Hi--

>A recent thread debated macro techniques. It seems to have died about the
>time I
>responded to a statement by one of the participants (Mac) that using
>extension tubes
>would result in light loss, but close up filters would not. When I
>challenged for an
>explanation, he retreated and the subject ended.
>
>In my challenge, I stated my understanding that any light loss with tubes
>had to be
>due to the magnified image placing less light onto the film plane. If the
>close up
>filter could provide equal magnification, then the light loss should be
>identical.
>
>I've never tested this hypothesis, but could you comment on my reasoning and
>fill in
>the gaps?
Jack Daynes <"jackd1"@san.rr.com>

The effective stop is the distance from the film to the lens center,
regardless of focus distance. If you take a 50mm f2 lens (50/2=25mm
aperture diameter) and add 50mm extension (for 11), the effective
stop is 50+50=100 100/25=f4, or two stops loss... If you take the
same 50mm f2 lens, and reach 11 without refocusing it, but by changing
its focal length (shortening it, with a close-up lens added) enough so
that 50mm from the film to the lens center focuses it at 11, it is still
50/25=f2 (it would be f1, if refocused to infinity by shortening the
film to lens center to 25mm...), so there is no light lost (but you can
guess that it may not be any too sharp wide open...;-).