The ability of the human eye and brain to deal with different levels of illumination far exceeds that of any emulsion or sensor. Indeed, the brightest light that the eye can handle is at least a billion times brighter than the faintest detectable glimmer. The eye examines only small areas of the field of vision at any one time, and can adapt rapidly to the prevailing intensity. The pupil contracts and dilates, and the sensitivities of the retina's cells and receptors change. Detail is detected at fractionally different times in shadow and highlight areas, and then used by the brain to construct a coherent image. However, films and sensors have to cope with light and dark areas simultaneously, and without any adaptation of chemistry. The resulting compromise exaggerates human perception of light and shade, and leads to contrast in the final image being more pronounced than that perceived through the viewfinder.
It is therefore vital to observe how a subject is illuminated. Contrast and tonal variation create the illusion of depth, so if light becomes too uniform three-dimensional qualities will be lost. Ideally the changes of intensity required to reveal form should be soft and gradual. Shadows created by harsh light are deep and sharply defined, and produce dramatic effects that must be carefully handled. Those created by diffused light are less obvious and generally more acceptable.
The exposure difference between shaded and directly lit parts of a subject can be five or six stops. A compromise that preserves detail in shadows and highlights must therefore be sought. Fortunately, modern cameras are extremely good at calculating the best exposure for a whole image. Only when very bright or dark tones dominate should it be necessary to intervene manually. Nevertheless, in extreme conditions, contrast can be too much for the film or sensor to handle.
The simplest solution, though not necessarily the best, is to choose subjects that are either in the shade or illuminated by diffused light. Contrast will be reduced and shadows will be softened. Photographs obtained under these conditions are often acceptable, but can also be flat and lifeless. Another approach is to photograph subjects in direct light and where possible use fill-in flash to control contrast. This works provided that the shadows are filled by an appropriate amount. Over-filled shadows look unnatural and give the image a washed-out appearance. Under-filled shadows are equally unsatisfactory because the problems of excess contrast and lost shadow detail remain. The perfect level of fill is obtained when the use of reflector or flash is barely detectable in the image. Contrast is reduced but not eliminated, and detail is retained in all areas.