David Pogue's Digital Photography: The Missing Manual

How Much Exposure?

Every camera has a light metering system built in. It's supposed to measure the brightness of the scene and to adjust the overall exposure accordingly.

Note

Exposure means "How bright or dark the overall shot comes out." It comes, of course, from the days of film, when it referred to how long you exposed the film to light.

There are times, though, when this circuit gets confused. Bright windows behind the Christmas tree will make the camera say, "Ooh, very bright scene. I'll darken things a bit to compensate"—but that makes the tree come out too dark, with all the detail lost in shadow. Or imagine a campfire at night. The camera will say, "Wow, this scene is 70 percent pure black! I'd better brighten it up"—and the photo will wind up so bright that the flames become pure, blown-out white, with none of the cool colors you saw with your eye.

Most cameras offer two ways to handle these situations. (Often, you'll make these adjustments after you've seen what the camera does on its own—which is botch the shot.)

Exposure Modes

First, you can change how the camera assesses the scene. All SLRs, and some smaller cameras, let you choose from a selection of metering modes (they're in the menus somewhere):

• Center-weighted. The name says it all: The camera assesses the light in the entire scene but gives special consideration to the exposure of the center chunk of the frame. Remember the Christmas tree example? If the exposure were center-weighted, it would have been more likely ...