A question of matching
As explained on our how the eye works page, sharp vision depends on the combined optical power of the cornea and natural lens matching the physical length of the eyeball, so light focuses precisely on the retina. Myopia and hyperopia are the two ways this match can go wrong. Neither is a disease — they're simply variations in eye shape and optical power that are extremely common and, in most cases, easily corrected.
Myopia: focusing too soon
In myopia, commonly known as nearsightedness, light entering the eye comes to focus before it reaches the retina, rather than exactly on it. By the time the light rays reach the retina, they've already crossed and started to spread back out, producing a blurred image. This typically happens because the eyeball is relatively longer than the eye's optical power calls for, or because the cornea and lens are focusing too strongly for that eye's length — often some combination of both.
The practical result is that distant objects appear blurry while close-up objects tend to stay clear, since near vision doesn't require as much extra focusing power to begin with. Myopia is corrected with "minus" powered lenses — glasses or contacts that diverge light slightly before it enters the eye, pushing the focus point back onto the retina where it belongs.
Hyperopia: focusing too late
Hyperopia, or farsightedness, is essentially the mirror image of myopia. Here, light entering the eye would focus behind the retina if it could — the eyeball is relatively shorter than its optical power calls for, or the focusing power is too weak for that length. Because the retina sits in front of where the light would naturally converge, the image formed on it is blurred.
Hyperopia is corrected with "plus" powered lenses, which add extra converging power to pull the focus point forward onto the retina. Mild hyperopia is sometimes partly compensated for by younger eyes using extra accommodation (see our natural lens page), which is part of why some people don't realize they're farsighted until later in life, when that extra accommodating power starts to decline.
Neither one is about the shape of the cornea's surface
It's worth distinguishing myopia and hyperopia from astigmatism. Myopia and hyperopia are about the eye's overall length relative to its focusing power — the amount of focusing power is off, but the shape of the optical surfaces is still essentially round and symmetrical. Astigmatism, by contrast, involves the shape of the cornea itself being irregular. The two can absolutely occur together — plenty of people are myopic and astigmatic, or hyperopic and astigmatic, at the same time. Read our dedicated astigmatism page →
Why this matters for cataract surgery
Before cataract surgery, your surgeon measures your eye's length and corneal power precisely in order to calculate the right power for your replacement lens. In effect, this calculation is designed to correct any underlying myopia or hyperopia at the same time the cataract is removed — many patients who wore strong glasses for nearsightedness or farsightedness for decades find their distance prescription is dramatically reduced or eliminated after surgery, depending on the intraocular lens power chosen. Explore your IOL options →