effective D/number

Definition: D/number of lens corrected for close focusing distances. * The f/number multiplied by the result of dividing the image distance by the focal length. * The focal length divided by effective diameter of aperture.
Only infinity counts
The correction for close-up work is needed because f/number is defined by the lens' focal length which is in turn defined relative to infinity. So if you focus much closer than infinity, you must make a correction: through-the-lens metering does that for you (but even then manufacturers have to exercise care over where they locate the meter). Any separate, off-camera metering, must take account of the error, in general, when focusing nearer than half a metre.

Related Terms: effective aperture

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