The MTF, or Modulation Transfer Function, is defined as the ratio of the image contrast to the target contrast, expressed as a function of spatial frequency. That is, MTF(u) = C'(u) / C(u). C is the contrast in the target, C' is the corresponding contrast in the image.

The modulation transfer function (MTF) is the most widely accepted measure of the spatial resolution response of medical x-ray imaging detectors and has proved to be a valuable tool for determining and comparing detector performance.

The production of point light sources and line light sources requires a certain precision and is relatively complex, so the method of obtaining ESF by edge imaging is generally used to calculate MTF, which is called slanted-edge method.

The MTF of a system may be measured with an interferometer by one of two methods: auto-correlating the pupil function of the lens-under-test or analyzing the PSF calculated by Fourier transforming the pupil wavefront.