At photopic luminance levels, the cone-cell variation of packing density across the retina provides a natural limit to the effective size of wide-field stimulus patterns. In some experiments, this eliminates the need for small test spots, which produce band-broadening effects in the spatial-frequency domain. Calculations of these effects are given, to aid in the design of such experiments.
D. H. Kelly, "Effects of the cone-cell distribution on pattern-detection experiments," J. Opt. Soc. Am. 64, 1523-1525 (1974)