F-Number
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A scale used to express the relative area of the aperture of a lens, simply the result of dividing the focal length of a lens by the effective aperture of the lens opening (the apparent size of the diaphragm seen from the front of the lens). The f-number increases by the multiple of the square root of 2, or 1.4142, from 1.0, 1.4, 2.8, 4, 5.6, 8, 11 and so on, allowing each to pass half the light of the aperture below and twice the light of the aperture above in the scale.
