It doesn't change length at all either for focus or focal length. No filter rotation of course. Comes supplied with a nice compact lens hood and a very sturdy (but not compact) padded case. Mounted on my Canon 40D it still goes easily into a Lowepro snoot bag so is scarcely bigger or heavier than the Canon EF-S17-55mm f2.8 that normally is on the camera. For what I have used it for the wide aperture sharpness and accuracy of AF have been beyond reproach. I don't do bench testing of lenses - if it works faultlessly on the camera thats good enough

The finish on the lens doesn't look very rub-proof and I guess that a working pro would soon have scruffy looking lens. This seems to be a common grumble with Sigmas. The supplied case is very bulky but would give excellent protection. I'm just very carefull with my gear.

I wish it had IS, but big aperture usually means adequate shutter speeds

I see all the press and general photogs seem to use 70-200mm f2.8 lenses for a lot of their work, also wedding shooters. Canon don't make a relatively similar size lens for the APS-C format. But Sigma do. The 50-150mm f2.8 EX DC lens is 80-240mm equivalent in full-frame. Close enough. I haven't had much luck with Sigma lenses in the past - but these were the cheap end models. The EX designation is supposed to be equivalent to Canon's L series. Well, maybe. Some Internet reviews were critical of the sharpness of the 50-150mm, and said it kept getting the focus wrong. I guess I wouldn't have bought but I found an auction item for an unused example where the guy was desperate for money. It happens these days. It has proved to be an ideal portrait lens and I have hardly used it outside the range f2.8 to f4. It is bitingly sharp anywhere in the focal length range at these apertures. I always try to stay just a little bit away from the extremes of focal length but again there has been no problem right at the ends.

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Only recently changed to digital (I'm one of the many thousands who gave up waiting for the EOS 5D MkII). The EOS 40D suits me well. Haven't for one moment regretted forking out for the Canon 17-55mm f2.8. IS My bag also has the EF-S 10-22mm which is very sharp. An elderly Canon 28-135mm IS lens which is usually more suited to fast moving kids, particlarly the wide end. And a Canon 100-400mmL IS for zoos and airshows - when I'm feeling strong enough to carry it all day!