Since diopter strength is based on the inverse of the focal length, a 2 diopter lens is not 2 meters but 1/2 meter, a 3 diopter lens is 1/3 meter and so forth. Lenses with a focal length – defined as the distance between the center of a lens and the point at which objects are brought into clear focus – of 1 meter (about 39 inches) have a diopter strength of 1. It's commonly applied to reader-strength eyeglasses or bifocals, as well as telescopes & other magnifying lenses. Diopter strength refers to the optical power of a lens.
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