The ‘DG’ after the Sigma’s name indicates that it can fit both 35mm film cameras and DSLRs. The ‘OS’ stands for Sigma’s own Optical Stabiliser technology, which uses moving elements within the lens to counter camera shake. The ‘HSM’ stands for Sigma’s HyperSonic Motors, which are used for autofocus.

On a 35mm or full-frame SLR, you’d call the Sigma a medium-long telephoto. On a non-full frame digital SLR, though, it equates roughly to a 240-800mm zoom. This makes it a super-telephoto, which is potentially perfect for capturing the most distant objects like jets at an air show, or pitch-based sports where the players are often a long way from the touchline or boundary.

Long lenses like this are tricky to handle, because the extreme magnification means that the slightest twitch sends your subject lurching across the viewfinder. That’s why some kind of Image Stabilisation system is such an advantage and, unless you’re using a tripod or a monopod, almost a necessity.

As soon as you half-press the shutter release, the Sigma’s OS cuts in and visibly steadies the image in the viewfinder. Sigma claims it lets you shoot up to four shutter speeds slower than usual without camera shake, and there seems no reason to doubt it. You can slide a switch on the lens to a ‘two’ position, which corrects vertical movement only – this will give better results with panning shots – or you can switch it off entirely. You’re not likely to want to do this with handheld photography but it’s a good idea for longer exposures on a tripod, because otherwise the OS will ‘drift’ slowly over time, creating some movement during the exposure.

Like other long-range zooms, this is a big, heavy lens. The jump up in size from, say, a 300mm zoom to a 500mm zoom like this one is substantial. For that reason alone it’s a good idea to make sure you really do need focal lengths this extreme, because lugging around nearly 2kg of glassware can get pretty tiring. Indeed, this lens weighs two, three or four times as much as the average DSLR body, and this alters the weight distribution considerably. Like other long telephotos, this one has a tripod collar which balances it much better on a tripod and, if you slacken it off and rotate it through 180 degrees, it makes a useful carrying handle.

The weight means that your arms can tire when you’re shooting handheld, so that image stabiliser really does earn its keep. The zoom action is, as we’ve come to expect from Sigma lenses, quite stiff, but this does at least eliminate any zoom creep.

The autofocus is fast and smooth, and Sigma’s HSM focusing system means that you also have full-time manual override – you just turn the broad focusing ring behind the main zoom ring. Accurate focusing remains a bit of a knack, as with all long lenses, not just this one. At these focal lengths there’s almost no depth of field at all, and the slightest movement on the part of your subject can send it out of focus. It’s often better to ‘pre-focus’ rather than try to follow a moving subject.

The Sigma felt very good in use. It was tested during a trip to a zoo and in a variety of conditions, and its magnifying power meant that it was possible to get some great animal portraits despite the distance between the visitors and the animals. It was disappointing, then, to get back and find that the pictures didn’t look particularly crisp, particularly those taken at longer focal lengths. The focusing was spot-on and the shutter speeds were usually high enough to prevent camera shake with or without the OS.

The lens tests confirmed this impression. At 150mm, the Sigma is not a bad performer, producing photos with fair to good levels of detail. But by 250mm the sharpness has dropped to the point where images look visibly soft, and at 500mm they’re softer still. A second set of tests confirmed the figures gained in the first – disappointing in a lens costing £750, whatever its specs and abilities.

With that in mind, you might be better off looking at the Sigma 50-500mm instead. It’s around £150 more and doesn’t have image stabilisation, but it’s a good deal sharper.

Final Verdict At this price you’re obviously not going to expect the razor-sharp image quality of pro lenses, but if you’re paying £750 for a lens, you’re probably going to expect better performance than this one provides OVERALL

Our lens reviewer, and technical expert, Rod is a veritable photographic encyclopaedia. His illustrious CV has seen him write for many mags, websites and journals.