Definition: f/number of a lens corrected for the light loss during transmission through the lens. * f/number of a perfectly transmitting lens which would give the same illuminance on the axis as that produced by the test lens. * Equals the f/number divided by the square root of transmittance (assuming a circular aperture) e.g. if transmittance is 50% (only half light entering system exits the system), square root of a half is 1/C2, so T-number is one stop more than the f/number, so a relative aperture of f/4 with transmittance 50% is a T/5.6 lens. * Also known as T-stop. Assuming the ideal The f/number of a lens is defined by simple geometry (one length divided by another) so it assumes that the lens passes all of the light entering it. But no lens does: each interface between media of different refractive indexes causes a loss. Modern lenses are amazingly efficient so losses are in practice very small and, at any rate, losses are automatically compensated by through-the-lens metering. T-numbers are important in film industry, where TTL metering is not common.

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