Photography

How your camera meters a scene

and why it matters that you know it

I’m sure you’ve noticed that your camera has the ability to meter a scene and decide what it thinks is the best exposure settings for that scene. This is how it works in any automatic mode or semi-automatic mode you might operate your camera in. But do you have any idea how it does that or why it’s helpful to know how?

It’s a curious fact that on average when you look at a scene in front of you, that scene is reflecting back 18% of the light that is falling on it. There are exceptions as with everything, such as a pure white snow covered landscape… but we’re talking average. That 18% of light reflected corresponds to middle gray, the mid-point between black and white. The light meter in your camera is designed with this in mind.

So, when your camera views a scene, the light meter doesn’t care about colours or details in the scene. The metering system converts the scene to black and white and then blurs the scene to remove all detail. What’s left is a blurry monochrome mush, but in that mush is varying degrees of gray. Dark shadows in the scene will be dark, almost black. Bright areas of the scene will be bright, almost white.

The camera knows (or believes) that a proper exposure would be middle gray and so it uses this monochrome mush to determine how much the exposure needs to be increased or decreased to make the average of the scene meet middle gray.