Near-sightedness
Near-sighted individuals cannot focus on objects far away. For an object that is far away, an image must therefore 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 (both f and di negative). By choosing the lens to make the image be at the far point, the individual will be able to focus on the image.
The images of closer objects will be closer than the far point, so the individual will be able to focus on them also. The prescription for the lens is given in diopters, i.e., 1/f where f is the focal length meters. The minus sign refers to the fact that it is a diverging lens. If the prescription is for eye glasses (rather than contacts), the eyeglasses rest a distance x from the eye. The image therefore needs to be only f - x, from the lens. In that case, the focal length should be chosen as
The distance x is approximately 2.0 cm.