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).
In our tests we found that measured ISOs from the Nikon D7100 match the marked ISOs within 1/6 stop accuracy, meaning ISO 100 indicated = ISO 100 measured.
Noise and Noise Reduction (JPEG)
At its default 'Normal' setting, the D7100's tendency to leave noise and retain detail produces measurably higher chroma and luminance noise levels than some of its recent model DSLR competitors. And, unsurprisingly, it lags behind the very impressive full frame Nikon D600, which you'd expect given its larger sensor. Looking at the image samples though, you can see that the D7100 still delivers crisp image detail up through ISO 3200, avoiding the over-processed look indicative of heavy-handed noise reduction attempts. At its extended ISO settings - approximately one and two stops above ISO 6400 - noise becomes very prominent, obscuring significant image detail.
RAW noise (ACR 7.4 , noise reduction set to zero)
In Raw mode, the D7100 displays noise levels consistent with its APS-C competition. However, since this is a per-pixel test, which would usually be a disadvantage for high pixel count sensors, its ability to produce the same per-pixel noise as the EOS 7D is impressive (since it means it'll produce lower noise if assessed at the whole image level). Faint noise patterns are visible as low as ISO 100. And by ISO 1600 chroma noise can be seen throughout. Yet image details hold up impressively well and noise doesn't become overly objectionable, until ISO 6400, at which point detail begins to be significantly obscured.