Photoreceptor diameter and spacing for highest resolving power.
The maximum center-to-center angular spacing delta phi of photoreceptors tolerated for reconstructing the highest spatial frequency passed by a diffraction-limited pupil of diameter D is delta phi - lambda/D square root 3 when the photoreceptors are packed in a hexagonal array, where lambda is the wavelength in vacuum. This spacing gives the maximum signal-to-photon noise ratio when the inner segments touch. The mean luminance required for an eye to achieve its highest resolving power is independent of eye size, provided the retina is designed to sample the highest spatial frequency passed by the diffraction-limited optics.
D006801:Humans; D008954:Models, Biological; D010786:Photoreceptor Cells / Q000033:anatomy & histology*; D014792:Visual Acuity