I believe the internal focusing changes the focal length. That would
make it even more difficult to calculate.
-jeff
----- Original Message -----
From: "Lama-Jim L'Hommedieu" <lamadoo@xxxxxxxx>
>
> 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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