What is spatial frequency in vision?

Spatial frequency describes the periodic distributions of light and dark in an image. High spatial frequencies correspond to features such as sharp edges and fine details , whereas low spatial frequencies correspond to features such as global shape.10 Ara 2015

“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.

That is, you are most sensitive for an intermediate range of spatial frequencies (around 4-6 cycles/degree), and less sensitive to spatial frequencies both lower and higher than this , much like the audiogram.