Sigma has gone out on a bit of a limb with the launch of its SD14 SLR and the unorthodox Foveon sensor it contains. The camera has 50 per cent more resolution than the SD9 and SD10 cameras that came before it, but it still only manages to deliver 4.7-megapixel images.

And Sigma expects us to fork out more than £1,000 for this! To understand why someone might pay this much for such a 'low-resolution' camera, you have to understand the sensor technology that underpins the SD14.

The pixels on camera sensors are sensitive only to light; they can't discern colours. To get around this shortcoming, conventional sensors employ individual pixels covered with red, green or blue filters in a regular rectangular array known as a 'Bayer' pattern.

A single pixel might only record red, but the blue and green data also needed to produce full colour information is taken from neighbouring blue and green pixels. The manufacturers call this technique 'de-mosaicing'. We might call this 'interpolation', as the image processor has to guess the colours.

De-mosaicing involves a lot of mathematical guesswork, which erodes fine detail and can introduce fringing and other pixel-level artefacts. We don't notice them much because we've got used to them and don't know any better.

The SD14's Foveon chip is totally different. Instead of a Bayer filter, the Foveon employs stacked layers that are responsive to different colours... a bit like colour film. The chip has a 'red' layer that's sensitive to red light, a 'green' layer and a 'blue' layer. Each of the 4.7 million pixels in the final image uses data supplied by the red, green and blue layers. There's no interpolation, no de-mosaicing and no guesswork involved.

The result of this different approach is an image quality with a level of clarity at a pixel level that is simply stunning. So much so that it could be a while before you notice the almost total absence of colour artefacts. Bare twigs against a bright sky are shown as perfect silhouettes, just like they are in the real world, and aren't ruined by those ghastly blue or magenta outlines.

With Foveon, the normal rules of resolution don't apply. Yes, the SD14's images may be 'only' 4.7 megapixels, but its images have such clarity and precision that you can't directly compare them to those produced by other cameras boasting more megapixels.

It's very important to understand what this Foveon sensor is capable of when evaluating this camera, because the SD14 itself is pretty ordinary. It's bulky and has a plasticky feel, though the quoted shutter life of 100,000 cycles indicates that it's solidly built. The autofocus isn't as snappy or as quiet as that of the Canon EOS 400D or Nikon D40x, say, and the camera takes a while to wake up from its 'sleep' mode, sometimes finally taking the shot the moment you give up and take the camera from your eye to see what's going on.

The 18-50mm Sigma lens that was supplied with our test camera benefited from a constant f/2.8 maximum aperture, but the focusing wasn't especially quiet, and it added a hefty £370 to the total cost. A far cheaper alternative is the 18-50mm f/3.5-5.6 Sigma-mount lens at £90.

The exposure options are pretty basic - the mode dial has just PASM markings with no scene modes available - but this could well appeal to photographers who've had enough of gimmicks and who'd prefer straightforward controls.