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 D7000 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 photography (remember that this test is performed in manual mode without reference to the camera's metering system).
Noise and Noise Reduction (JPEG)The D7000 offers four noise reduction settings - Off, Low, Norm (default) and High, with the Off setting only applying noise reduction at ISO 1600 and higher, and even then with a lower intensity than than in the Low setting. In reality you have to look pretty closely to spot a difference between any of the noise reduction settings up to ISO 800. Above that it's the usual compromise between controlling noise and retaining fine detail. Having said that the D7000'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 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 all in all the Nikon D7000 is arguably APS-C camera that so far offers the best performance at these extremely high ISO settings.
Looking at the graph, there is a sudden leap in noise from the D7000 at ISO 12800 compared to the Sony SLT-A55 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 D7000's conventional capture.
RAW noise (ACR 6.3 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 800 the differences are marginal. At the higher settings the Nikon 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 D7000'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 the playing field is pretty level - as we'd expect from cameras which share the same (or at least an extremely closely-related) sensor, the Sony SLT-A55 and Nikon D7000 give very similar measurements in RAW mode.