There is measurable blurring of the retinal image even under ideal viewing conditions, and the visual system is subject to spherical and chromatic aberrations. Figure 10 shows the measured distribution of light across the retina when viewing a sharp edge between luminances of two different levels. The blur of the edge is several minutes of arc wide. Visual objects are normalized to the distance from which they are viewed. The size of an object is described by the angle formed when lines are drawn from the eye to the edges of the object.
Different wavelengths come into focus at different distances behind the lens of the eye.
This is called chromatic aberration. Chromatic aberration of the eye. Figure 11 shows the relative refractive differences of various wavelengths. The zero point of the ordinate, at 587 nanometers, is arbitrary.