Here's a sort of explanation given by Elliott S! Maggin one of the major Superman comic writers in his book "Miracle Monday".
"I figured out how my eyes work. I've got this weird optic nerve, see? It's got an active mode along with the passive mode everybody else's optic nerve has, which is why I can project heat and X rays with my eyes, besides just seeing through them. Anyway, all I have to do to intensify the magnification of that microscope is divert the active mode impulse of my - Are you following this, Pa?"
I'm sure its pseudoscience at its best, but it sounded impressive considering the fact that we have to remember that Clark isn't human and probably does have quite a few internal differences.
-Michael (who has come to consider the possibility that he read waaaayyyy too many comics as a kid)
