The ciliary muscles contract and relax adjusting the thickness of the lens and therefore its focal length. The cornea and the lens combine to focus images on the fovea of the retina.

By changing the shape of the lens near and far objects can be focused with precision at fovea. Many visual disorders occur as a result of the lens losing its ability to accomodate. The natural aging process causes the lens to harden and less able to alter its shape.
Prebyopia is a condition which impairs vision of near objects and is caused by the lens losing its ability to change its shape. This condition can be easily corrected by the use of spectacles prescribed by an optometrist. These spectacles are often called reading glasses.

Spectacles are prescribed for near-sighted and short-sighted eyes. Glasses distort the image and compensate for the inability of the lens to change its shape.

Click to see the image formed by glasses.