- what is visible;
- the range of sensitivity of the photo-receptive medium (film or photo-sensor); and
- the expected appearance of the subject.
Exposure can concentrate or diffuse attention
The viewer's eye gravitates towards areas of perceptually 'normal' exposure from regions of underexposed dark and overexposed bright. Therefore:
- higher contrast directs the eye; whereas
- lower contrast lends the eye greater freedom to roam over the entire surface.
Exposure affects the perception of form
High contrast conditions break up the visual continuity of form, whereas low contrast conditions preserve it. Take, for example, two horizontal elements: one above and at an angle to the other, illuminated from above. High contrast will mean that the continuous form of the lower element will be visually broken by dark shadow cast by the upper element. Low contrast would allow the lower element still to be seen as intact with the casting of a lighter shadow.
Exposure affects colour saturation
Over-exposure leads to desaturated (muted, bleached, washed-out) colours. The practice of exposing to keep the highlights within the range of the film - the dominant procedure still today - came about with the introduction of Kodak's Kodachrome film in 1935. Kodachrome was intolerant of overexposure, but rewarded underexposure with rich saturated colours. 'Exposing for the highlights' is still relevant in the world of digital photography because overexposure results in unattractive digital clipping. In this respect, digital sensors share similar limitations to Kodachrome (discontinued in 2009).
Graphical features of exposure
- Silhouettes are the result from complete underexposure of the foreground, denying any foreground detail so that the outline of the dark shape tells the story.
- Flares also deny detail, but are achieved through overexposure, and are commonly used to unify or increase the activity of a composition.