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-308), middle gray matched. We estimate the accuracy of these results to be +/- 1/6 EV (the margin of error given in the ISO specifications). By our tests, the Nikon P7100 measured sensitivities are about 1/3 stop higher than indicated (i.e. images are fractionally brighter than expected for any given set of exposure values so ISO 100 = 125, etc). This means that, given identical exposure settings, the P7100 at ISO 100 should give you the same result as ISO 125 on another camera with a standard ISO rating.
Noise and Noise Reduction (JPEG)
ISO range noise comparison
The P7100 uses the same CCD sensor used in the P7000 as well as the Canon G12. However, in JPEG mode the P7100 exhibits less noise across the board at all ISO settings compared to the P7000. This is due to new noise reduction processing which Nikon claims is more sophisticated than that used by the P7000. At the low end of the ISO settings, the P7100 even shows less noise than the G12. However, Nikon's fairly heavy-handed noise reduction removes much of the fine detail at higher ISO settings, and compared to the Canon G12 the P7100 appears just to be smearing away noise, resulting in images that look somewhat plastic when viewed at 100%.
Raw noise (ACR 6.6 noise reduction set to zero)
Here we look at the raw files processed through Adobe Camera Raw (in this case version 6.6). Images are brightness matched and processed with all noise reduction options set to zero. Adobe does a degree of noise reduction even when the user-controlled NR is turned off.
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.
Image quality is good up through ISO 800, but at 1600 and above noise begins to obscure most of the fine detail. The P7100 uses the same sensor as its predecessor, and as we would expect the noise is nearly identical between the two, with the exception of ISO 6400 where the P7100 appears to be applying some noise reduction to the raw file.