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Review by Markus Stamm, published August 2011
Introduction
The market segment of entry level kit lenses is no doubt owned and well populated by the original manufacturers. For almost any system you can get a low cost lens that offers a large enough zoom range to cover most everyday needs, in case of Nikon and Canon even combined with optical stabilization. The build quality is far from stellar, but these lenses often perform on a suprisingly high level, at least regarding sharpness.
Any third-party manufacturer trying to compete in this segment will have a hard time to undercut the low prices and still offer a usable product. Sigma tries it with better specs instead: the 18-50 DC OS is a bit less than a stop faster throughout the zoom range than Nikon's kit lenses, but also retails at a somewhat higher price of around 180 EUR/200 USD at the time of this review.
Anyone familiar with the Nikon 18-55mm DX lenses will immediately notice two additional obvious differences: the Sigma lens is quite a bit larger and substantially better built. The lens does not change its physical length during zooming and focusing, which is a rare combination of features for an entry-level lens. Unfortunately the focus ring rotates during AF operation, which stands a bit in contrast to the otherwise very good construction. On a modern lens with OS and HSM, this approach feels a little outdated.
Despite the HSM drive, FTM (full time manual override) is not available and the lens needs to be switched to MF to focus manually. The focus ring is not damped and the focus path is very short, making exact manual focus a bit of a challenge. In addition, the focus ring works in "Canon" style, which is the exact opposite of how these rings work on Nikkor lenses. However, for most buyers manual focus is certainly not high on the priority list anyway.
The large rubberized zoom ring is well damped. It rotates the same way as on Nikon zoom lenses (twist clockwise to zoom in).
The optical construction is surprisingly complex for a lens in this class, featuring no less than 16 elements, including 3 aspherical and 2 SLD elements. Thanks to an inner focusing (IF) design the front lens does not rotate, so using a polarizer remains easily possible.
Thanks to its HSM drive the AF of the Sigma is very silent. The AF speed is ok in general terms, but certainly better than on many other offerings in this segment.
The lens features Sigma's optical stabilization system (OS). In our field tests up to three stops longer shutter times were possible in the field (given a steady subject). Actual results will vary depending on the photographer, of course.
The Sigma is a G-type lens and does not have a dedicated aperture ring.
| Specifications |
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| Equiv. focal length | 27-75 mm (full format equivalent) |
| Equiv. aperture | f/4.2-f/6.8 (full format equivalent, in terms of depth-of-field) |
| Optical construction | 16 elements in 12 groups inc. 3 aspherical and 2 SLD elements |
| Number of aperture blades | 7 (rounded) |
| min. focus distance | 0.30 m (max. magnification ratio 1:4.1) |
| Dimensions | 74 x 88.6 mm |
| Weight | 395 g |
| Filter size | 67 mm (non-rotating) |
| Hood | petal-shaped, bayonet mount (supplied) |
| Other features | Optical stabilization (OS), Hypersonic AF drive |
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