Review by Klaus Schroiff, published July 2010

Introduction

The build quality of the Sigma lens is pretty good although it stays a little short of the high expectations that we associate with a Sigma EX ("Excellence") lens. It's made of tightly assembled combination of metal and high quality parts so there's nothing wrong here. Like its predecessor (AF 18-50mm f/2.8 HSM DC EX macro) it uses a mono-cam system (one inner lens tube) to extend the lens when zooming towards the tele setting. The front element does not rotate The zoom ring has a smooth, pleasant action whereas the focus ring has basically no friction in manual mode. The focus ring is coupled to the AF mechanism so it rotates during autofocus - this is somewhat disappointing for a high end product actually.

The 17-50mm f/2.8 EX DC HSM OS features the slightly simplified version of Sigma's HSM ("hypersonic AF motor"). It is still fast and nearly silent but full-time manual (FTM) override is not possible in one-shot AF mode. The AF speed via in contrast AF (Live-View) is on the slow side. Manual focusing is delicate due to the extremely short focus path - it just takes a turn of ~30 degrees to focus from infinity to the close focus limit. This approach improves the AF speed for sure but you need to have some haptic skills if you want to do it all manual. The focus accuracy of the tested sample was fine on our test DSLR.

A major new feature is, of course, the OS ("Optical Stabilizer"). As already reported in previous reviews it is impressively efficient and as good as the corresponding genuine manufacturer implementations. Sigma claims a gain of 4-stops which seems to fit here based on our field impressions.

Finally here's a comparison of the "gang of three" fast standard zoom lenses with image stabilization support: