Optical transfer function (OTF)
From The Glossary of Digital Photography
A complicated function used in microscopy to measure the ratio of the image contrast to the specimen contrast. Other variables, such as spatial frequency, are involved as well. If the optical system were perfect, the OTF would be equal to one (or 100%). In worst-case scenarios, the image contrast can be reversed from that of the specimen so that dark areas appear light instead of dark, for example.
