I Compared: Tamron 24-70 f2.8 VC versus Canon 24-70 f2.8 versus Canon 24-105 f4 IS

I'm a Nikon shooter, not a Canon shooter -- but I think you ranked the Tamron way too close to the Canon 24-70L in terms of corner sharpness. Across the board, it was as weak as the EF24-105, in corners.

In center sharpness -- it was very close the Canon 24-70L, which is true. (Especially considering that the Canon 24-70L appears to have much more chromatic aberration -- if you look closely between the fences, there is a lot of color-shift on the Canon 24-70L, whereas the Tamron has much less).

I think people who want a 24-105 know who they are -- thats when you need reach. 70mm is a terrible length to end at, but the price for sharpness. I wish there was a supersharp 20-135, but there just isn't one. So if you need reach, you need the 24-105 IS.

That being said, Tamron is not a corners sharpness lens, its an optically stabilized f/2.8 lens, and would be a wonderful alternative to the Canon or the Nikon 24-70 f/2.8s. I saw reviews which indicate that the IS/VR on the Tamron is extremely good -- and one of the best for videos, whereas the non-IS/VR lenses are incredibly jumpy So, that's something to consider as well, if you are into video.

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Sincerely, GlobalGuyUSA