Camera Metering Quiz: Spot Metering and Histograms

Center weighted metering reads the whole frame but gives most weight to the central area, on the assumption that the main subject is there. It is not the same as spot metering, which reads only a small spot.

Spot metering reads only a small area, so you can meter the exact part of a high contrast scene that matters, such as a lit face against a dark background. In even lighting any metering mode gives a similar result, so the precision is not needed.

Cameras meter to render an average scene as middle gray. Snow is far brighter than middle gray, so the meter reduces exposure to compensate, turning the white snow a muddy gray in the resulting image.

Applying positive exposure compensation overrides the meter's tendency to darken bright scenes. Adding one to two stops pushes the snow back toward white without overexposing it to featureless blown-out brightness.

A dark subject in a dark setting averages out to a very low overall brightness, which the meter interprets as underexposure and compensates by brightening the scene. The dark subject ends up rendered as gray rather than black.

Applying negative exposure compensation counteracts the meter's brightening of a predominantly dark scene. Dialing in one to two stops of negative compensation restores the black subject to its actual dark tonality.

Highlight-weighted metering sets exposure from the brightest important areas of the scene, so they stay just below clipping. It is useful for stage lighting and bright skies, at the cost of darker shadows that may need lifting later.

The right edge of the histogram represents pure white. A spike pressed against that edge means many pixels have hit maximum brightness and lost all detail: the highlights are clipped. Underexposure would pile up on the left side instead.

Exposure bracketing captures a series of frames at different exposure values around the metered setting. The spread can be merged into an HDR image, or the best single exposure can be chosen later, providing insurance when conditions are uncertain.

In aperture priority the aperture you chose stays locked, so the camera can only reach the brighter or darker target by changing shutter speed. If Auto ISO is on, it may change ISO instead or as well.

In manual mode you fix both aperture and shutter speed, so with Auto ISO the only thing left for the camera to vary is ISO. Exposure compensation shifts the brightness the camera aims for, and it gets there by raising or lowering ISO.

Camera meters are calibrated to render any scene as if its average reflectance equals middle gray, roughly 18 percent reflectance. Scenes that are predominantly light or predominantly dark need exposure compensation to override this assumption.

Clipped highlights cannot be recovered, while slightly dark shadows can be brightened in editing, at the cost of some noise. Exposing a little under protects the highlights, then lifting the shadows restores the balance. Overexposing and trying to recover often fails because the clipped areas have no data.

Zebras are diagonal stripes the camera draws over areas brighter than a level you choose. They show where highlights are about to clip or have clipped, so you can lower exposure before taking the shot. Focus peaking is a different tool that outlines sharp edges.

A meter assumes every scene averages middle gray, so it is fooled by bright or dark subjects. Choosing a metering mode and applying exposure compensation are how you tell the meter what the scene really is, rather than letting it decide on its own.