Crawley's original definitions.
In order to discuss present-day films usefully, it is necessary to define four terms - acutance, resolving-power, sharpness, definition. Acutance, as we have just seen. is a scientifically measurable quantity which is directly related to the measurement of density gradients. Now when acutance is high, sharpness will be good; therefore one might say that acutance is the scientific measure of sharpness, which latter is a visual or psycho-visual effect. ' Sharpness' is usually associated with the overall impression of a print or projected image, judged from normal viewing distance for its size; 'definition' on the other hand may have to be judged from a closer viewpoint, and refers to the extent to which fine detail is recognisably rendered. When the acutance with which fine detail is rendered is high, then definition will be pronounced good. Resolving power or resolution refers directly to the ability of a lens, film, or developer, or any combination of these three, to record detail, and like acutance is a scientifically measurable quantity, so long as the conditions and criteria used are stated, Just as acutance is the measure of visual sharpness and efinition, so resolution is the measure of the actual fineness of detail rendered.
His summary in a later shortened version:
" Sharpness "-the overall impression of a print or projected image, measured scientifically as "acutance ", seen from normal viewing distance.
" Definition "-the extent to which fine detail is recognisably rendered in a print, etc. When acutance of fine detail is good, then definition is good.
" Acutance "-the contrast at the edge of significant detail, a scientific measurement of the density gradient at that point.
" Resolving Power "-the scientific measurement of the actual fineness of detail recordable by a lens, film, or developer, or any combination of these three.