This slide illustrates both diffraction and monochromatic aberrations in the eye. Each image shows the calculated point spread function for a range of pupil sizes in the same eye. Iíll explain how we compute these patterns later.
The eye has fairly good central optics. For small pupils, aberrations are low. So, you might think that the problem is solved by going to a small pupil. But the problem is that smaller pupils give more diffraction. So for a small pupil, the point spread function is very round, but it is also quite large.
Diffraction theory tells us that larger pupils should give us smaller point spread functions but near the margins, aberrations start to dominate so that we can not take advantage of the higher numerical aperture offered by a larger pupil.
In this image, the optimal pupil size lies somewhere between 2 and 4 mm, depending on your criteria.
If you can correct for the aberrations, then the best image quality is through the largest possible pupil.