Compared to...

Studio scene comparison (higher sensitivities)

The DP2 is not unbearably noisy up to ISO 800. You get rather better results processing the RAW files but, as already discussed, the supplied Sigma Photo Pro software gives no control over noise reduction so you can do nothing to get rid of the color blotches that start to appear at ISO 800 and above. This blotching is particularly noticeable in shadow regions.

At ISO 1600 and up it simply can't compete with the Olympus E-P1. It does a reasonable job of retaining detail but contrast and saturation drop quite badly. It avoids smoothing all the detail away as the Sony does. Indeed the Sony doesn't produce results that are as far ahead of the Ricoh as you'd like, given that its sensor has eight-and-a-half times the surface area.

ISO 3200 is a step too far for the DP2, with shadow areas clipped to black to try (unsuccessfully) to hide chroma noise.

ISO 800

Sigma DP2 (JPEG) Sigma DP2 (RAW)
Olympus E-P1 Sony A330
Ricoh GR Digital III  

ISO 1600

Sigma DP2 (RAW) Sony DSLR-A330
Olympus E-P1 Ricoh GR Digital III

ISO 3200

Sigma DP2 (RAW) Sony DSLR-A330
Olympus E-P1