SIMPLE GUIDE TO PRESCRIPTION LENSES

EMMETROPIA - NO PRESCRIPTION

An eye which sees normally. Light rays from  infinity are brought to a focus on the retina by the cornea and lens.
 

HYPERMETROPIA - LONG SIGHTED (+VE PRESCRIPTION)

Long sight. Light rays from infinity are brought to a focus behind the retina.

A convex lens will converge the rays to focus on the retina.

MYOPIA - SHORT SIGHTED ( -VE PRESCRIPTION)

Short sight. Light rays from infinity are brought  to a focus in front of the retina.

A concave lens will diverge the rays to focus on the retina.

PRESBYOPIA

The weakening of the eye focusing muscles (usually around mid forties) which causes problems with near vision and the need for reading glasses, even when distance vision is still adequate.