CSF Parameterization. The spatial contrast sensitivity function, which describes reciprocal contrast threshold as a function of spatial frequency, can be described by four parameters: (1) the peak gain, max g (2) the peak frequency, max f (3) the bandwidth (fullwidth at half-maximum),b , and (4) the truncated fall-off on the low-frequency side. The qCSF method rapidly estimates the CSF by directly estimating these four parameters.