Nikon D5100 High ISO RAW
We've recently started looking at RAW files converted with dcraw, an excellent freeware raw converter. dcraw usually offers timely support for the latest cameras, but more importantly, it does not apply any noise-reduction, sharpening or other corrections such as geometric distortion correction to the output files. (We found that Adobe Camera Raw still applies some limited noise-reduction when its NR settings are set to zero, and it also applies other corrections depending on the make and model of the camera). There will always be differences between RAW converters, in terms of the sort of de-mosaicing algorithms they use (the processes by which they convert the separate Red, Green, and Blue data sets to an array of full-color RGB pixels), but dcraw seems to use a fairly generic algorithm that delivers good sharpness with relatively few artifacts, and can be counted on to not apply any noise reduction if you don't want it to. (That said, looking at the ISO 6,400 and above examples below, it's clear that dcraw's de-mosaicing approach does have some tendency to produce rectilinear artifacts in response to high noise levels.)
Below are crops from Nikon D5100's 14-bit RAW files compared to RAW files from other consumer SLRs, converted with dcraw.
The crops above compare the Nikon D5100's RAW noise performance to that of other similarly priced consumer models. All cameras were equipped with our Sigma 70mm f/2.8 EX DG Macro reference lenses, stopped down to f/8 for maximum sharpness. As you can see, the Pentax K-r produced RAW files with the lowest noise, though it also has the largest pixels (lowest resolution), giving it a slight edge in light gathering capability per photosite. The K-r also applies some subtle noise reduction at ISOs above 1,600 that can't be disabled, which leads to cleaner images but also to some blurring. The Nikon D5100 comes in a close second, while the Sony A580 arguably comes in third, and the Canon T3i fourth. Excellent performance from the Nikon D5100.
Here's a comparison with the D5100's two immediate siblings in the current Nikon SLR lineup:
Here, RAW noise performance is similar, though the Nikon D3100 appears to be slightly noisier than both the D5100 and D7000, despite having larger photosites. The Nikon D7000 seems to have just a slight edge over the D5100, but it really is close between the two.