Consider a thin (spherical) convex lens intended to form an image of an object at “optical infinity”—that is, very far away (say, a star)—located to the left of the lens. The image will be formed to the right of the lens—say, at a distance f, measured in meters. This distance is referred to as the focal length of the lens. The power of the lens is then P = 1/f, where the unit for P is reciprocal meters, which is referred to as the “diopter” (abbreviated “D”). For example, a lens that images starlight 0.5 m to its right has a power of P = 1/0.5 m = 2.0 D (Figure I-8).
Figure I-8 Image formation for an object at infinite distance (such as a star) by a simple convex lens. The distance from the lens to the image is f, the focal length of the lens; the power of the lens is given by P = 1/f, where P is given units of diopters (D), equivalent to reciprocal meters.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.