Chromatic Aberration
The eye cannot simultaneously focus on the three regions of the spectrum where the cone-pigment absorptions peak since refraction at the cornea and lens is greater for short wavelengths than it is for long wavelength. Thus, it is said that the eye is not corrected for chromatic aberration. The medium- and long-wavelength peaks are quite close together and therefore the lens optimally focusses light of about 560 nm on the retina. Since the short-wavelength cones receive a slightly blurred image it is not necessary to provide the same spatial resolution that is provided by the other two sets of cones. The retina contains approximately 40 long-wavelength cones and 20 medium-wavelength cones for every single short-wavelength cone.
The rods and cones are not evenly distributed on the retina. The central part of the retina, the fovea, contains only cones whereas at greater eccentricities there is a greater preponderance of rods. In the fovea the cones are densely packed and it is this part of the retina that provides the greatest spatial resolution under normal viewing conditions.