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basicly you can use any Sigma lens on hand.
The 18-200 works good at the long end (i.e. 70-200)
I got the best results with the Sigma 150mm Macro so far.
It also depends on what you are going for,if you are at home and can arange
your set-up, the distance does not matter you can go close and use shorter
If you are outside and go for insects, you may stay a bit away,and therefor
a longer lense gives you more distance.
There is no "best lense" for all (macro)purposes.
If you plan serious macrowork,you should look for a 105 or 150 Sigma macro lense in the long term.
just do use a manual bellows! I did so before I made my own automatic bellows.
This one was done with an old M42 Bellows and a 35 y.o. PORST 70-260mm M42-Zoom. The only problem might be, to focus with the lens slowed down.
The comparatively dark vewfinder has never annoyed me all that much.
The picture was taken with my SD9 freehanded using a flashlight.