> usher99@xxxxxxx wrote:
>
> Must measure with Big Foot at 1:1 as it uses a combo of
> standard
> extension and floating elements--down side of straight extension is
> loss of light of course.
> Mike
>
>From my understanding the light loss is directly related to
magnification, so
>whether that 1:1 is achived by changing focal length, lens extension
or a bit
>of each is irrelevant.
...Wayne
Wayne Harridge
Well it is a good point that the no free lunch principle applies--more
mag means more light loss and a smaller fraction of the total image
circle is used.
The inverse square law should apply.
The exposure correction factor should be able to be calculated from
(M/P+1)^2 where M is the magnification and P is the lens pupillary
magnification --I think about 1 for a standard lens).
For IF lenses things are complicated. . These lenses focus by moving
the rear node away from the film/sensor this results in some shortening
of the focal length and a change in pupillary magnification results.
This should lead to slightly less light loss than a linearly focusing
lens. I believe this to be true and have read it from several sources.
It seems it should be possible to compensate
in front to keep the FL/working distance the same, but I have no clue
whether such a lens has been made like that.
I can tell you the VF is much brighter at 1:1 with Big Foot than the
viv S1 90mm F2.5 with matched TC at 1:1.
I would defer to Dr. Focus or Dr. No-Flash on such matters as either
have forgotten more about such things than I have learned.
Mike
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