I described my experience with Canon 200mm F2.8 FD lens previously in another thread..
http://www.iceinspace.com.au/forum/s...ad.php?t=51866
It is sort of quite good at full aperture, excellent at F5.6 - BUT with external diaphragm (with internal iris it shows significant lateral CA, while with external one - just before front element - it is almost non-existent).
I paid for it $250 on ebay.
Now, I also did the same experiment with Tamron 200mm F2.8.
It behaves just slightly worse than Canon in terms of lateral CA (almost nothing, and I am talking about pixel-peeping at the corners of the frame).. but this was at a price of $60 at ebay (and my fiddling with mechanical modification).
I will do something similar with yet another 200mm lens soon (Sun.. is it Pentax?) that I obtain on trash market for $25. I expect the same results as with Tamron.
The crops are sized 200%, directly from DPP, from lower left corner of the frame, they are attached below (this is where the resolution of Canon FD lens is worse, it mildly suffers from astigmatism here while it is visibly better at the opposite corner). First two images were taken with external diaphragm set to f5.6 (the third image was taken with Tamron, internal iris at f5.6. Canon behaves better with its internal iris, but lateral CA was still very visible.)
BTW, I strongly believe this is the only right way to evaluate the lens suitability for astro photography and to compare them.
And if I knew all this above earlier, I would not buy this Canon.. yes it is better (less CA), but not THAT MUCH better.
So here you go...
However, for what I intend to do (photometry and later spectrometry) all this is not that important.
One more thing to note: All those lenses discussed above are designed for full frame sensor (24x36 film)- so smaller sensor of my Canon 400D is not showing what is happening that far away from optical axis (and I do not really want to know this
)