Shel Belinkoff writes:
>OK, let's look at it from this perspective. First, I don't
>have a 50mm macro that I can use for this project. I do have
>the A100/2.8 and the A*200/4.0 macro lenses.
>The item I want to photograph is about 11mm in diameter. I
>would like to fill as much of the frame with it as possible,
>i.e. about 2X magnification. I haven't been able to do that
>with just the lens alone or the lens with 68mm worth of tubes,
>that's why thoughts of greater extension are running through
>my mind.
Shel,
Here's my calculation, FWIW.... To the extent that simple optics formulas
apply (I think they do), your setup with the A100/2.8 will require an
effective distance from lens to object of 150 mm, and an effective distance
from lens to film plane of 300 mm. If you're using a macro lens, then it's
probably already pretty well-corrected for spherical aberration, so the
corners/edges shouldn't be too far out of focus (assuming your object is
flat enough to be within the depth of field). I don't know off-hand what
formulas to use for depth-of-field calculations. That 300-mm extension will
probably form a pretty serious field stop, however, so you'll lose a LOT of
light. The case with the A*200 is worse, since the distances are doubled.
I'd be interested to hear if your experimentally determined measurements
come anywhere near these simple calculations. I like that quick-and-dirty
idea someone suggested with the toilet paper tube. You could probably get a
quick idea with a really bright object (e.g., lens held about six inches
from a tungsten lamp filament, and imaged at 2X magnification onto a piece
of wax paper held about a foot behind the lens). I can't wait to see your
February PUG submission!
Bill Peifer
Rochester, NY
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