Quote:
Originally Posted by giordano
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.
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In the formula I gave above
f is the focal length, not the extension, and
N is the numerical f-stop. So the formula takes care of the problems related to extension. It depends only on geometric reasoning. There are in fact minor deviations depending on the type of lens. So two lenses of 50mm focal length (ceteris paribus) can have slightly different DoF, but without interest for practical purposes.