FIGURE 236: (A) Generalized scheme of the respective magnitudes of aberration-free (diffraction-limited) and average real (i.e. aberrated) eye, on axis. Geometric blur size and RMS wavefront error have nearly identical rate of change, thus both scales apply to the same plot. With all eye aberrations included, diffraction dominates for pupil diameters smaller than ~2mm, and for larger pupils eye aberrations. Geometric size of point-image blur - as aberrated ray spot blur, or diffraction blur conventionally represented by the Airy disc - is smallest at ~2mm pupil diameter. For larger pupils, it is enlarged due to eye aberrations, and for smaller pupils due to diffraction. With defocus corrected, which applies to the telescopic eye, eye is diffraction limited for pupils diameters smaller than ~3.5mm. Relative magnitude of higher-order aberrations increases with the pupil size, becoming comparable to the defocus-corrected eye (i.e. plagued only by primary astigmatism) at about 7mm pupil diameter.
(B) Foveal eye contrast transfer  for 5.7mm pupil diameter without correction, with corrected 2nd order aberrations, and with corrected monochromatic aberrations (based on Guirao et al. 2002). The latter effectively shows contrast transfer drop due to eye's chromatic aberrations; at this pupil size, eye chromatism (longitudinal and lateral error combined) cause more contrast loss than secondary spectrum in 200mm f/5 achromat (shown in the corresponding relative range of frequencies). Spatial frequency - given by 1/ν, with the frequency ν determined by the inverse of the wavelength-to-aperture ratio, in radians - is, by default, limited to 60 per degree, i.e. one arc minute, despite the theoretical cutoff frequency being 200 per degree, or 0.3 arc minute (as shown on graph C, this theoretical resolution doesn't materialize, due to the contrast level at these frequencies falling below the minimum required by the eye).
(C) Foveal eye contrast transfer for three different pupil sizes, with 2nd order aberrations - defocus and primary astigmatism - corrected. Since eye defocus error is cancelled out by defocusing the eyepiece, these plots are close to the actual monochromatic (around e-line) contrast transfer for the telescopic eye, with eye astigmatism being relatively insignificant at this error level. The MTF for larger pupil sizes indicates considerably higher cutoff frequencies, but the actual cutoff frequency is similar for all three pupil sizes, being determined by the minimum contrast level required by eye for resolution.
(D) Foveal MTF for the same three pupil sizes with spatial frequency normalized to 1. A comparison with MTF for 24mm König eyepiece for identical exit pupil sizes shows that eye aberrations lower contrast more than the eyepiece aberrations for all three pupil sizes. At 2mm exit pupil diameter, the König is essentially aberration-free on axis, and nearly so even at 4mm exit pupil (with the diameter of eyepiece exit pupil is given by the ratio of eyepiece f.l. and system's focal ratio, these exit pupils correspond to an ƒ/12 and ƒ/6 system, respectively). Since the only axial aberration of an eyepiece is spherical aberration, which increases much faster with the pupil diameter than the astigmatism/coma/spherical/trefoil mix of the eye, its contrast transfer at 6mm exit pupil diameter (i.e. with ƒ/4 system) is comparable to that of the eye with 4mm pupil (C and D based mainly on Liang and Williams 1997 and Thibos et al 2000).
(E) Polychromatic and monochromatic diffraction spots for four eye pupil diameters. At 1mm eye pupil diameter, the two patterns are essentially identical, indicating near-zero eye chromatism; at 2mm, the difference is only slight. At 4 and 6mm, polychromatic pattern becomes noticeably fainter, indicating loss of energy due to chromatic error. Patterns are generated by OSLO, with the eye model specified in above table; real eye pattern at larger pupil sizes - as depicted in the dark circle - commonly shows significant random asymmetry, caused by random local deviations of eye surfaces from the rotationally symmetrical conic surface. Change in nominal size of the conspicuous portion of diffraction blur is much less pronounced than its change relative to the Airy disc for corresponding pupil sizes, shown at right. The blur is relatively large with respect to the cones (~2.3μ-10μ from foveola to the outer retina, respectively) and rods (~2.5μ-5μ), hence with the limiting resolution determined entirely by the blur size.