One of the upgrades introduced with the SD15 is a new processor which is supposed to provide faster performance, but I have to say that I couldn't detect any significant improvement. It still starts up quickly, and in single shot mode it can shoot at just over one frame a second in both Raw and JPEG modes, although in the former the buffer fills up after approximately 16 shots and it slows down. In continuous shooting mode is can manage 3fps, but only for 21 frames . This is a bit on the slow side by modern DSLR standards; the Nikon D5000 can manage 5fps, while the Pentax K-7 can shoot at 5.2fps.
The autofocus system is also a bit primitive, lacking the cross-type sensors used by most other manufacturers. It is comparatively slow and is also inclined to hunt around, especially at long ranges or in dim light. The SD15 does feature a decent AF assist lamp, a bright white LED on the front near the handgrip, but this only helps at short range.
Battery duration is also not as good as it might be. The camera is powered by a large 1500mAh Li-ion cell, which one would think should be big enough. Sigma makes no particular claim for its performance, but I found that it only lasted for approximately 170 shots, with few using the built-in flash and with minimal use of the LCD monitor.
Of course the main claim that Sigma makes for the Foveon X3 sensor technology is that it produces superior image quality compared to other types of sensor. The colour reproduction in good light is certainly exceptional, with rich vibrant hues, outstanding tonal range and good detail even in highly saturated areas. However it does have a couple of drawbacks. Dynamic range in low light conditions is lower, and image noise in JPEG mode is much worse than any of its rivals, showing obtrusive noise and saturation reduction even at 200 ISO. Image noise in Raw mode can be corrected to some extent in either the supplied software or more effectively in Adobe Camera Raw, but it's still a problem.
However the main drawback with the SD15 is the sheer lack of resolution. Sigma can call it 14.06 megapixels until they're blue in the face, but there's no getting away from the fact that the final image size is only 2640 x 1760 pixels, 4.64 megapixels, and that's where it counts. The vastly superior resolution of most modern APC-C CMOS sensors, as well as their superior light-gathering ability, especially from the new back-illuminated designs, simply trumps any advantage that the three-in-one Foveon sensor design imparts. Yes, it is certainly much better than a conventional 4.6 megapixel sensor, but it's competing with technology from five years ago. These days even the cheapest entry-level DSLRs have more than double that resolution. Considering its price tag of nearly £800, the SD15 simply can't compete on features, performance, versatility or image quality with smaller, lighter cameras costing half as much, which ultimately makes it pretty poor value for money.
There's no denying though that the SD15 can take a very nice picture under the right circumstances, and I do believe that the Foveon X3 sensor holds some promise, as demonstrated by the Sigma DP2s compact, so I will hold out a hope that the 15.3-megapixel Sigma SD1 due out next year will finally realise the potential that the technology has been tempting us with for so long.
Although the Foveon sensor technology certainly has potential, as it stands the SD15 is only an incremental upgrade from the three-year-old SD14 and can't compete with the current range of consumer DSLR cameras. It is expensive, the design is bulky and heavy, it has relatively poor performance, and it simply doesn't produce the kind of image quality to compare with its rivals, especially at higher ISO settings.