Not an easy decision at all. I'm not going to suggest you which one, instead, I'll just say what I think you should know (if you don't know that already).


First of, if this is going to be your only lens, note that 50mm might be a bit short for some situations.


I did not try the Tamron (for Canon) lens but people seem to complain about its AF accuracy. With that said, I did try the Tamron for Nikon (on a Nikon body of course) at the shop (in a pretty dark room I have to say) and the AF seem too slow to me, but I'm not a Nikon user and maybe something was not set correctly so take my previous statement with caution.


Assuming that it is a slow lens (AF wise), then comparing it to the 17-85, the difference is between missing the shot altogether (with the Tamron) or taking a dark/noisy image with the slow (aperture wise) Canon 17-85. Personally, I'd go with a noisy picture rather than missing the shot.


So pros for the Canon:


1. Fast and silent AF


2. More reach - 85mm





Pros for the Tamron:


1. Fast lens - f/2.8


2. Nice bokeh (I guess, <span style="text-decoration: line-through;"]but didn't see this myself. Stupid me LOL... I did see it on clemmb's picture above - very nice I have to say!) thanks to the f/2.8 aperture


3. Better IQ I believe (that's what people say. clemmb's picture above seem great to me although the eyes are not tack sharp IMO, but that could be for a different reason, we can't conclude anything from that regarding the AF accuracy without knowing more)





Note that you can compensate for the slow aperture of the 17-85 using high ISO with your 40D, but you can't compensate for the inaccurate (and perhaps slow too) AF of the Tamron or for its relatively short 50mm at the long end.





Here is a picture I took with my 50D + 17-85:


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And here is another one (I think one of my sharpest so far):


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Ok, last one:








Note that unsharp mask was used for all the above pictures (the Cat had been unshaped for 4 or even 5 times though).


If you want, I can send you the original file of any picture you ask for. I shoot in RAW, but the bird above was taken when I first got the camera and was still shooting JPEG.