Definition, as the term is used in the present paper, is the quality aspect of photographs that is associated with the clarity of detail. Resolving power is used in its customary sense, and sharpness is defined as the impression received by an observer viewing well-resolved elements of detail; its objective correlate is acutance, which has been defined quantitatively elsewhere. A series of negatives was made of a scene, the lens being moved longitudinally between exposures to vary the resolving power and the acutance in the negatives. A corresponding set of negatives was made of resolving power and acutance test objects. Positive transparencies were made from both sets of negatives. The transparencies of the scene were judged for definition, and numerical values were assigned by statistical methods. Neither acutance nor resolving power correlated well with definition except when the resolving power exceeded the limit set by the eye, in which case acutance correlated fairly well. A practical correlate of definition is proposed which consists of acutance multiplied by an exponential function of resolving power.
GEORGE C. HIGGINS and ROBERT N. WOLFE, "The Relation of Definition to Sharpness and Resolving Power in a Photographic System," J. Opt. Soc. Am. 45, 121-125 (1955)