Acutance: A numerical value that correlates to some extent with subjective image sharpness. The term "acutance" is generally reserved for photographic edge sharpness as measured with images formed in light-sensitive materials. as distinct from optical images. Acutance is determined by printing a sharp knife-edge in contact with the material being tested using a point source of light or a collimated beam. Scatter of the exposing light in the emulsion layer causes the developed image of the knife to be diffused producing an s-shaped density distribution. Acutance is the geometrical average of the slopes at different points on the edge trace divided by the density difference between the limits of the trace.