[photo-3d] reverse rings / macro question

[boris@xxxxxxxxxxxx]

Bob:

I've been looking for a prime lens to do quality macros with my cameras
(Pentax bayonet system).  I got rid of those 40-80zoom macros lenses.  I
didn't get enough magnification out of them, and I think the image quality
was not as good as needed.

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?

I want the macro capability for jewelry/product photography.  Sharpness at
high f-numbers (small apertures) is important.

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!

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?)

Thanks for any advice

Boris



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


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

Boris Starosta                        boris@xxxxxxxxxxxx
Stereoscopic Art & 3-D Photography    http://www.starosta.com
usa - 804 979 3930                    http://www.starosta.com/3dshowcase