Definition: Measure of refractive power of lens: calculated as 1000/focal length in millimetres where a positive value indicates a converging lens, a negativeve value indicates a diverging lens. * Unit: D. * Strictly there is a factor for refractive index of the image space -- as it is usually is air, the factor is 1. * Used to specify supplementary lenses used for eyepiece correction and for close-up attachments and for prescribing spectacles. * Negative power dioptres are used for myopia (short sight), positive for hypermetropia (long sight) and as reading glasses to help with presbyopia.

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