[photo-3d] RE:reverse rings / macro question

["Brown, Fritz - LABS" <brownf@xxxxxxxxxxxxxxxx>]

;A NY mail order house lists a NEW Sigma 50/f2.8 macro lens for $159.-
;
;That seems quite a bit lower than the 250-350 I typically see for such a
;macro lens.  Is Sigma a cheapo off brand, or do you think this might be a
;good value?

The best bang for the buck currwntly is the Vivitar 100 f/3.5 macro.  All
reports from people that are actually using the lens (as opposed to press
releases masquerading as reviews) are that the lens is very sharp.  It runs
in the $120 neighborhood at places like B&H, Adorama, and Exactly.

;ANother question: under the heading macro, I see "reversing rings"
;advertised, for something like $10.  I assume these are for turning a prime
;lens around, for shooting through the lens backwards.  Retrofocus?  As I
;would do macros infrequently, is that a good solution for doing macros?  It
;certainly seems cheaper, sounds good to me!

There are a couple of ways to do macro with a reversed lens.  One is to use
the lens reversed so now your retrofocus design short lens becomes a
telephoto design close focused lens.  The results obviously depend upon the
quality of the lens you start with and some times things such as field
flatness become a problem.  

The $10 reversing rings you mention are designed to reverse a lens on the
end of another lens so it acts basically as a closeup diopter attachment.
Using a lens for this is often better than using an actual diopter lens
because the quality of design of a lens is better than that of the
relatively cheap diopter lens (that isnt meant to disparage the quality of
the high end multielement diopters made by the likes of Canon et al.).  The
big problem you will run into though is vignetting,  particularly at higher
magnifications.  It takes a really fast lens to be a good one for reversing
and you have to stop your main lens way down to get the entrance pupil
smaller than the exit pupil of the reversed lens.  Otherwise vignetting will
occur.  As an example my 50 f/1.4 vignettes when reversed on my 210 f/4 even
when stopped down to f/32.  If you can get away with using the central
portion of the image and discarding the periphery,  then the vignetting
becomes less of a problem.

One more thing on reversing lens on each other.  The on film magnification
is the focal length of the main lens divided by the focal length of the
reversed lens.  In my example of a 50mm reversed on a 210mm, the on film
image is 4.2 times life sized.  Fo the work you mention, 


;Finally, what's the difference between getting a macro in 50mm f.l. as
;opposed to something like 100mm f.l. ?   (For stereo I imagine the 50 to be
;better, as one could get a decent stereobase, without having to toe-in the
;cameras as much, because the camera would be used closer to the subject.
;Are the higher f.l. macros mainly for shooting macro at a distance?)

The longer the focal length,  the more distance you have between the lens
and the subject.  As an example, at 1:1 magnification a 50mm lens will be
100mm from the subject while a 100mm lens will be 200mm from the subject.
That extra distance will be really useful for example when you are trying to
get your lights positioned without casting a shadow from the lens.

;Thanks for any advice

You are welcome

;Boris

-Fritz

;P.S. to all other than bob, this post is 3-d related because I am a 3-d
;photographer!

Keep both eyes open and everything is 3d

;- Science is the part of culture that rubs against the world.
;-
;-                                     Stanislaw Lem, _His Master's Voice_

What demon posessed me that I behaved so well.
                           -H.D. Thoreau