Available for Sigma, Canon, Nikon, Sony and Pentax DSLR mounts, the Sigma 17-50mm is a fairly substantial lens that weighs in at 569 grams without any lens caps attached. The ‘EX’ abbreviation in its full name means it belongs to Sigma’s range of professional-grade lenses. As such it gets the signature gold band and gold lettering common to all lenses with the EX denomination, along with a classy matt finish on the lens barrel. Picking it up for the first time, it undoubtedly feels like a quality optic.
While the outer casing, lens barrel, zoom and focus rings are all constructed from plastic, it's toughened and feels solid enough to withstand a few gentle knocks and scrapes. You wouldn’t want to knock it with any great force though, and you certainly wouldn’t want to drop it either. If you don’t have a compartment in your camera bag that it’ll fit snugly into, Sigma supplies a padded case to keep it safe when not in use. There’s even a locking switch on the lens barrel that can be used to stop the lens from extending beyond 17mm when in storage. It isn’t weather-sealed though, so you’ll need to keep it away from direct contact with water, sand and dirt.
Measuring 87mm at its most compact the lens extends to 113mm when fully extended. Compared to a standard 18-55mm kit zoom this might seem rather large, but measured against other professional-grade lenses of this kind, it's very much par for the course. Internally, the lens is constructed from 17 Elements in 13 Groups, including two “F” Low Dispersion (FLD) glass elements to keep fringing on high-contrast borders to a minimum while boosting contrast and definition.
The front element doesn’t rotate, so attaching circular polarisers and other alignment-sensitive filters such as circular ND Grads isn’t a problem. It’s generally considered good practice to attach a UV filter to the front of a lens, not just to block out haze but also to protect the front element of your valuable new investment should you have an accident. While these generally aren’t all that expensive, the Sigma 17-50mm’s relatively large 72mm lens thread diameter does mean that additional filters (polarisers especially) will cost substantially more.
Cutting through the naming jargon - the lens's ‘EX’ designation signifies pro-grade quality, its ‘DC’ designation marks it out as being specifically for APS-C cameras and ‘HSM’ refers to the Hyper Sonic Motor autofocus mechanism inside the lens. Unlike Sigma’s regular motor-driven autofocus system, HSM has no downward gearing and is powered by ultrasonic waves, making it quicker to start and stop and very quiet during operation. This last characteristic is definitely a plus if you’re using the lens in a sound sensitive environment like a church wedding service.
The final designation is ‘OS’, which stands for Optical Stabilizer and is designed to eliminate the effects of camera shake. Sigma claims its technology offers a four-stop advantage for handheld photography. During testing we found that we were able to consistently achieve acceptably sharp results for stationary subjects at 1/20sec at 17-30mm, and approximately 1/30-1/40sec at 50mm. Of course, these results will differ slightly from person to person, depending on how still they can hold a camera.