There are no perfect lenses in nature. Aberrations of some kind will always be there. But why healthy human eyes circumvent this issue? Or, are there any aberrations we don't "see?"
The Wikipedia article Aberrations of the eye discusses this. The article is really about non-normal aberration i.e. the sort of thing that makes us have to wear spectacles, but it also mentions the aberrations in normal eyes. The conclusion is that normal eyes are actually pretty good with just a small amount of spherical aberration. The lens and cornea form a compound lens system and correct for most aberration.
It shouldn't be surprising that the eye is pretty good as poor eyesight wouldn't have been a good strategy for survival in the days when our ancestors had to avoid sabre tooth tigers and their friends!
There is a comprensive but turgid article on the performance of normal human eyes here.