> > Effective f# = (m+1)*(marked f#), where m is magnification
> > In stops, it's log((m+1)**2)/log(2)
>
> Yes. Rearranging the 1st equation, you get.
>
> marked f# = (Effective f#) / (m+1)
>
> The effective f# is what the film will experience.
> The marked f# is the lens marking.
Just curious -- where does this come from? Is it based on light loss
bouncing around inside the extension tubes, or less light on the film
because the subject area is smaller, or something else entirely?
-- dan
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