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Meaning of relative aperture

rel'ative ap'erture

Pronunciation: [key]
— Optics. Optics.
  1. the ratio of the diameter of a lens, esp. a camera lens, to the focal length; the reciprocal of the f number or focal ratio of the lens.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.