV values in exposure compensation as stops, translate the brightness difference between two areas into stops and a ratio, work out with the inverse square law how many stops moving closer to or further from a small light source changes, and use this calculation to set the light falling on a face.

The question

At a shoot, you might hear an experienced photographer say to an assistant: "Open up a stop." Or in an editing lesson: "This photo is two stops dark." The same word is used to describe an aperture, a shutter speed, an ISO value, a lamp, even a photo itself. It is as if photographers had a secret currency everyone understands.

What is the thing photographers call "1 stop"? It is actually simple: light doubling or halving. In the previous lesson we called this change a "step". In this lesson we give that step its name, and talk about how we measure the three settings and light itself with the same unit, and why this unit is so useful at a shoot.

Context

You know what happens when you go from f/4 to f/5.6 in the aperture series, from 1/125 to 1/250 in the shutter speed series, and from 400 to 200 in the ISO series: each time, the result darkens by half. In Lesson 8.7 you built equivalent combinations by counting these steps. Photographers call this common step a stop. The word comes from the metal plates with holes of different sizes used on old lenses to set the aperture; these plates that "stopped" light were called stops.

The stop's greatest convenience is that it translates everything into the same language. When you stop the aperture down one stop and raise ISO one stop, the brightness doesn't change; to know this you don't need to memorize which number matches which. The same unit also measures light sources, the difference between the lit and shadow sides of a face, and exposure compensation. Once you start thinking in stops, light turns into something spoken of in steps rather than numbers.

Simple explanation

Why does one stop mean twice?

A stop is light doubling or halving. Two stops means four times, three stops eight times, four stops sixteen times. So stops add, while light multiplies: two stops plus one stop makes three stops, and the light goes up two times two times two, that is, eight times. That is why saying "three stops darker" instead of "eight times darker" is both shorter and easier.

Another reason the stop rests on doubling is the way our eye perceives light. The eye perceives differences in light roughly as ratios: in a dark room, putting a second candle next to a candle makes a clear difference, but in a bright hall the same candle changes almost nothing. Doubling light feels like a roughly similar step whatever the level. The stop counts light the way the eye looks at it, in steps that look equal.

How do you read the three settings and the in-between steps?

You can put the full-stop series of the three settings side by side. In aperture f/2, f/2.8, f/4, f/5.6, f/8; in shutter speed 1/1000, 1/500, 1/250, 1/125, 1/60; in ISO 100, 200, 400, 800, 1600. In each series one rung is one stop. Opening the aperture one stop, lengthening the shutter speed one stop and raising ISO one stop brighten the result by the same amount. Which setting you touch depends, as in Lesson 8.7, on which cost you want to pay.

Most cameras put two more in-between values between these full stops, and each click of the dial becomes a one-third stop. In aperture, between f/4 and f/5.6, there are f/4.5 and f/5; in shutter speed, between 1/125 and 1/250, there are 1/160 and 1/200; in ISO, between 100 and 200, there are 125 and 160. Three clicks make one stop. These fine steps let you bring the light to exactly the point you want. On some cameras the click can also be set to half a stop; the camera's menu shows this.

How are EV and exposure compensation linked to stops?

On cameras and phones, exposure compensation is usually shown in EV units: +1 EV, −2/3 EV and so on. One unit of EV, short for exposure value, is one stop. In exposure compensation, +1 EV means a photo one stop brighter than the camera suggests, and −1 EV one stop darker. The marks on the scale on the screen are often spaced at one-third-stop intervals.

This lets you think of compensation in numbers. If a photo you take in snow comes out gray, one or one and a half stops of plus compensation often brings the snow back to white. If a face in front of a bright window stays dark, two thirds of a stop or one stop of plus compensation opens up the face. If a face in front of a dark background glows too much, a few clicks in the minus direction are enough. When you speak in stops, you can say "open up a third of a stop more" instead of "open up a little more", and repeat the same correction on the next shoot.

How are light sources and contrast measured in stops?

The stop is used not only for the camera's settings but also for light itself. If the lit side of a face gets twice as much light as the shadow side, there is one stop of difference between them; if four times, two stops; if eight times, three stops. This ratio is called the contrast ratio and is written one to two, one to four, one to eight. Thinking of this difference on the same face in stops is a measurable way of describing how hard or soft a light models a face.

On a face lit by a small light source, for example a bare bulb or a flash, distance is also worked out in stops. According to the inverse square law, when the distance doubles, the light falling on the face drops to a quarter, that is, it falls by two stops; when the distance halves, it rises by two stops. Dividing the distance by about 1.4, for example bringing the lamp from 1.4 meters to 1 meter, gains one stop. This calculation holds for small sources: right next to a big window or a large softbox, light doesn't fall off this fast, because at that distance the source no longer behaves like a single point. Skin tone turns into a measurable adjustment here: bringing the lamp a little closer to open up a dark face left in darkness gains that face stops of light without touching any setting on the camera; if the bright areas of a light face are overflowing, moving the lamp a little further away brings them back.

Visual evidence

Imagine lighting a friend sitting at a table in a dark room with a single bare bulb. The bulb is two meters from their face, off to the side. The settings on the camera are fixed. In the photo your friend's dark skin is buried in darkness; the edge of their cheek and the tip of their nose can barely be made out, and the shadow side is completely black.

You bring the bulb to one meter; the distance has halved, which means the light falling on the face has gone up four times: you have gained two stops. In the photo taken with the same settings, the face is now lit; the warm brown of their skin, the slight sheen on their cheekbone and the line of their eyebrows can be made out. There is still a clear difference between the lit side and the shadow side; the shadow side is about two stops darker than the lit side, around one to four. When you hold a white sheet of paper on the shadow side of the table, that half brightens and the difference drops to one stop. The same bulb, the same settings; every step you read the light in stops opened up the face one rung more.

Common mistakes

Summary

A stop is light doubling or halving; stops add while light multiplies, and this scale fits the way the eye perceives light. In the aperture, shutter speed and ISO series every full step is one stop, each click of the dial is often a third of a stop, and every EV unit in exposure compensation is a stop too. The same unit also measures the contrast between the lit and shadow sides of a face and the light gained by moving closer to a small source; when the distance halves, two stops are gained, and this is the measurable way of opening up or toning down a face of any tone without touching the camera.

Mini assignment

In this assignment you will measure and change light in stops.

1. Turn the dial on your camera one click, watch how the aperture, shutter speed and ISO displays change, and confirm that three clicks make one stop.
2. In a dark room, photograph a friend lit by a single small lamp, with the lamp two meters away, at fixed settings.
3. Bring the lamp to one meter, shoot again with the same settings, and see whether the difference in brightness is close to two stops, that is four times.
4. Reduce the difference between the lit and shadow sides by holding a white sheet of paper on the shadow side of the face, and compare the two frames.

Check yourself

• When someone says "three stops", can you say without hesitating how many times the light that is?
• When you brought the lamp closer, was the increase in brightness on the face as much as you expected?
• When the lamp came closer, did your friend's skin open up in its own color and texture, or did the bright areas overflow?

Glossary

Mini quiz

1. Three stops more light is how many times as much?

• A) Three times
• B) Six times
• C) Eight times
• D) Nine times

2. What does +1 EV mean in exposure compensation?

• A) One stop brighter than the camera suggests
• B) One stop darker than the camera suggests
• C) ISO going up by one
• D) The aperture opening by one millimeter

3. How does bringing a small lamp from two meters to one meter from a face change the light falling on the face?

• A) It doubles it
• B) It doesn't change it
• C) It halves it
• D) It quadruples it, that is, raises it by two stops

4. If the lit side of a face gets four times as much light as the shadow side, how many stops of difference are there between them?

• A) Four stops
• B) Two stops
• C) One stop
• D) Three stops

Answers

1. C — Each stop doubles the light; three stops is two times two times two, that is, eight times.
2. A — Each unit of EV is a stop; the plus direction brightens the exposure the camera suggests by one stop.
3. D — According to the inverse square law, when the distance halves, the light goes up four times; this is two stops.
4. B — One to two is one stop of difference, one to four two stops; four times the light is doubling twice.

Summary card

• One stop = twice or half the light.
• Stops add, light multiplies: three stops is eight times.
• In aperture, shutter speed and ISO every full step is a stop; each click is often a third.
• +1 EV compensation = one stop brighter.
• With a small source, halving the distance gains two stops; open up a face stop by stop with a lamp.

Bridge to the next lesson

Now you can count light in stops and think of the three settings in the same language. But will you do all this calculation by hand at every shoot? No. In one of the camera's modes you choose only the aperture, and the camera works out the shutter speed for you. The next lesson explains why professionals use this mode so often.