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Comprehensive
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astigmatism an irregularity in the curvature of a lens, including that of the eye, that prevents rays of light from meeting in a single focal point, resulting in an indistinct or distorted image. [1/2 definitions]
bifocal in optics, having two focal lengths. [1/3 definitions]
compound microscope a microscope in which two lenses of differing focal lengths are mounted in an adjustable tube.
diopter a unit of measure of the refractive power of a lens, equal to the power of a lens with a one-meter focal length.
f-number the relative aperture of a camera lens, representing the light it will transmit, which is obtained by dividing the focal length of the lens by the diameter of the aperture.
hope the source of, or focal point for, that which is wished or desired. [1/6 definitions]
multifocal combined form of focal.
omphalos a central or focal point. [1/2 definitions]
trifocal of eyeglasses, having three lenses of different focal lengths. [1/2 definitions]