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The nearsighted eye: the eyeball is too long and the image is focused in front of the retina

Nearsightedness is corrected by use of concave lenses, which help the eye lens to focus properly on the retina

If the eyeball is too long, the image is focused short of the retina and appears blurred, causing nearsightedness. This can be corrected by using spectacles of the concave variety, which diverge the light entering the eye and help the eye’s convex lens to focus (form a focal point) on the retina properly.

On the other hand, farsightedness occurs because the eyeball is too small and the image is formed behind the retina. A convex spectacle lens introduced in front of the eye will converge the light rays entering and guide them onto the retina properly.

The formation of an image behind the retina also occurs when the eye’s lens loses its power of accommodation and reaches a point where it cannot assume the deeply curved shape necessary for focusing on nearby objects. Usually affecting those passing middle age, this condition is known as presbyopia, “old sight,” requiring convex segments in the spectacle to correct the weakness.