There are several conditions that can cause this intricate visual system to not work properly, including:
LASIK can fix the first three conditions, but it cannot fix presbyopia.
In a normal eye, light rays project an image directly onto your retina. If the eye is too long, however, or if the cornea bends light rays too much, then the image is focused in front of the retina instead. In this case, the person is said to have myopia, or nearsightedness, 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.
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 onto the retina. In this case, the person is said to have hyperopia, or farsightedness.
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 others. 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 being focused behind. Depending on the amount of astigmatism, this person might need corrective lenses to see clearly at a distance and/or up close.