Glasses are made of two lenses and a frame. The strength of glasses, their optical power, is measured in diopter (dbt), this value is the reciprocal of the lens focal length in meters. Convex (bend outward) lenses have a positive diopter value and correct farsightedness. Concave (bend inward) lenses have a negative diopter value and correct nearsightedness. Farsighted people see better at distances than at close objects, this is common in higher age.
P = 1 / f
Glasses strength = 1/(reading distance in meters) - 1/(minimal seeing distance in meters)
Calculation of focal length, optical power and diopter of glasses and the estimation of the right strength of reading glasses.