Hi BF:
a little late to this thread, but in case you are still out there in cyberland and interested in this glass, here are a few brief observations.
first, as you have mentioned, there are quite a number of reviews on this lens on the web. most are quite complimentary, but as you have mentioned, not much specifically relating to bird photography. However if you surf the DP review. com lens forums you will see some shooters who have posted bird pics.
as far as reviews are concerned, if you havent seen this one, the short comment on "competitive with any 70-300 on the market" or words to that effect, is telling:
http://www.lensrentals.com/rent/canon/lenses/telephoto/tamron-70-300-f4-5.6-di-vc-usd-for-canon
since these folks are a lens and camera rental outfit, personally i give their opinions a lot of weight.
i have this tammie coupled to a Canon T2i, and frankly I was very, very well pleased w/ what I saw right out of the gate. "Amazed" might be a more appropriate term considering the price. Very sharp wide open and all the way out, and on the Canon 18mp sensor quite a room for heavy crops of the wee feathered beasties like Kinglets and Gnatcatchers, maybe up to 1/2 a frame or so before the image starts to show some softness. Really finely etched contour feather details along the body of these small birds. I compared it to a Canon 300 f/4 IS L I had on hand, not quite an A/B resolution chart test, but same birds, same day and while the L lens was a sharper, certainly not 3X the price sharper, as the cost might indicate. I found in 5X7 up to 8X10 inch, which is about as big as I care to print, the quality of the image was a wash betweem the two. The IS is rock solid, no question less drift of the image compared to the Canon, which is of course c. late '90's technology. There was a big difference in autofocus speed also, as the Tamron was much snappier to lock focus. The only real softness was noticed when doing insane crops of birds at large distances, where the L lens (and the Sony G, see below) held together better, but this seems a fair compromise in this level of glass.
I wound up returning the L lens, which may have had some mechanical defects that resulted in an inordinate amount of color fringing (decentered elements?). It is possible my copy was bad.
I recently took both this lens and a Sony 70-400 G lens to Costa Rica, and took a dump truck load of pics, mostly birds. Again, the G lens won out in terms of absolute bleeding edge retrieval of feather-level detail. But after all the prints were done, looking at the output of these two lenses boht on a monitor and on paper, if it was not for the extra reach of 400mm I do believe I would have been happy w/ the results from the Tammie alone. It certainly is easier to tote.
A big two thumbs up from me!
regards,
UTC
