The Diopter

The power of a lens (D) in diopters is defined as the reciprocal of the focal length measured in meters.

The powers of two thin lenses in contact add.

The optical strength of spectacle lenses is quoted in diopters. A +2 lens is a convex meniscus lens with a focal length of 50 cm.

Adding a -0.5 lens to a +5 lens gives a +4.5 combination - which is how opticians traditionally found the best formula for the client.

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