Review of Sigma 17-50mm F2.8 EX DC OS HSM (for Sony Alpha mount)
Overall the Sigma 17-50mm F2.8 EX DC OS HSM is a very good lens, especially as a kit lens replacement. This Sigma 17-50mm has a constant aperture of f/2.8, whereas kit lenses drop to f/5.6 at 50mm. Sigma have given this lens an EX designation, which means the company considers this a superior product. DC means this lens was made for APS-C digital sensors, HSM means Hyper Sonic Motor, which quietly focuses the lens. There is no OS (Optical Stabilizer) in this made-for-Sony copy, the camera body does the image stabilisation, and it seems to work adequately with this lens.
Physically this lens is mostly plastic, but it still seems strong. The focus ring is very smooth, but it has a very short throw. The zoom ring is stiff enough that it doesn't need the zoom lock (active at 17mm). It uses 77mm filters, the front element doesn't rotate during focus. Similar lenses from other manufacturers have smaller filter sizes, I guess the designers gave away some of the benefit of a smaller front element in exchange for a better optical package.
The lens hood is excellent, except I wish they put corrugations on the inside of it. It has markings to help align the hood, either forward or reversed. On the other end of the lens, the mount is metal. The lens weights 565g according to Sigma's website.
Sharpness: Wide open images are sharp in the centre and a bit soft in the corners. Lights in the corner of the frame at f/2.8 show a fair bit of coma (shaped like bat wings), but for real world use I don't see this as a problem. The corners are much better at f/4, and f/5.6 is near perfect.
Vignetting: Wide open at 17mm has very strong vignetting (1.5 stops), but ACR fixes it. Correcting barrel distortion removes a lot of the corners too, to the point where vignetting is not much of a problem at 17mm. Vignetting isn't really an issue at other focal lengths.
Bokeh: At 50mm f/2.8 the bokeh is good, bokeh balls fade away nicely. It's hard to judge other settings because the bokeh size is small, especially at 17mm, but the bokeh effect is still pleasing. See the 2nd picture for some bokeh samples.
LCA (Lateral Colour Aberration): The lens displays some red/cyan (or maybe magenta/green) fringing at 17mm and 24mm, very slight CA at 35mm and none at 50mm. Your choice of RAW converter should fix this easily.
Flair: I found a bit of rainbow coloured flair at 17mm, but that was with the sun in the frame so it's unlikely to be an issue.
Operation: You must switch to MF to manually focus this lens. Turn off the lock switch to zoom.
In conclusion the Sigma 17-50mm f/2.8 can provide the performance of professional lenses without the expence, but you have to work around small issues.
Verified purchase: YesCondition: Pre-owned