The lens of the eye focuses the light entering through the pupil. With approximately 10 to 20 diopters of the eye's total refractive power (of about 63 diopters), the eye lens - along with the cornea - is an essential component of the eye for sharp vision.

The human lens is an elastic converging lens that can be adjusted to different distances by the inner eye muscles (ciliary muscle). The lens is convex (curved outward) and can change its curvature and thus its refractive power through accommodation.

Irregularities in the lens of the eye can also lead to refractive errors such as nearsightedness, farsightedness and astigmatism. In addition, the lens becomes increasingly inflexible from the age of 40 and can therefore adjust less and less easily to near vision - this is referred to as presbyopia. Clouding of the lens in old age is also common - this is then referred to as a cataract.