The Sigma 18-50mm f/2.8 EX DC Lens contains a nice general purpose focal length range, is well-built and does not cost a fortune. Like Canon’s EF-S Lenses, the 18-50 DC is compatible only with 1.5x/1.6x crop factor digital SLR cameras. Being sized for a smaller-format sensor allows a smaller and less expensive design to be used. Here’s the review summary from TheDigitalPicture:

“For many, Canon’s biggest advantage is the 3-stop image stabilizer that the Sigma 18-50mm f/2.8 EX DC Lens lacks. If your subject is not moving (or you are), this is a really nice feature. The Canon also features fast, silent and accurate USM (Ultrasonic Motor) focusing with FTM focusing - also a big advantage. Price is a huge difference between these two lenses. I personally think the Canon is worth the price difference over the Sigma 18-50mm f/2.8 EX DC Lens, but if this lens is not within your budget, keep reading …

The Tamron 17-50mm f/2.8 XR Di II Lens is very, very similar to the Sigma 18-50mm f/2.8 EX DC Lens. These lenses are more similar than different. They are very similarly sized and shaped. They function very similarly.

The differences? The Sigma flares more easily. The Sigma shows more CA. The Tamron is noticeably wider (17mm vs. 18mm). The Tamron focuses faster but the Sigma focuses more quietly. The Tamron focuses much more accurately - My keeper rate is far higher with the Tamron. My Sigma has a weak left side - For sharpness, the Tamron beats the Sigma easily on the left side, but the Sigma is slightly better on the right. The Sigma rings rotate the Canon direction, the Tamron rings rotate the opposite direction but with less effort. The Sigma is less expensive (but not by much). The Sigma’s requires its lens hood to be removed to install/remove the lens cap. My choice would definitely be the Tamron 17-50mm f/2.8 XR Di II Lens over the Sigma.

For the price, the Sigma 18-50mm f/2.8 EX DC Lens is a good value. It is definitely an upgrade from the Canon EF-S 18-55mm f/3.5-5.6 Kit Lens.”

Website : Sigma 18-50mm f/2.8 EX DC Lens Review from TheDigitalPicture