"I'm getting my eyes lasered" is a remark becoming almost as common as "I'm getting my hair done," and it's often uttered with the same mixture of excitement and nonchalance. Millions of Americans are undergoing the laser in the hope of kissing their glasses goodbye — and giving themselves a more youthful appearance to boot.

The popularity of LASIK, short for Laser-Assisted In-Situ Keratomileusis, is no mystery. Promising almost instant, nearly painless vision correction for a few thousand dollars, it's nearly irresistible. And to those for whom the technique has been successful, the results seem nothing short of miraculous. But before exposing your only set of eyes to the procedure, know that Lasik isn't foolproof, and isn't for everyone.

Even Ken Moadel, a Manhattan ophthalmologist, will say as much, and he loves LASIK. He'd better; he's performed more than 7,000 LASIK operations since the technique was approved by the FDA in 1995, and he anticipates doing thousands more this year alone. "The wonderful part about this procedure is you're taking a lifelong ailment — having spent 20 or 30 years waking up and putting on a pair of glasses every morning — and then with a five-minute procedure, and results within 12 to 24 hours, you cure them of that ailment."

Who's a Candidate for LASIK?

"You should be at least 18 years of age, but there really is no upper age limit, as long as the eye is otherwise healthy," says Moadel. "There are certain conditions — types of eye diseases or eye shapes — which would exclude a patient as a candidate."

Limitations on who can undergo LASIK successfully stem from the fact that the technique does one thing, and does it well, unless there are confounding conditions in the eye before or after surgery. To understand how LASIK works, a brief refresher on eye anatomy will help.

We've all seen the textbook line diagrams illustrating how light beams entering the eye from different angles meet at a single point, creating a focused image. If that focal point happens to be right on the retina, a sort of projection screen in the back of the eye, the image is clear. If, however, it falls short when looking at faraway objects (a condition called myopia or "shortsightedness"), the image projected onto the retina will be blurry. If the focal point is somewhere beyond the retina when viewing nearby objects (a condition called hyperopia or farsightedness), the effect is also a blur.

Bending or refracting incoming light so the focal point is right where you want it is accomplished in a camera by the lens. The human eye has a lens that does the same thing, but incoming light is also bent (before it ever hits that lens) by the cornea, a dome-shaped outside surface of the eyeball covering the pupil. In fact, the lens provides less than half the eye's focusing ability; the cornea does the bulk of the work.

Made mostly of collagen and water, a healthy cornea is circular and perfectly clear. However, one that protrudes too far will result in a too-short focal length, or myopia. If the cornea is too flat, the opposite condition of farsightedness results. And if the cornea's shape or surface is uneven, called astigmatism, incoming light will also be distorted.