Sight is corrected by using lenses infront of the eye to focus an image onto the retina. The lens can be held in a frame as spectacles or placed onto the eye itself as a contact lens.

Generally spectacle lenses fall into 3 groups

Most common type, may be reading, distance, VDU or general purpose. Younger patients (under middle 40's) probably only need one prescription for everything. As we get older we loose our ability to change the focus of our eyes and we then need a stronger lens for reading (a "reading addition").

Instead of having 2 separate pairs of spectacles for distance and reading, bifocals have two parts to them, an upper or distance part and a (usually smaller) lower part for reading. There are many shapes and styles of bifocal segment designed for differing tasks.

A more modern attempt to recreate the smooth change in focus that the natural eye exhibits. By very clever variation in the curves on the lenses there is a gradual change in power as one looks down the lens. There are several designs and generally fall into budget, standard and deluxe versions.

Single vision
Bifocals
Multifocals

Distance prescription
When the eye is at rest it should focus light from a distant object onto the retina. When it does it is called emmetropic. If it does not then the person sees object blurred unless the eye can focus itself or a lens is placed over it to focus the image accurately. The strength of this lens is called the eyes Distance Prescription. The eye can be long sighted (hypermetropia) or short sighted (myopia) and may also have an error called Astigmatism.

Reading addition
A young eye can adjust its focus onto close objects. As the eye ages it slowly looses this ability and by about 45years it cannot focus at a normal reading distance. A reading addition is a plus lens added over the top of the eye's distance prescription to bring close objects into focus.