Diopter converter
Convert lens power between diopters and focal length. A +2 reading lens focuses at half a meter.
That's about the same as reading glasses. That brings light to a focus about 500 mm away.
What it means
How strong is that lens?
Prescriptions carry a cylinder and an axis as well, for astigmatism. This page converts the sphere, which is the main number.
Every unit
Your lens in every unit
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Three things worth knowing
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Diopters = 1 ÷ meters
A 250 mm focal length is +4 D, a 50 mm camera lens is +20 D. Short focus means strong lens.
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Adding beats dividing
Monoyer proposed the diopter in 1872 precisely because optometrists stack trial lenses. In diopters that is addition; in focal lengths it is a fraction of fractions.
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Reading glasses are not magnification
A "+2.00" pair is not two times magnification. As a magnifier held at the eye it is about 1.5×; the number is a diopter, not a ratio.
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Background
About diopters
A diopter is one divided by the focal length in meters, so a +2 D lens focuses parallel light 500 mm away and a +4 D lens at 250 mm.
The reason optics uses it rather than focal length is that diopters add: put a +2 in front of a +3 and you have a +5. Focal lengths would need a formula.