The Human Eye & Glasses

Power of accommodation is the eye lens's ability to change its focal length so that both near and far objects form a sharp image on the retina. The ciliary muscles make the lens thicker for near objects and thinner for far ones.

Myopia, or short-sightedness, is when a person sees near objects clearly but distant ones look blurred. The image of a far object forms in front of the retina, because the eyeball is too long or the lens too strong. It is corrected with a concave lens of suitable negative power.

Hypermetropia, or long-sightedness, is when a person sees distant objects clearly but near ones look blurred. The near point is farther than 25 cm and the image of a near object forms behind the retina. It is corrected with a convex lens of suitable positive power.

Presbyopia is the gradual loss of the eye's power of accommodation with age, as the ciliary muscles weaken and the lens stiffens. The near point moves away, so reading becomes hard. It is corrected with convex reading glasses, or bifocals if the person is also short-sighted.

The near point is the closest distance at which the eye can see clearly, about 25 cm for a normal young adult. The far point is the farthest distance it can see clearly, which is infinity for a normal eye.

The distance from the eye lens to the retina is fixed at roughly 2.5 cm in a grown-up eye. Since this 'screen' can't move, the eye focuses by changing the lens's power instead: about 40 D for far objects and about 44 D for objects at 25 cm.