Take the reciprocal of the focal length measured in metres.
The optical power of a lens (or mirror) with a focal length on 2.5 metres is 1/2.5 = 0.4 dioptres.
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Diopters are a unit of measurement used to quantify the focusing power of a lens, which is important in optometry and ophthalmology. They indicate how much a lens converges or diverges light. Positive diopters indicate converging lenses (helps nearsightedness), negative diopters indicate diverging lenses (helps farsightedness), and zero diopters indicate no optical power.
To convert meters to micrometers, you multiply by 1,000,000. So, 0.056915 meters is equal to 56,915 micrometers.
To convert centimeters to meters, divide by 100. So, 569 centimeters is equal to 5.69 meters.
To convert from feet to meters multiply by 0.3048 21 feet = 21 x 0.3048 meters ~ 6.4 meters
To convert kilometers to meters, you multiply by 1000 (since there are 1000 meters in a kilometer). Thus, to convert 3.5 kilometers to meters, you would multiply 3.5 by 1000 to get 3500 meters.