Influences of subject and background brightness

People photographed against a white background appear rather dark

Even when lighting conditions remain constant, the exposure will vary if you are shooting a subject (person or object) against a white, black, light or dark background. For instance, if you are taking a portrait shot from the bust upwards, the person will appear darker if the background is light, and vice versa. The proportions of the screen occupied by the subject and the background affect the output of the light sensor, which takes an average of the entire scene. This means that the difference in brightness between the subject and the background influences the exposure. This is not a problem for center-weighted light sensors as long as the subject is in the center, but it will affect the results if the subject is off-center.

People photographed against a dark background appear rather bright