The 'EX' in the name denotes this lens's superior optical and build quality, the 'DG' means that it's optimised for the smaller sensor areas of most DSLRs but will still work on a full-frame camera, and the 'HSM' stands for 'Hyper-Sonic Motor', Sigma's proprietary high-speed autofocus technology.

This lens is available in Nikon, Canon, Sony, Pentax and Sigma mounts. On a full-frame SLR it acts as the kind of fast, fixed focal length 'standard' lens popular years ago before zooms took over, and on an APS-C sized sensor it's equivalent to around 80mm. This has long been considered an ideal 'portrait' focal length because it produces a more flattering perspective than shorter focal lengths, let still lets you work in the average-sized room. Indeed, back in the days of film photography, fast 80mm lenses cost a fortune, so if you're looking for a high-speed portrait lens, the Sigma could be something of a bargain. But more on this shortly.

The maximum aperture of f1.4 is only half a stop larger than the f1.7 'standard' lenses which were popular years ago, but this extra half-stop has always brought with it extra bulk and extra expense.And the Sigma is pretty bulky. The barrel's not that long, but it's very fat, mainly because of the huge front lens element. There's no Zoom ring of course, so the only controls are a manual Focus ring near the front and the Auto/Manual focus switch on the side.

You still get full-time manual focus override even when the lens is set to AF mode, and Sigma's HSM focusing is quiet and smooth and almost as good as Canon's USM system.

The viewfinder image produced by the f1.4 maximum aperture is bright, as you'd expect, though the focusing screens in today's DSLRs are designed for brightness anyway, so you may not see that much difference in practice.

The build quality and handling of this lens are really good - easily as good, in fact, as camera makers' own brand optics. The performance, though, is a mixed bag. Our test chart shows average to good resolution at medium apertures but very poor resolution wide open. This was a surprise, and prompted us to re-test the lens two more times, but it made no difference to the wide-open performance. You would expect fast lenses like this to soften at maximum aperture, but not to this degree.

However, shots taken out of doors were much better, and a few more tests suggested that this lens does not perform well wide open at shorter focusing distances - our test targets are shot at a distance of 2m, which doesn't appear to suit this lens's optical properties at all.

Much of the time, then, this lens will perform better than our test chart suggests. However, if you are using it as a portrait lens at or around this distance, you should think about stopping down to f2.8 or f4 as a minimum.

Apart from the concerns about close-range performance, this is a well-made lens, which is nice to use. Before you buy, though, check whether your own camera maker offers a cheaper alternative. Canon's 50mm f1.4, for example, is cheaper than the Sigma, and there's a 50mm f1.8 that is just £100.

Final Verdict A brilliant, prime lens that's just a little too expensive 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.