Review by Klaus Schroiff and Markus Stamm, published May 2013

Introduction

That alone may not be all THAT exciting - we've seen many fast standard zoom lenses by Sigma come and go - but this one is a bit more special. Within the recent years, Sigma announced a couple of lenses featuring their new FLD glass and the new 17-50mm f/2.8 OS is among them. FLD glass has an optical characteristic similar to fluorite glass. Such glass elements can be used to compensate optical aberrations (defects) more efficiently than conventional "special" elements such as Sigma's more commonly used SLD glass. We were already very impressed by the performance of the Sigma AF 8-16mm f/4.5-5.6 DC HSM which also benefits from FLD elements so there's some well founded hope that the AF 17-50mm f/2.8 EX DC HSM OS stands out from the ordinary as well.

So, let's have a look at how the lens performs on our current DX test camera, the Nikon D7000.

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.

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 single-shot AF mode. Still, thanks to HSM the lens is fully compatible with all Nikon DSLRs, including the entry-level models.

Both the zoom and the focus ring work in "Canon" style, which is the exact opposite of how these rings work on Nikkor lenses. If you're used to the Nikon way of zooming it may take a while to feel comfortable with the Sigma. And if you try to focus manually, you'll have to remember that the focus assist in the viewfinder assumes a Nikon lens and consequently suggest the wrong direction of focus ring movement. However, 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.

An important 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.