I figured someone would "best" me somehow. :)
At first I thought this would be a very minor correction. Then I thought of
the 50mm f/3.5 macro. When that helical is racked all
the way out for close focusing the lens extension (LE) would be well worth
correcting for with slide film.
I guess if you have internal focussing, like the AF camp usually does, LE is
essentially zero?
Lama
From: "John A. Lind" If
> [the lens is] focused closer than [infinity], the lens is extended further by
> its internal
> helical, in addition to the extension tube, and that extension must be
> added as well. The equation I use is similar, but also takes into account
> extension of the lens with its own focusing helical, FE:
>
> FF = (FL + LE + E)/FL
FF = f-stop multiplier factor
FL = focal length (nominal, when at infinity)
E = Extension tube length in same units as LE and the focal length
LE = lens extension
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