Quote:
Originally Posted by jaapv
I will be happy to be corrected if I am wrong,it is becoming a bit abtruse, but numerical f-stop I think means calculated f-stop, as opposed to the one indicated by the f-scale. So it should be actual lens diameter divided by real (=extended) focal length. The f=extended focal length in that formula afaik.
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FWIW I think so too. At a magnification of 1:1 the "focal length" is doubled and the numerical aperture halved (requiring two stops exposure compensation if you're not using a TTL meter) so at magnification of 1:2 there would be a difference of just over 1 stop: set the aperture ring to f/11 and get an effective aperture of about f/18.