If the contrast sensitivity function (CSF) is measured when the eye is not in focus for the grating (e.g. due to a lack of correction or an erroneous correction), the CSF will suffer appreciably and fall much more rapidly with increasing spatial frequency towards zero contrast sensitivity. At this point the grating can no longer be resolved and appears a uniform grey. At spatial frequencies greater than this threshold value there may occur a phenomenon known as spurious resolution in which the CSF rises above zero. Thus, the grating may become visible again at higher spatial frequencies than that at which it first disappeared. Example: spurious resolution can be observed when looking at a radial grating (or star sector target) in which the spatial frequencies increase towards its centre. If the grating is held close to the eye with the accommodation relaxed one can see a grey annulus (and sometimes two) separating a zone (or two) of spurious resolution. See contrast sensitivity function.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann