Now, let's look at why some people need corrective lenses.
We'll start with nearsightedness. In a normal eye, light rays project an image directly ON your retina. But if the eye is too long, or if the cornea bends light rays too much, then the image is focused in FRONT of the retina. In this case, the person is said to have myopia, or near-sightedness, which means they can see clearly up close, but need corrective lenses to see clearly at a distance.
Corrective lenses bend the light in a way that causes it to be focused directly onto the retina again. Now, let's look at farsightedness. If the eye is too short, or if the cornea does not bend the light rays enough, then the image is focused behind the retina - instead of on the retina. In this case, the person is said to have hyperopia, or far-sightedness.
Depending on the amount of farsightedness, this person might need corrective lenses to see clearly both at a distance and up close. Again, the corrective lenses are shaped to bend the light so that it is focused directly onto the retina. If the cornea has an uneven shape to it, then some light rays will be bent more than other light rays. This is called astigmatism. These corneas are shaped more like a football or a barrel. This may lead to some light being focused in front of the retina and/or some light behind. Depending on the amount of astigmatism, this person might need corrective lenses to see clearly at a distance and/or up close.
Presbyopia is condition that affects EVERYONE sometime after the age of 40. Presbyopia is a gradual loss of the ability to focus on objects up close. Although this procedure corrects your distance vision, you will still eventually need glasses for seeing up close. We will discuss this more a little later.