Myopia = Nearsightedness -A focusing error caused by a cornea that too round / steep or an eyeball that is too long. Light rays passing into the eye are focused in front of the retina, the image is seen as blurred. This focusing error is corrected by lenses or surgery that redirect light rays so the focal point falls on the retina.
Hyperopia = Farsightedness - A focusing error caused by a cornea that too flat or not round/steep enough or an eyeball that is too short. Light rays passing into the eye are focused behind the retina, the image is seen as blurred. This focusing error is corrected by lenses or surgery that redirect light rays so the focal point falls on the retina.
Astigmatism - A focusing error caused by a unround or non-sperical cornea. If you imagine the cornea as the surface of a ball, a spherical cornea is like a bowling ball - smooth and completely round.
A cornea that has astigmatism has the surface of a football - smooth, but more round in one direction that the other.
This DIFFERENCE in roundness is astigmatism. The difference causes one part of the cornea to focus light ray differently that the other. resulting in TWO focus points. The vision from astigmatism can be described a blurred, warped, smeared, stretched, or double. This unfocused vision can be INDEPENDENT from effects from focus problems of nearsightedness (myopia) or farsightedness (hyperopia).
Presbyopia (aging eye) -When an eye becomes older, near focusing unit (lens, ciliary body & zonules) becomes less efficient. The point of near focus ability fades and near objects become blurry. Near object need to be place away from the eye to bee seen clearly. This focusing problem can occur independently of or along with the others mentioned above (myopia, hyperopia, astgmatism). reading glasses or bifocal glasses, contacts or surgery can correct this problem.