Why do short-sighted people need glasses for up-close vision if they have difficulty with seeing things far away?

Myopic (near- or short-sighted) eyes are typically a larger diameter than "normal" eyeballs. The focusing parts of the front of the eye (cornea, natural lens, and pupil) do not "know" this, so they focus a distant object too soon inside the eye: the image is in front of the retina instead of right on it. This means that the default focal distance is not out there at the horizon where it should be, but is much closer to the eye itself. Just how close depends on how big the eyeball is, i.e. how much nearsightedness you have. In a highly nearsighted eye, the focal distance can be just a few inches (or cm) from your face while in a moderately nearsighted eye (say, -2.50 Diopters), the default focal distance can be a very convenient 40 cm from your eye -about normal reading distance.

In the latter case, you still cannot see at all well off at a distance without glasses, but you will be able to see great at a typical 40 cm reading distance if you take off those glasses. This is not of much use in a young person who can accommodate, i.e. use the muscle surrounding the natural, internal lens to squeeze it into the right shape for near focus. But it can be very useful to an older patient with Presbyopia, i.e. whose natural lens has hardened to a degree the muscle just can't do its job anymore. A 50 year old -3.00 D. myope (nearsighted person) can just take off his glasses and read just fine despite the Presbyopia because his eye's default focal distance is 33cm. The myopia neutralizes the effects of the presbyopia!

But if that same person has LASIK, or gets an (IOL) intraocular lens during cataract surgery that is set to focus at distance, then he will lose that nice near vision and have to wear reading glasses -even though that was not the case before surgery.

Are you curious if you're a candidate for a vision correction procedure? You can take our self evaluation test here

Written by J. Trevor Woodhams, M.D. - Chief of Surgery, Woodhams Eye Clinic