Incident metering vs. reflective metering

Incident metering vs. reflective metering

All in-camera light meters have a fundamental flaw: they can only measure reflected light.

This means the best they can do is guess how much light is actually hitting the subject.

If all objects reflected the same percentage of incident light, this would work just fine, however real-world subjects vary greatly in their reflective.

For this reason, in-camera metering is standardized based on the luminescence of light which would be reflected from an object appearing as middle gray.

If the camera is aimed directly at any object lighter or darker than middle gray, the camera's light meter will incorrectly calculate under or over-exposure, respectively.

A hand-held light meter would calculate the same exposure for any object under the same incident lighting.

In photography, painting, and other visual arts, middle gray or middle grey is a tone that is perceptually about halfway between black and white on a lightness scale.

In photography, it is typically defined as 18% reflective in visible light.