Like the modulation transfer function of man-made imaging devices, the contrast sensitivity of the human eye can be measured with sinusoidal grating targets of various spatial frequencies. Criterion-free psychophysical methods permit us to regard the contrast sensitivity as a direct measure of the subject's visual performance, independent of subjective factors. Under these conditions, not only the shape of the contrast-sensitivity function but also its absolute values show good agreement among normal subjects. However, the most interesting properties of this function cannot be attributed to the optics of the eye, but must be understood in terms of the image-processing activities of the visual pathways. The contrast-sensitivity function varies with the size, brightness, motion and flicker of the test target, with the adaptive state of the subject's retina, and with his eye-movements. Most of these effects can be explained in terms of known neurophysiology.