Spherical aberration is one of the most important problems that can occur after laser eye surgery, in particular with high myopic corrections.
For lenses made with spherical surfaces, rays which are parallel to the optic axis but at different distances from the optic axis fail to converge to the same point. The peripheral light rays are bent more than the central ones as in the following diagram:
A spherical surface has a "Q value" of 0. A surface which is a parabola has the peripheral part of the lens relatively flatter than the centre and so bends the peripheral light rays less, eliminating this spherical aberration. Such a cornea has a negative Q value and has a prolate shape A parabola has a Q value of -0.5. The human eye of a young person has a Q value of -0.5, which is made up of the cornea (Q= -0.25) and the lens of the eye (Q= -0.25) added up together. The over 40y age group has a rounding out of the lens, so its Q value becomes near O. Hence older people have more natural spherical aberration as their Q value is only that of the cornea i.e. -0.25.
There is a nice demonstration of spherical aberration at the Olympus web site (http://www.olympusmicro.com/primer/java/aberrations/spherical/). The Hubble Space Telescope suffered from spherical aberration when first launched. This was solved by using "adaptive optics", similar to that now being used in excimer lasers. See the Hubble page on this web site: http://www.birkacre.freeserve.co.uk/hubble_space_telescope.htm.)
After a myopic PRK or LASIK, the Q value becomes positive with increased spherical aberration. The cornea then has an oblate shape No normal human cornea is oblate or has a positive Q value. However, all the modern lasers have "blend zones" that smooth off the mid-peripheral "knee" that has a high local Q value and this lessens the induced spherical aberration. e.g. The Technolas 217 laser has true optical zones up to 7mm with a blend zone at least 3mm bigger than this. (the cornea is only about 12.5mm diameter). Similarly the Nidek EC5000 has optical zone up to 6.5mm and the blend zone is adjustable up to 10mm.
Spherical aberration is not really a problem with low myopic corrections but can be a problem with some patients having higher corrections e.g. about about -5 D. The laser manufacturers are tying to improve the shape of the ablation profile to lessen this problem. All the "custom ablations" done by various lasers have totally "aspheric" profiles that have, in theory, no aberrations. However, they can take off more tissue, which can again be a problem with higher corrections as there may not be much to spare. Spherical aberration is not normally a problem in good light but in low light. See night vision: (http://www.birkacre.freeserve.co.uk/night_vision.htm) and lasik complications: http://www.birkacre.freeserve.co.uk/lasik_complications1.htm).