Contrast transfer function
(Science: microscopy) a mathematical function that expresses the ability of an optical or electronic device to transfer signals faithfully as a function of the spatial or temporal frequency of the signal.
The modulation transfer function is the ratio of percentage modulation of a sinusoidal signal leaving to that entering the device over the range of frequencies of interest. The modulation transfer function is usually presented as a graph of modulation transfer function versus log (frequency). For a square wave signal, the function is known as the CTF.