Specifically:
Sigma 18-50mm f/2.8 EX DC Macro (not the previous non-macro version. HSM is only on Nikon fit)
Versus:
Tamron 17-50mm f2.8 SP XR Di II LD VC Lens (not the cheaper one without vibration reduction)
Currently the Sigma is slightly cheaper and offers a macro ability but my question is only about sharpness.
I'm not sure if it matters, but I'm only interested in the Canon mount versions.
Here are some reviews, with data for the Tamron, but not the Sigma: http://www.popphoto.com/Reviews/Lenses/Lens-Test-Sigma-18-50mm-f-2.8-EX-DC-Macro http://www.popphoto.com/tamron/2010/03/lens-test-tamron-sp-17-50mm-f28-xr-di-ii-vc-af
And here with data for the Sigma, but not the Tamron: http://lens-reviews.com/Lenses/Sigma/Sigma-18-50mm-f2.8-EX-DC-Macro.html http://lens-reviews.com/Lenses/Tamron/Tamron-SP-AF17-50mm-F2.8-XR-Di-II-LD-Aspherical-IF.html
...other than those and other reviews you can find with Google, you might want to check Flickr for full size photos you can inspect in their original sizes to compare the lenses. Just make sure you get a large sampling of data since some people are better with sharpening than others and lack of focus, low light, and motion can all make a lens look worse than it is.
Comparing two very similar lenses like the Sigma 18-50mm f/2.8 EX DC Macro and the Tamron 17-50mm f2.8 SP XR Di II LD VC Lens in terms of sharpness can be difficult.
Often one lens will perform better at a particular focal length and aperture, while the other will perform better at other focal length and aperture combinations.
Even at the same focal length and aperture, one may be sharper at the center but not as sharp at the edges as the other lens.
When two lenses this close are considered, copy-to-copy variation for either lens may be more significant than representative examples of both models compared to each other.
As other answers have stated, there doesn't seem to be many places that have reviewed both of these exact models, so even comparative test charts like this one at The-Digital-Picture that uses the earlier non-Macro version of the Sigma aren't definitive. The non-Macro Sigma, by the way seems to be sharper overall than the Tamron, but not by much when the different magnification sizes of the charts are taken into account. (The non-VC Tamron, on the other hand, is clearly sharper than either.)
I did find a straight up comparison at DxO Mark. When I first selected each lens, the default camera body was the 7D for both. I was then able to select each lens as tested on a Canon 50D.
At the widest aperture of f/2.8 both lenses are very similar at the widest focal lengths. From 35mm to 50mm the Tamron is slightly sharper in the center but not quite as sharp on the edges. Center sharpness is very close for both at f/5.6 across the focal length range. At the edges at f/5.6 the Tamron is barely sharper at the widest focal lengths but is progressively sharper on the edges from 35mm to 50mm. At f/11 it is pretty much a dead heat from 17/18mm to 50mm. The Tamron measures an almost undetectable amount sharper at 24mm and 35mm.
Overall, the Tamron tested a little sharper than the Sigma on the 50D. In the real world I think copy to copy variation could well be greater than the differences measured by DxO Labs. When both were tested on the 7D, the Tamron clearly did better at f/5.6, but the Sigma compared better at wider focal lengths and apertures than when tested on the 50D. The Tamron also tested slightly better in terms of CA and Vignetting, but also had slightly more pincushion distortion at 17mm than the Sigma at 18mm.
The significant differences between the two lenses
I've not used the Sigma, but I own the Tamron 17-50 VC. It really is quite sharp, just as sharp as the than the 50mm 1.8 prime, comparing both at f2.8, in my experience
Also, when you're using slower shutter speeds, the VC does help prevent motion blur from camera shake.
As always, it's a lot sharper at f4 than f2.8, but all lenses are.
I have the Sigma 18-50 f2.8 (non-HSM, non-MACRO) Made in Japan and the Tamron 17-50 f2.8 (non-VC) Made in China, both copies are for Pentax K-mount.
I would have to warn you about Tamron. The QC is very inconsistent and it really depends on whether you're lucky and get a sharp copy, or like me, who has a back-focusing copy with inconsistent edge distortion. I've sent out the lens to Japan for calibration, hoping it will arrive back sharp and well calibrated.
It is hard to talk about Tamron as there are currently 3 kinds of this lens: Made in Japan Assembled in China Made in China
If you're comparing a properly calibrated Tamron (which I had several years ago), performing the way it should to Sigma it will: -Tamron will focus slower then Sigma -Tamaron will be superior in producing less CA -Tamaron will be sharper wide open -I noticed that Sigma produces a very very slight warm tint which is pleasant in portrait photography.
However, unless you're able to get hands on a Made in Japan or Assembled in China (with Japanese parts) lens that will be properly calibrated, go for the Sigma.
That is strictly my point of view and coming from my previous experience over the last 4 years.
I have not used either, but this website has quite a bit of data, you might try your comparisons there: Tamron sigma
I saw a new lens comparison website a month ago and it looked promising, but for the life of me, I can't find it now. I hope someone can post a link to it.
