For The Non-Conformist
Sigma's 18-50mm f/2.8 EX DC is a wide, constant aperture autofocus zoom lens for use only with digital SLR cameras using APS-C sized sensors. The zoom range is ideal for covering events, going from wide enough to cover a scene or a table of guests, to a 'mild' telephoto good enough for portraits, so it is currently my most used lens. In the studio, it can find use for full-body portraits and groups, and its image quality holds up to scrutiny.
Canon and Nikon both make excellent 17-55mm f/2.8 lenses, but Sigma's lens is a viable, if unusual, alternative, sized compact enough to be similar in dimensions to a kit lens, yet having the image quality and large aperture one would expect of a bigger lens.
The Sigma 18-50/2.8 is an EX lens, which denotes high-quality construction. The lens feels solid and the weight is considerable for its size, making it feel packed, and it is, with fast glass. The special Sigma EX 'crinkle' finish is a love-it-or-hate-it look but has the advantage of being not easy to scratch. It has a 67mm filter thread. The lens mount is metal.
For the price, Sigma provides you with a nice padded EX Sigma bag, a sturdy petal bayonet lens hood that can be reversed on the lens for storage, a nice rear lens cap and a not-so-nice front lens cap. I prefer the latest Nikon front lens caps because they are easier to remove even with a lens hood attached.
The Sigma 18-50/2.8 has a large zoom ring. On my copy it was at first stiff but with use the action becomes smoother. It extends as you zoom from wide to tele. There is no sign of lens creep after 6 months of use, but there is a locking mechanism anyway that keeps the lens locked at its widest setting.
The focus ring is smaller but wide enough to be easily used, and the action is smooth. The distance scale is printed on. The lens operates by internal focusing so the front element does not rotate as it focuses, making it easy to use a circular polarizer or graduated ND filter with this lens.
Both rings rotate anticlockwise, which is familiar for Canon users but catches out Nikon users.
The Nikon version does not have a switch for Manual to Automatic focusing modes, though the Canon and other versions have it. It does not have a full-time autofocus override mode as with Sigma DF (Dual Focus) or HSM, Nikon AF-S or Canon USM.
AF speed, since it does not have micromotors, is dependent on the camera body being used, but it is fast enough on a Nikon D50. AF action is unfortunately also noisy being without the benefit of Silent Wave Motor technology.
The lens focuses very close, closer than the Nikon 18-70mm (kit lens), but it is not a macro lens.
The lens does not have an aperture ring so will be incompatible with some cameras.
The Sigma is sharp through the zoom range, especially in the center area. The sharpness looks good at f/2.8 and excellent at f/4 and smaller. Light falloff shows up at the large apertures but is less noticeable past f/4.
At 18mm to around 35mm there is noticeable barrel distortion that may be undesirable for some photos with straight lines. This can be corrected in postprocessing. PTLens has a profile for this lens for most cameras.
The quality of out of focus area ("bokeh") is not especially remarkable, and is not helped by a 7-bladed aperture that shows out of focus point lights as geometric shapes rather than solid circles. Still, the rendering is not that objectionable.
As with any large aperture lens, it is not a good idea to shoot at maximum aperture at a high contrast subject (such as a window or doorway letting light in, because chromatic aberration or purple fringing shows up at f/2.8, but on this lens it easily goes away as you stop down. If you must shoot at f/2.8 in such conditions, the fringing can also be corrected in software.
Lens Mount Compatibility
Sigma has versions of this lens for easily all the DSLR lens mounts, including the major ones such as Nikon F and Canon EF-S (and recently Olympus FourThirds), and also Konica Minolta G and Pentax K, and of course the Sigma mount.
The Sigma 18-50mm f/2.8 is by no means the best lens in its class. That distinction in the Nikon world still goes to the Nikon 17-55 f/2.8 DX. But factor in the costs and the Sigma delivers value for the price-sensitive. On its own, it is a great lens to have as a compact yet fast and durable all-around travel lens.
For more concise technical reviews, check out:
Photozone reviews the Sigma 18-50mm f/2.8
ePhotoZine reviews the Sigma 18-50mm f/2.8
[ Sample photos were shot with the gear being reviewed. Photos of the product were shot with a Sony V3. ]
For The Non-Conformist