Metering, and why it lies
Your camera's meter assumes every scene averages to mid-grey. Knowing when that assumption fails explains most automatic exposure mistakes.
Your camera measures the light before it chooses an exposure. That measurement rests on one assumption, and understanding the assumption explains nearly every automatic exposure mistake you will see.
The assumption
A meter assumes that whatever it is pointed at averages out to middle grey — a tone reflecting about 18% of the light falling on it.
Point a camera at anything and it will choose an exposure that renders that thing as middle grey. Point it at a white wall and it gives you a grey wall. Point it at a black cat and it gives you a grey cat.
For most scenes this works, because most scenes contain a mixture of light and dark that genuinely does average out near the middle.
Where it fails
Two situations, and they fail in opposite directions.
A bright background. A person photographed against a bright sky: the meter reads mostly sky, decides the scene is very bright, and cuts the exposure. The sky comes out correct and the face comes out dark.
A uniformly bright or dark scene. A snow field is not mid-grey — it is snow. The meter reads it, renders it as mid-grey, and hands you a photograph of dirty slush. A black cat filling the frame comes out grey for the same reason.
Metering modes
How much of the frame the camera measures:
- Average or evaluative reads the whole frame. Right most of the time.
- Centre-weighted reads the whole frame but favours the middle. A reasonable default.
- Spot reads a small area only. Right when the subject is much brighter or darker than everything around it.
Changing the mode fixes the first failure — measure less of that bright sky and the reading improves. It does nothing for the second, because when the whole frame is snow it does not matter how much of it you measure.
Exposure compensation
exposure compensation tells the camera its reading is wrong and by how much, without leaving an automatic mode. Set +1 and every photograph comes out one stop brighter than the meter wanted.
Rough starting points:
| Scene | Compensation |
|---|---|
| Snow, sand, a white wall | +1 to +2 |
| A subject against a bright sky | +1 to +2 |
| A dark subject filling the frame | −1 to −2 |
| A spotlit subject on a dark stage | −1 to −2 |
It only does anything in an automatic mode. In full manual the camera is not choosing the exposure, so there is nothing to compensate — you simply set what you want.
Try both failures
The whole room
Indoors and hand-held, with depth to hold and not much light to hold it with. Sharp front to back, steady in the hand, correctly exposed, and as clean as you can manage.
Loads an interactive 3D camera simulator.
Key takeaways
- Meters assume the scene averages to middle grey, and expose to make it so.
- A bright background makes the camera underexpose the subject; the metering mode can fix it.
- A uniformly bright scene comes out grey, and only compensation can fix it.
- Compensation does nothing in manual, because nothing is being chosen for you.
Common questions
Why did my snow come out grey?
Because the meter assumed it was mid-grey and exposed it accordingly. Snow should render about two stops brighter than mid-grey, so it needs positive exposure compensation.
Which metering mode should I leave it on?
Centre-weighted or evaluative for most things — they are right most of the time. Switch to spot when the subject is much brighter or darker than its surroundings, which is exactly when the others fail.
Next steps
The exposure triangle
Three controls decide how bright a photograph is, and each one changes something else as well. That trade-off is the whole of exposure.
ISO and noise
ISO buys brightness when there is not enough light, and pays for it with noise. Knowing when that trade is worth it is the skill.
The letters on the mode dial
P, A, S and M are not difficulty levels. Each hands you a different control and does the arithmetic on the rest.