Lenses have the ability to bend light or change its direction. A minus-powered lens causes light to diverge once it has passed through, whilst a plus-powered lens does the opposite by converging the light towards a point. A minus lens is generally thicker at the outside edges than at the centre and a plus lens is thicker at its centre than at the edge. This applies to both spectacle and contact lenses.

As the short-sighted eye focuses light in front or short of the retina it makes sense to use a minus lens to take the focus back to the retina. It changes the direction of light before it enters the eye by making it more divergent. This, coupled with the normal focusing of the short-sighted eye, serves to place the focal point back on the retina assuming the correct strength of prescription has been used.

The long-sighted person's focus is too far back so a plus-powered lens is ideal in order to create more bending of light. This convergence of light occurs in front of the eye and when the effect is coupled with that of the eye's normal focusing the focal point is brought forwards onto the retina.

The visual experience of presbyopia is the same as that for long-sight such that near objects are blurry. The difference is that long-sighted individuals have a prescription for their distance as well as near vision and that prescription is always plus-powered. Presbyopes have a more plus-powered prescription up close only, whilst their distance vision could be perfect, short-sighted or even astigmatic. So whatever the individuals distance prescription may be, the near prescription is invariably always different.

The astigmatic eye possesses two main areas of curvature causing light to focus shorter along one curve and further back along the other. Since minus lenses push the focus of light further back and plus lenses bring focal points forwards, a combination of the characteristics of these lenses in a single lens would solve the problem of astigmatism. This lens form is termed toroidal and in contact lenses the term toric is often used. Disposable contact lenses have virtually eliminated the problems previously associated with contact lenses. Frequently replaced, fresh lenses reduce the incidence of eye discomfort and infections.