Tamron SP AF17-50mm F/2.8 XR Di-II LD Aspherical [IF] Review
Image Quality - 17mm Corner
The following images are 100% crops from the top left corner of an image shot using and EOS 20D. They have not been processed in any way (other than cropping).
The Tamron SP AF17-50mm F/2.8 shows very little chromatic aberration (note that latteral CA shouldn't really depend on aperture) whereas there is distinct green/purple fringing with the Canon EF-S 17-85. Even there it's not excessive, but less is better! The Tamron is softer wide open at f2.8 than stopped down a stop to f4 and gets even better when stopped down to f5.6. I'd rate this as good performance, but as you might anticipate, stopping down to at least f5.6 would be desirable if you're looking for maximum corner to corner sharpness.
Image Quality - 17mm Center
Below are shots of the same tree branches, but this time placed in the center of the frame:
As you'd expect, center sharpness and contrast is higher than that found in the corners of the image. Again performance is good wide open, but gets better when stopped down to f4 and even better when stopped down to f5.6. Shots from the Canon EF-S 17-85 aren't bad, but they're not as good as those from the Tamron lens.
Image Quality - 50mm Center
The following images are 100% crops of the image shown above. The area in the red box shows the approximate crop position They are exactly as they came out the the camera (EOS 20D). Again the Tamron SP AF 17-50/2.8.was compared with the Canon EF-S 17-85/4-5.6 IS.
All of the images show very similar sharpness. The one shot with the Tamron SP AF 17-50/2.8 wide open at f2.8 maybe has a touch less contrast and may be very slightly less sharp, but remember these are 100% crops and on most monitors will be the equivalent of sections of a print that's something like 24" x 36". Good performance from both lenses.
I also looked at how well the Tamron SP AF 17-50/2.8 performed with a Tamron 1.4x TC (Tamron 1.4x C-AF MC4) added. Note that this is an older Tamron 4-element 1.4x TC bought about 10 years ago so it's possible newer models may have different (better?) performance. AF was still very good with the TC attached and all images were shot using AF for focusing.
I think it's clear here that wide open with the Tamron 1.4x TC the image is lower in contrast and not quite as sharp as the image stopped down 1 stop. The Canon EF-S 17-85/4-5.6IS was set to 70mm and shot wide open (which is f5.6). I think it's better than either shot with a TC, but that's not really surprising. Again these are 100% crops and so represent a section of a large print. The Tamron SP AF 17-50/2.8 yields pretty good images at 50mm with a 1.4x especially if stopped down a stop. It's certainly a very usable combination and one which yields better results than upsizing the image shot without a TC. The use of a TC could be useful to bridge the gap between the 17-50/2.8 and a longer zoom like a 70-200/2.8.
Image Quality - 50mm Corner
The following images are 100% crops from the top left corner of an image shot with the EOS 20D. Care was taken to make sure that focus was kept on the target (i.e.I didn't focus in the center and then recompose!).
There's really not a huge difference between corner and center performance at 50mm, which is good. Again the Tamron does pick up a little contrast when stopped down, but even wide open it's pretty good. It's clear here that the Tamron lens is slightly sharper the the Canon in the corner of the frame.
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