Clear vision occurs when light enters a clear, unobstructed eye through a normal cornea and lens and is focused precisely on the fovea in the macula in the center of the retina. Young eyes can accommodate using the ciliary muscles to bend the flexible crystaline lens to allow light rays from near and far objects to focus.

Refractive errors disrupt the accurate and precise focus in the eyes. Nearsightedness results from a long eye or a strong cornea blurring distance images. Farsighted eyes are too short, or the cornea too flat so near objects are more blurry. Astigmatism results from a non-round surface on the cornea or lens rendering images in one meridian blurrier than another. Presbyopia occurs in older eyes that are not able to accommodate near images. Refractive error changes with age.

Optical correction includes glasses, bifocals, contact lenses, cataract implants and corneo-refractive surgery. Careful refraction of the eyes combined with expert fitting of optical devices leads to the best visual acuity.