Glance at the summary firstThe summary from the end of the lesson, as a road map before you start reading.
The camera's light meter measures the light reflected from the scene and assumes every area it measures is middle gray; that is why it darkens snow and white surfaces and brightens blacks. Incident light metering, or a mid-toned reference such as a gray card, is the most reliable way out of this deviation; shifting toward plus for light tones and toward minus for dark tones is a quick correction. A face is also a reflecting area to the light meter: skin darker than a mid tone is pulled lighter, and lighter skin toward gray; that is why you need to correct the reading for the skin's tone and zoom in to check the result.
Learning goal
You will be able to explain that the light meter inside the camera measures the light reflected from the scene and tries to pull every area it measures toward a middle gray. You will tell apart reflected light metering from incident light metering, which measures the light falling on the subject, show why white, black and mid-toned subjects push the reading in different directions, learn to take a correct reading with a gray card or a mid-toned surface, and when you make a face the metering point, know in which direction the reading shifts depending on the skin's tone, and correct it.
The question
In Lesson 8.1 we saw the camera turn snow gray in a snowy garden. The same happens the other way round: when you photograph a black cat on a black sofa, both the cat and the sofa turn a dark gray. If white and black both come out gray, it is as if in the camera's eyes there is only one color in the world.
Why do snow photos come out gray? Because the camera's light meter doesn't know what it is looking at; it only knows how much light is coming in. In this lesson we will talk about how the light meter measures light, why it tries to pull everything toward a middle gray, and how to use it correctly by knowing this behavior in advance.
Context
In the exposure module we thought of metering as a needle: the camera looks at the scene's brightness and makes a suggestion; we correct this suggestion with compensation or manual settings. This module goes inside that needle. Understanding metering means knowing before the shot where the camera will go wrong; understanding the histogram and dynamic range means seeing whether the result really is right.
Before cameras had them built in, separate handheld meters were used to measure light; many cinema and studio photographers still use them today. The light meter inside the camera, on the other hand, measures the light that passes through the lens. The difference between the two approaches is simple but important, and it explains why the camera turns snow gray. This lesson describes that difference and the assumption the camera reads the scene with.
Simple explanation
What does the light meter measure?
The camera's light meter measures the light that reflects off the objects in the scene and enters the lens. This is called reflected light metering. The problem is this: the amount of reflected light depends on two things. One is the amount of light falling on the subject, the other is the subject's own color. A white sheet of paper sends back most of the light falling on it; black velvet very little. In the same light, white paper looks bright and black velvet dark.
The light meter can't tell these two apart. When it sees a bright reflection, it can't know whether it comes from strong light or from a light-colored object. Some handheld meters solve this problem from the other direction: they are taken next to the subject and pointed toward the camera to measure the light falling on the subject. This is called incident light metering. This reading isn't affected by the subject's color; a white wedding dress and a black suit give the same result if they are in the same light.
What is the middle gray assumption?
Because the light meter has to make a decision, it makes a simple assumption: it accepts that every area it measures is mid-toned. A mid tone is a value neither light nor dark, right in the middle of the grays. Photographers often describe this tone with a gray card that reflects eighteen percent; the exact calibration of cameras differs a little from manufacturer to manufacturer, but the idea is the same. The camera exposes the area it measures so that it looks this middle gray in the photo.
In most scenes this assumption works, because when light and dark areas come together the average falls roughly close to a mid tone. But if most of the scene is light-toned, for example snow, white sand or a whitewashed facade, the light meter mistakes this light tone for strong light and darkens the photo; whites turn gray. If most of the scene is dark-toned, for example a black background or a dark forest, the camera takes this for too little light and brightens the photo; blacks turn gray. In the camera's eyes, everything wants to be middle gray.
How do you take a correct reading?
Knowing this behavior is the key to using it correctly. The most reliable way is to have the camera meter off a mid-toned surface. You put a gray card next to the subject, in the same light, fill the frame with it to take a reading, and fix these settings in manual mode. If you don't have a gray card, you can use a mid-toned surface in the scene: green grass in the sun, a faded stone wall or a mid-toned fabric often work.
A faster solution is to correct the camera's suggestion knowing the scene's tone. If the area you measured is white, you know the camera pulls it toward gray, and you shift the exposure toward plus; snow or a white wedding dress usually wants between one and two stops of plus. If the area you measured is black, you shift the exposure toward minus. These amounts aren't exact; they change with the scene and the camera, and you need to check the result on the screen. But knowing the direction, that is plus for light tones and minus for dark tones, gets you very close to right on the first try.
What happens when a face is the metering point?
Many photographers take the reading from the face in portraits. This makes sense, because the face is the most important place in the photo. But to the light meter, a face is also just a reflecting area, and the light meter wants to pull it toward middle gray too. Every skin sends back a different share of the light falling on it; some skins are darker than a mid tone, some lighter. That is why a reading taken from a face deviates in different directions depending on the skin's own tone.
When you meter a face that is darker than a mid tone, the camera takes it to be too dark and brightens the photo; the skin looks lighter than it really is, sometimes ashy and pale. In this case, shifting the exposure a little toward minus brings back the skin's own depth. When you meter a face that is lighter than a mid tone, the camera takes it to be too bright and darkens the photo; the skin slides toward gray. Here a little plus returns the skin to its own lightness. Incident light metering or a gray card gives a starting point with no deviation at all: they measure the light, not the skin. In every case, the final decision is to zoom in on the face and look.
Visual evidence
Imagine photographing a baker in a white apron in front of a stone oven in a bakery. The mouth of the oven is pitch black, the apron bright white, and the baker's skin dark brown. You narrow the camera's metering to a small point of the frame; we will look at this option in detail in the next lesson. First you meter the apron. The apron has turned gray, the baker's face is almost black, and the inside of the oven can't be made out at all. Then you meter the black mouth of the oven. This time the inside of the oven is gray, the apron has blown out pure white, and the baker's face is pale and too bright.
Finally you put a gray card next to the sack of flour on the counter, meter the card and fix the settings in manual mode. The apron is white but its folds can be made out, the mouth of the oven is black but its stone texture shows, and the baker's dark brown skin appears in its own warmth, with the flour dust on the forehead and the lines around the eyes. The same light, three readings; only the gray card's reading kept the scene's real tones.
Common mistakes
Common mistakeThinking the light meter recognizes the scene.
The light meter sees only the amount of reflected light; it can't tell white from black, or strong light from weak light. The fix: tell it yourself whether the area you are measuring is light or dark.
Common mistakeMetering a white or black area and accepting the result as it is.
The camera pulls both toward gray. The fix: shift toward plus for light tones and minus for dark tones, or meter off a mid-toned surface.
Common mistakeThinking a reading taken from a face is right for every skin.
The light meter pulls a face toward middle gray too, and deviates according to the skin's tone. The fix: start with a gray card or incident light metering, or correct the reading for the skin's tone and zoom in on the face to check.
Summary
The camera's light meter measures the light reflected from the scene and assumes every area it measures is middle gray; that is why it darkens snow and white surfaces and brightens blacks. Incident light metering, or a mid-toned reference such as a gray card, is the most reliable way out of this deviation; shifting toward plus for light tones and toward minus for dark tones is a quick correction. A face is also a reflecting area to the light meter: skin darker than a mid tone is pulled lighter, and lighter skin toward gray; that is why you need to correct the reading for the skin's tone and zoom in to check the result.
Mini assignment
In this assignment you will see the light meter's middle gray assumption with your own eyes.
- Photograph a white sheet of paper, a black fabric and a mid-toned surface separately, filling the frame, in auto or A mode; see that all three turn roughly the same gray in the photo.
- Photograph the same three surfaces again with compensation, shifting the white toward plus and the black toward minus, and note how close they come to their real tones.
- Take a reading with a gray card or a mid-toned surface, fix the settings in manual mode, and photograph white, black and mid-toned objects together in the same light.
- Take a reading from a friend's face, then from the gray card, and compare the skin's tone in the two frames.
Check yourself
- In the auto frames, were you able to see the white and the black turn gray?
- In the frame fixed with the gray card, did white stay white and black stay black?
- Between the reading from the face and the reading from the gray card, in which direction did the skin's tone differ?
Glossary
| Term | Short definition |
|---|---|
| Light meter | A tool, or the metering system inside the camera, that measures the amount of light and suggests an exposure. |
| Reflected light metering | The method that measures the light reflected from the subject into the lens; it is affected by the subject's color. |
| Incident light metering | The method that measures the light falling on the subject; it isn't affected by the subject's color. |
| Middle gray | The tone, neither light nor dark, that the light meter tries to pull every area it measures toward. |
| Gray card | A reference card that reflects in a middle gray tone, used for taking a correct reading. |
Mini quiz
1. What does the camera's light meter measure?
- A) The light falling on the subject
- B) The light reflected from the subject into the lens
- C) The subject's temperature
- D) The distance to the subject
2. When you fill the frame with bright white snow, what does the camera do?
- A) It leaves the snow white as it is
- B) It brightens the photo
- C) It darkens the photo to pull the snow toward middle gray
- D) It changes the white balance
3. What is the difference between incident light metering and reflected light metering?
- A) It isn't affected by the subject's color
- B) It works only at night
- C) It always gives a darker result
- D) It is found only on phones
4. When a reading is taken from a face darker than a mid tone, how can the skin look, and what do you do?
- A) It looks too dark; shift toward plus
- B) It doesn't change; do nothing
- C) It shifts toward blue; change the white balance
- D) It can look lighter and paler than it really is; shift the exposure a little toward minus
Answers
- B — The light meter inside the camera measures the light entering through the lens, that is, reflected from the subject; that is why the subject's color affects the reading.
- C — The light meter takes the area it measures to be middle gray; it reads the white snow as too much light and darkens the photo, and the snow turns gray.
- A — Incident light metering measures the light falling on the subject; white and black subjects give the same result if they are in the same light.
- D — The light meter brightens the photo to pull the dark face toward middle gray; a little minus compensation brings back the skin's own depth.
Summary card
- The light meter measures reflected light; it doesn't know what it is looking at.
- It pulls every area it measures toward middle gray: white darkens, black brightens.
- A gray card or incident light metering measures the light, not the color.
- Plus for light tones, minus for dark tones.
- A face is a reflecting area too: correct the reading for the skin's tone and zoom in to check.
Bridge to the next lesson
You have seen that the light meter pulls every area it measures toward middle gray. But which part of the frame does the camera measure: the whole frame, the middle, or a single point? This choice was the source of the difference we saw between the baker's apron and the mouth of the oven. The next lesson describes where the camera looks at the scene from, that is, the metering modes.