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diopter

A unit of length used to express the focal length of lenses; currently, chiefly of eyeglasses, magnifying glasses and close-up attachments for photography. The diopter value is the reciprocal of the focal length in meters: a 3-diopter lens has a focal length of ¹/₃ meter. The greater the number, the greater the magnification.

examples

The last 8 μm of the cone has a power of 0.2 megadiopters, apparently representing the most powerful lens known to man.

Malcolm J. Scoble.
The Lepidoptera: form, function, and diversity.
Oxford: Oxford University Press, 1992.
Page 30.
citing
D.-E. Nilson, M. F. Land and J. Howard.
Afocal apposition optics in butterfly eyes.
Nature, vol 312, pages 561-563 (6 December 1984.)