What is a diopter?
A diopter is the unit of measurement of the optical power of a lens or curved mirror. It is the strength of the magnifying lens measured at one meter. Four diopters (4.0D) represent a 100% increase in magnification or 1x power magnification. So, a 20 diopter (20.0D) measurement is approximately equal to 5x power magnification. A rough formula to convert from magnification power to diopters is to multiply the magnification power by four.
There are two answers to that question. In common photographic usage, diopter is synonomous with “close up lens”. A standard close-up lens is a single element lens that screws into the accessory thread of your camera lens, like a filter. These lenses allow you to photograph small objects, close up. The strength of close up lenses is measured in diopters. Common values are +½, +1 and +2. For high quality work, you probably won’t want to use a diopter stronger than +3. The second and technically more accurate answer to your question is that diopter is a measure of lens power, often used regarding opthalmic (eyeglass) lenses. A lens with a power of 1 diopter has a focal length of 1 meter. Close up lenses are marked as “+1 diopter”, for example, so they’ve come to be known as “diopters”, in common usage.
below) because the unit of measurement of the power of a lens is the diopter, are simple lenses that look rather like filters, and screw into the filter ring at the front of a lens. The effect of adding one of these supplementary lenses is that it increases the magnification that the main lens can achieve. With the lens focused at infinity, the magnification achievable is the ratio of the focal lengths of the two lenses; for example, if the supplementary lens has a focal length of 50mm (this would be a 20 diopter lens) and the primary lens is 100mm focal length, then the combination will give 2:1 magnification (twice life-size) with the primary lens focused at infinity.
