In article <311B7E60.6692@med-ph.uni-sb.de>, peper@med-ph.uni-sb.de says... (quoted posts deleted)

>For professional macro photography there is only one advice: buy
>a Macro lens, or else use an enlarging lens fitted to tubes or
>a bellows system. These lenses are especially designed for this
>job, and result in sharp images. They are far superior to any other
>lens designed for a different job. The optical requirements for a
>macro lens are much harsher than with normal lenses.
>
>If you use an infinity lens (your normal equipment) together
>with extension tubes, you move your lens out of the proper range
>it was designed for. You certainly add a lot of abberations, although
>no new glass is in the path.
>
>If you use additional close up lenses, then both elements work in
>their designed range, both from infinity to focal length.
>However, the close up lens has exactly to do the optical job as
>the high quality camera lens, and you never would consider
>the close up itself a good camera lens alone. So why consider a two >lens element (close up) sufficient for transforming focal length
>to infinity, and the second part (infinity to film plane) is done by a
>6 element lens? You may use a second high quality camera lens in retro
>position in front of the normal. This is optically best, but you get
>vignettation.
>
>For the occasional macro use a close up lens is ok, and better than
>the normal camera lens with tubes (unless you have a SYMMAR etc, with
BN:---The f1.8-2 normals are quite good on short tubes.
>symmetric design). Don't use the macro in zoom lenses.
>For professional work, use the professional macro lenses. The brand is
>not important.

I would generally agree with the above, but, since I was doing a large
bug-shoot a couple of years ago (and being an incurable lens fanatic),
I tried many combinations to get into the macro range and found several
notable exceptions to the above. While macro lenses did generally
perform the best (and extension tubes and single-element close-up
lenses added to standard lenses performed relatively worse), the Sigma
achromat (it comes with the 90mm macro, but can be purchased from the importer separately, if you ask nicely...;-) added to some lenses
(Nikkor 50mm E, 85mmf2, and 200mmf4) gave results when stopped down
that approached the macro lens performance (adding converters produced higher magnification with little loss in quality). In the case of the 200mm f4 non-macro Nikkor (compact AI version), I was able to get performance that was superior to the macro lenses I tried (Nikkor 55mm, 60mm, and 105mm, Vivitar 90mm Series I, and Sigma 90mm [all excellent]) when the Sigma achromat and a Nikkor 1.4x, 2x, or PN-11 long tube were combined for about 3x magnification. Reversing a 50mm E on the 85mm
and 200mm lenses did not work as well as the Sigma achromat. Adding
converters and/or tubes and the Sigma or Nikkor 6T achromats to the macro lenses generally worked well. Adding achromats and/or converters to normal-to-short tele lenses worked reasonably well. Adding tubes to
some short teles produced very poor results. Enlarging lenses work
well, but they are often off-color. Reversed lenses were O.K., but disappointing. Adding achromats or tubes to zoom lenses, even very
good ones, did not generally produce first-class performance
(especially at frame corners). Any combination that caused the loss
of the lens auto-diaphram feature was awkward to use.
The best combinations for 1:1 and greater magnifications (all f11-16): -- 60mm Nikkor (+ TC14A/TC200)
-- Viv. Ser. I 90mm + 1:1 optics (+ TC14A/TC14B-C + tube/TC200 - with
or without Nikkor 6T added to any combination)
-- 200mm f4 non-macro AI-AIS Nikkor + Sigma achromat (+ TC14A/TC14C + tube/TC200/PN-11 tube)
Hope This Helps