Near-sighted individuals can not focus on objects far away. For an object that is far away, an image must be produced at the individual's far point. A diverging (concave) lens is used for this purpose.

If the object is very far away, the image will appear at the focal point of the lens. By choosing the focal point as the far point, the individual will then be able to focus on the image.

The images of closer objects will occur inside the focal point, so the individual can focus on all distant objects. The prescription for the lense is the inverse focal point, where the focal point is measured in meters. The minus sign refers to the fact that this is a diverging lens. If the prescription were for eye glasses (rather than contacts), the eyeglasses rest a distance x from the eye, which means that the image need only be f - x, from the lens. In that case, one can chose the focal length as

The distance x may be on the order of one centimeter.

Examples     Optical instrument's index