What is exposure?

Learning goal

You will be able to define exposure as the total amount of light reaching the sensor during a shot. You will explain this amount by comparing it to a bucket filling from a tap, tell apart that the aperture is how wide the tap is open, the shutter speed how long the tap stays open, and ISO how much the reading of the water collected is raised; you will show what overexposure and underexposure lead to in a photo, know why the camera's metering is sometimes wrong, and make it a principle to set exposure by the most important area of the scene, very often a face.

The question

In a photo you took in the garden on a snowy day, everything has turned a dirty gray; the snow isn't white, and the child's face is almost pitch black. On the same day, in a photo you took in front of a window in a dark room, the opposite happened: the face is bright white, as if looking into a lamp. In both, the camera was in auto mode and was supposed to take the "right" photo.

Why does a photo sometimes come out pure white and sometimes pitch black? Because every photo is formed by a certain amount of light reaching the sensor, and this amount is either too much, too little, or just as much as you want. This measure is called exposure. Throughout this module you will get to know the three settings that build exposure, the aperture, the shutter speed and ISO, one by one, and at the end you will use them together. This lesson lays the foundation: what is exposure, and why does it sometimes come out wrong?

Context

In Level 1 you learned to look at a photo, read light, build a frame and edit. Most of the time you left exposure to the camera or the phone and tidied up the result in editing. Lesson 7.3 showed that editing has a limit: an area blown out to pure white doesn't come back, and an area that is too dark gets grainy when it is opened up. Level 2 begins with taking control of this limit before the shot.

In the lesson explaining how a camera works, Lesson 3.2, we compared the aperture to the pupil of an eye and the shutter to an eyelid. This module turns that comparison into a measure. Photographers call these three settings together the exposure triangle, because when you change one, you usually need to change another to keep the same brightness. Each corner of the triangle also has a side effect: the aperture affects the zone of sharpness, the shutter speed affects movement, and ISO affects grain. We will see these side effects one by one in the next lessons.

Simple explanation

How is exposure like a bucket?

You know there are millions of small light collectors on the surface of the sensor. Think of each one as a small bucket. When you press the shutter button, a tap opens above these buckets and light starts to fill them. When the tap closes, how much light has gathered in each bucket is measured, and the brightness of the photo at that point is set accordingly. A bucket that fills a little becomes a dark point; a bucket that fills a lot becomes a bright point.

Two things decide how much light fills a bucket. One is how wide the tap is open: this is the aperture. The bigger the opening, the more light flows per unit of time. The other is how long the tap stays open: this is the shutter speed. The longer the time, the more light gathers. Exposure, in the strict sense, is the total amount of light set by these two together; the brighter the scene itself is, the more light flows with the same tap and the same time.

ISO stands in a slightly different place in this comparison. It doesn't change the amount of light filling the bucket; it decides how much the reading in the bucket is raised afterward. Raising the reading of a bucket that has filled a little brightens the photo, but it also raises the noise in the bucket along with it; this shows up as grain. That is why all three settings together decide the final brightness of the photo, but the light actually gathered is the work of the aperture and the shutter speed alone.

What do overexposure and underexposure lead to?

A bucket can only hold a certain amount of water. If the tap stays open too long or too wide, the bucket overflows; you can no longer know how much water overflowed. The same happens on the sensor: when a collector fills up, the light that comes after it isn't recorded and that point becomes pure white. This is called overexposure. The clipped highlights in Lesson 7.3 are exactly this: the buckets have overflowed, and the detail is lost.

The other way round, if the tap opens too briefly or too narrowly, the buckets don't fill enough and the photo comes out dark. This is called underexposure. In the dark areas the information isn't completely gone, but the small amount of light at the bottom of the bucket is mixed with noise; when it is opened up afterward, grain appears. That is why a good exposure tries to fill the buckets as much as possible in the important areas without making them overflow. Which area is important, you decide: sometimes a face, sometimes a sky, sometimes a single window in the darkness.

Why is the camera's metering wrong?

Before every shot the camera measures how bright the scene is and sets how much the buckets will fill accordingly. This is called metering. The metering system doesn't know what the scene is; it sees only its brightness and tries to bring the scene to an average brightness, a middle gray. In most scenes this guess works, because most scenes are a mix of bright and dark areas.

But if most of the scene is bright, for example snow, a beach or a white wall, the camera finds this brightness too much and darkens the photo; the snow turns gray. If most of the scene is dark, for example a street at night or a black background, the camera brightens the photo; the blacks turn gray and small bright areas turn pure white. The exposure compensation you learned in Lesson 3.5 is the fastest way to correct this mistake: a little plus in snow, a little minus against a dark background. At the end of this module you will be able to make the same correction by hand, choosing the settings yourself.

How do you set exposure by a face?

When photographing people, the most important area is very often the face, and faces reflect different amounts of light in the same light. Dark skin sends back less of the light falling on it; that is why the pixels of the face fill less. When the camera looks at the overall brightness of the scene, it can leave a dark face in darkness; because the buckets haven't filled enough, grain and an ashy color appear on the face when it is opened up afterward. Setting exposure by the face, that is, filling the face's buckets enough, keeps the color and texture of the skin.

On light skin the danger is the opposite: the buckets of the places that reflect light most, such as the forehead, nose and cheekbones, overflow easily, and these areas turn into pure white patches. If there are people with both dark and light skin in the same frame, you need to set exposure at a point that keeps the detail of both: filling the buckets of the dark skin as much as possible without making the shines on the light skin overflow. This sometimes means changing the light, for example placing the people again relative to the window; exposure isn't only a setting, it is a bargain made with light.

Visual evidence

Imagine a dark-skinned child in a dark blue coat building a snowman in a snowy garden. The sun is behind the clouds, and everything is filled with a soft whiteness. In the photo you take in auto mode, the camera has found this whiteness too much: the snow is a dirty gray, and the child's coat and face are almost pitch black; the smile on their face can't be made out.

You shift exposure compensation in the plus direction, by about a step and a half, and shoot again. The snow is now white, but the footprints on it and the rough surface of the snowman can be made out. The child's face has brightened; their dark skin with cheeks reddened by the cold, their shining eyes and the snowflakes on their gloves can be seen. Once more, this time you shift it much too far toward plus: the snow turns into a pure white, flat void, the footprints disappear, and the edges of the snowman look as if they have melted. The same garden, the same child; three frames, three different levels of bucket filling.

Common mistakes

Summary

Exposure is the total amount of light reaching the sensor during a shot; just like a bucket filling from a tap, this amount is set by how wide the tap is open, that is the aperture, and how long it stays open, that is the shutter speed, while ISO raises the reading afterward. If the buckets overflow, overexposure leaves pure white areas that don't come back; if they don't fill enough, underexposure leaves dark, grainy areas. Because the camera's metering pulls the scene toward an average gray, it goes wrong in snow and against a dark background; setting exposure by the most important area of the photo, very often a face, and keeping the detail of every skin is the foundation of this module.

Mini assignment

The first assignment of Level 2: measuring exposure with your eye.

1. Find a scene that is mostly bright, such as an object in front of a white wall or a sunny beach, and shoot it once in auto mode.
2. Shoot the same scene twice more, shifting exposure compensation first one step toward plus, then one step toward minus.
3. In a scene that is mostly dark, for example a face in front of a dark background, take the same three frames.
4. For each of the six frames, note in one sentence whether the most important area overflowed or stayed too dark.

Check yourself

• In the auto frames, did you notice the camera darkening the bright scene and brightening the dark scene?
• In which frame was the detail of the most important area kept best?
• In the frames with a face, can the color and texture of the face be made out?

Glossary

Mini quiz

1. What does exposure describe?

• A) The colors of the photo
• B) The size of the photo
• C) The total amount of light reaching the sensor
• D) The sharpness of the photo

2. In the bucket comparison, what does the aperture correspond to?

• A) How wide the tap is open
• B) How long the tap stays open
• C) The color of the bucket
• D) The temperature of the water

3. Why does the snow turn gray in a photo of a snowy scene taken in auto mode?

• A) Because snow is really gray
• B) Because the sensor doesn't work in the cold
• C) Because the photo is too sharp
• D) Because the camera tries to pull the bright scene toward an average gray and darkens the photo

4. If there are two people with dark and light skin in the same frame, how should exposure be set?

• A) Only by the light skin, leaving the dark skin in darkness
• B) So as to keep the detail of the dark skin as much as possible without making the shines on the light skin overflow
• C) Only by the dark skin, leaving the light skin pure white
• D) By the background, ignoring the faces

Answers

1. C — Exposure is the total of light filling the buckets through the width of the tap and how long it stays open; it is the foundation of a photo's brightness.
2. A — The aperture sets the opening; the bigger the opening, the more light flows per unit of time, just like a tap opened wider.
3. D — The metering system doesn't know what the scene is; it finds the brightness too much and pulls it toward the average, which is why the snow turns gray.
4. B — Bright areas on light skin overflow easily, while the buckets of dark skin fill slowly; a balance that keeps the detail of both faces is sought.

Summary card

• Exposure is the total amount of light reaching the sensor.
• Aperture is the width of the tap, shutter speed how long it stays open, ISO the raising of the reading.
• An overflowing bucket is pure white and doesn't come back; a bucket that fills a little is dark and grainy.
• The camera pulls the scene toward an average gray; it goes wrong in snow and against a dark background.
• Set exposure by the most important area, very often a face.

Bridge to the next lesson

The first setting that decides the light filling the bucket is the tap itself: the aperture. Inside the lens there is an opening that grows and shrinks, just like the pupil in your eye. The next lesson shows how this opening works, and why photographers describe it with a number that seems backward: why a small f-number means a big opening and more light.