Sigma specialises in lenses that have a wide range of focal lengths and despite being a 7x zoom this is actually on the modest side by the manufacturer’s standards.
All three lenses feature Sigma’s own Optical Stabiliser (OS) technology, which is claimed to allow the use of exposures up to four times longer than would normally be possible before blurring becomes apparent. The 18-125mm zoom also features two hybrid aspherical elements, a third moulded glass aspherical element and a fourth element that uses super-low dispersion glass.
This variety of exotic optics should produce some impressive MTF results but that is not uniformly true. Although at least one aperture exceeds 0.3 cycles-per-pixel at every focal length tested, and wide-open performance is especially good at shorter focal lengths, the same is not true for longer focal lengths. There is also some chromatic aberration at both ends of the zoom range but much less in between.
Sigma makes much of this zoom’s close-up capability, which provides a generous maximum reproduction ratio of 0.38x. By coincidence, the same digits raise a curious anomaly in that Sigma has specified the lens with an f/3.8 maximum aperture but the Canon EOS 40D used for testing never recorded anything wider than f/4.
The AF/M and OS on/off sliders are both located too close to the camera body to be reached comfortably. This has the advantage of leaving more room for generous zoom and focusing rings but works against the fact that it is necessary to move the AF/M slider to manual focusing when manual intervention is required. Predictably, this also means that the manual-focusing ring rotates in AF mode and care must be taken not to obstruct its movement.
Overall this is a useful lens that justifies its weaknesses with an on-the-street price that is at least £100 less than other camera manufacturers’ comparable lenses.
There is a bit too much scatter in the wide-aperture results but the figures overall are good down to f/16.