>>I think you'll find that the exposure compensation required will be
the same
>>for the same magnification regardless of the means to achieve it.
...Wayne
Perhaps you are referring to the known relationship for extension :
Effective F-Stop = F-Stop x (1 + Magnification / Pupil Magnification)
Minimal effective light lost with the CU diopter lens, but if one
wishes the same DOF, must stop down to the same effective aperture used
for the same mag when using extension--then I fully agree. So the VF is
indeed brighter if using a CU lens compared to extension. Big Foot for
example does some odd internal machinations with floating elements that
result in 1/2 stop less loss of light than it should at 1:1. John Shaw
said some lenses do this, but I wasn't sure it could really be true but
is seems to be so--perhaps it is mostly FL shortening? as the working
distance at 1:1 with Big Foot is less than it should be for 125mm. The
80mm macro is symmetric with PMF of 1, so that makes it easy. The PMF
even if known gives me headache to think about as it also changes with
focusing distance. Nykon cams will often give the true effective
aperture in the VF with macro lenses--cooool.
Let's save the full DOF thread for quiet winter days, Mike
--
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