How to calculate spatial frequency of an image

“Spatial frequency” refers to the number of pairs of bars imaged within a given distance on the retina . One-third of a millimeter is a convenient unit of retinal distance because an image this size is said to subtend one degree of visual angle on the retina.

In mathematics, physics, and engineering, spatial frequency is a characteristic of any structure that is periodic across position in space . The spatial frequency is a measure of how often sinusoidal components (as determined by the Fourier transform) of the structure repeat per unit of distance.

A spatial frequency is a number of cycles in space or per unit distance .