Work out lens power (dioptre) instantly with clear inputs, formula shown and shareable results.
Lens power in dioptres is the reciprocal of the focal length in metres, so a 0.25 m converging lens is +4 D and a diverging lens of the same strength is -4 D. Optometrists use dioptres because powers of lenses in contact simply add, which makes prescriptions and trial-lens stacking straightforward.
Lens power
P = 1 / f, with f in metres
Sign convention
converging lenses positive, diverging negative
Because powers add. Stacking a +2 D and a +1 D lens gives +3 D, whereas focal lengths would need reciprocals combined.
A diverging lens with a 33 cm focal length, correcting short sight so distant objects focus on the retina.