It needs to be kept in mind that the light or white subject matter is composed in the frame to mainly have white tonalities to make it the high key picture. Therefore, it is advisable to light the subject and have a white background without detail. A white wall will do with uneven light falling on the subject.
Faces of living things or people, high key images are used quite often for headshots to give the clean and sharp feel to these images of the subject. The dark areas or solid blacks have to be less as compared to the white ones.
In visual perception, contrast is determined by the difference in the brightness of the object and other objects within the same field of view.
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