Modulation transfer function (MTF)
From The Glossary of Digital Photography
A measurement of an optical system's frequency response. It is also sometimes called modulated transfer function or spatial frequency response. It is the measurement an optical system's spatial frequency response often done by photographing line pairs, expressed as line pairs per millimeter in film systems or line widths per picture height with digital cameras. The higher the spatial frequency or MTF, the finer the detail that can be resolved. MTF can be used, for example, with a scanner or similar system to measure the system's frequency response.
