Sigma AF 18-50mm f/2.8 EX DC Aspherical IF - Review / Test Report
Lens Reviews - Canon EOS (APS-C)

Review by Klaus Schroiff, published August 2005


In an age of cheap kit zooms the Sigma AF 18-50mm f/2.8 EX DC Aspherical IF (one of those names ...) is a potentially interesting third-party alternatives and more than that it is the first and only high-speed zoom in its range (at the time of this review). At a price around 400US$/EUR the penalty on your bank account would remain also pretty manageable. It is a DC ("Digital Camera") lens specifically designed for the reduced image circle of today's APS-C DSLRs where the field-of-view resembles a ~28-80mm equivalent on classic full frame cameras. Remember that the depth-of-field remains dependent on the original focal length so in comparison DOF is a little deeper at the same aperture settings. The lens is available in Canon EOS, Nikon, Minolta, Pentax and native Sigma mount.

Despite the large max. aperture and thanks to the DC design the lens is very compact with a size of just 74x84mm and a weight of 445g which is quite a difference compared to full frame, high-speed standard zooms which tend to reside in the 700-1000g league. As such it is a good match for the latest miniature DSLRs. The lens comes with a petal-type lens hood as well as a soft bag. The optical construction is made of 15 elements in 13 groups with 2 aspherical and one SLD (Special Low Dispersion) element. The aperture mechanism features 7 aperture blades. With a min. focus distance of 0.28m the max. magnification is 1:5 at 50mm. The lens extends during zooming (see below) reaching its max. length at 50mm. The filter size is 67mm.

For whatever reason Sigma did not release any HSM standard zoom to date (a licensing issue ?) and the 18-50mm f/2.8EX follows that tradition with a classic AF micro motor. Speed-wise there's no need for an ultrasonic drive whatsoever - due to a very short focus path (~45 degrees) the AF performance is very fast. On the downside this design results in rather delicate manual focusing. Manual focusing is not possible during AF mode so you've to use a AF-MF switch for this purpose. Thanks to internal focusing (IF) the front element does not rotate during focusing. Unfortunately the focus ring does rotate in AF mode which isn't really something to be proud of these days. Typical for most micro-motor drives focusing generates some noise but the level remains acceptable.

Similar to other EX ("Excellence") lenses the build quality is very decent but a little worse compared to the more pricey EX lenses such as the 50-500EX. The lens features the EX crinkle finish and broad rubberized control rings. The focusing action is pretty smooth whereas zooming is a little on the stiff side (typical for EX zooms). The zoom mechanism is very tight without any wobbling of the inner lens tube. We haven't experienced any problems with zoom creeping during the field tests but Sigma has provided a switch to lock the lens at its 18mm setting (why only there ?).

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