The actual sensitivity of each indicated ISO is measured using the same shots as are used to measure ISO noise levels, we simply compare the exposure for each shot to the metered light level (using a calibrated Sekonic L-358), middle gray matched. We estimate the accuracy of these results to be +/- 1/6 EV (the margin of error given in the ISO specifications). We found that measured ISO from the D5100 is roughly 1/3EV higher than indicated - so ISO 100 = ISO 125 (approx). This holds true throughout the entire ISO range, but a discrepancy this small has little practical impact upon everyday photograph.
Here, we're looking at the D5100's noise performance compared to three of its closest competitors, the Canon EOS 600D, Sony SLT-A55 and Panasonic Lumix DMC-G2 at default noise reduction settings.
Up to ISO 1600, there isn't much to separate these models, but once we get into the higher ISO settings it is clear that the Panasonic G2 is the weakest of the bunch. Results from the other three cameras are very similar until ISO 6400, at which point the EOS 600D and Sony A55's output start to look noticeably smudgier than the D5100.
Looking at the graph, the A55 actually gives lower noise readings than the D5100 at default settings, but it is clear from the samples that the Sony is simply applying much more noise reduction, and detail is blunted as a result. Also worth noting is that although image quality is impressive, the ISO 25,600 image from the A55 is not a 'straight' shot, but was captured using multi-exposure NR - the only option available at this ISO setting. This explains the lower noise and higher detail compared to the D5100's conventional capture.
Noise and Noise Reduction (JPEG)
In reality you have to look pretty closely to spot a difference between any of the D5100's noise reduction settings up to ISO 800. Above that it's the usual compromise between controlling noise and retaining fine detail. Having said that, as we'd expect from our experience with the D7000, the D5100's NR algorithms appear to focus heavily on chroma (color) noise and by doing so manage to deliver good detail even at higher sensitivities.
The NR Norm setting arguably provides the best balance between detail and clean images. ISO 6400 output is perfectly usable at normal print sizes, only when you go even higher does the chroma noise become more intrusive. Having said that, even at the HI1.0 (ISO 12,800) setting high contrast detail is still acceptable and, despite the visible luminance noise, images are perfectly usable at smaller print sizes. Only at the very highest sensitivity setting (Hi2.0) things go visibly downhill but the fact that this option is even available in a nominally entry-level DSLR is impressive in itself.
RAW noise (ACR 6.4 noise reduction set to zero)
The amount of NR applied 'under the hood' is not high, but it does vary by camera (Adobe is attempting to normalize output across different sensors), so inevitably we are still looking at a balance of noise and noise reduction, rather than pure noise levels. However, the use of the most popular third-party RAW converter is intended to give a photographically relevant result, rather than simply comparing sensor performance in an abstract manner.
All the cameras show signs of noise even at the lowest sensitivity settings (remember these samples have noise reduction turned down to zero in ACR) but up to ISO 1600 the differences are marginal. At higher sensitivities the more advanced sensors of the D5100, A55 and EOS 600D clearly outstrip the Panasonic G2, but up to ISO 6400, in this environment, there isn't much to chose between the three models.
At the higher settings the Nikon's output is a little cleaner than most cameras in this class but again, the differences are almost too small to be relevant for many photographers. However, the fact that all these ACR converted files look pretty similar when there's more difference in the JPEG output suggests that the Nikon D5100's JPEG engine is doing a slightly better job than some of the competition at higher ISOs.
A glance at the graph shows us that apart from the Panasonic Lumix DMC-G2, the playing field is actually pretty level.