Sigma 10-20mm 1:4-5.6 EX DC HSM review

The 10-20mm F4-5.6 EX DC HSM is Sigma's offering in the APS-C ultrawide zoom sector, first announced in February 2005. It was introduced amongst a flurry of announcements as manufacturers raced to provide wideangle zooms for the burgeoning APS-C format DSLR market, with similar offerings also appearing in short measure from the likes of Tamron and Tokina (not to mention all of the major camera companies). It has since become a very popular lens, due in no small part to offering the joint-widest angle of view of all the APS-C ultra-wide zooms, while being available in mounts to fit all of the major DSLR systems. Equally it's not hurt by having a very competitive price; it's notably much cheaper than the camera manufacturers' own offerings in this sector.
The optical formula features 14 elements in 10 groups, including three Super-low Dispersion glass elements for the minimization of chromatic aberrations, and three aspherical elements to reduce distortion and other aberrations. Users of Canon, Nikon, Sigma, and Four Thirds cameras benefit from an ultrasonic-type 'HyperSonic Motor' (HSM) for fast and silent autofocus, while the Pentax and Sony mount versions use the 'screw-drive' coupling from the camera body. The 'DC' designation indicates that this lens is designed for DSLRs with APS-C or smaller sensors, and isn't suitable for use on a full-frame body (the image circle isn't large enough, and vignetting will occur).
Of course the market doesn't stand still, and Tamron (arguably Sigma's biggest direct rival) has recently revised its own offering in this sector with the SP AF 10-24mm F3.5-4.5 Di-II LD Aspherical (IF). In the Four Thirds mount, Olympus has also entered the arena with its very impressive Zuiko Digital ED 9-18mm F4-5.6. And while Sigma has itself recently announced a new constant maximum aperture 10-20mm F3.5 EX DC HSM, the indications are that this will sell alongside the slower model as a higher-priced premium product. So how does the 10-20mm F4-5.6 stand up in the face of its new competitors?
Headline features
Ultra-wideangle zoom (15-30mm equivalent on 1.5x DX / APS-C DSLRs)
F4-5.6 maximum aperture
Available for Canon, Nikon, Sigma and Four Thirds mounts with built-in HSM focusing motor, and Pentax and Sony mounts with focus driven from camera body (non-HSM version)
For reduced frame (APS-C / DX / Four Thirds) format DSLRs only