Many factors affect the definition of a photographic print, and although definition in photography is used to indicate the quality aspect of a photograph that is associated with the clarity of detail, no way has yet been devised to evaluate it in other than relative terms. The precise effects of the factors on which it depends, such as resolving power, graininess, sharpness, and contrast, have not been fully determined. Furthermore, the problem of evaluation is complicated by the fact that a subject having fine detail, such as an aerial view of a city, looks unsatisfactory unless the definition is exceptionally good, whereas a close-up of a person may be considered a good likeness even though the detail definition is poor.
Any photographic subject may be thought of as an aggregate of a vast number of single points, and a perfect lens should, of course, image each object point as a point of light correctly located on the film. However, because of the limited powers of the human eye, it is unnecessary that each object point be imaged as a true point, and in practice a small patch of light is indistinguishable from a point if it is smaller than some limiting size. We shall make use of this concept when discussing depth of field (page 84), and we are now applying it to discuss the factors that limit the definition obtainable in a negative or in a positive print.
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