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 D600 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 D600's JPEG output delivers impressive noise performance throughout its ISO range. You can see from the graph that among its full frame rivals, only the Canon EOS 5D Mark III displays measurably lower chroma and luminance noise levels. Yet a look at the image samples here reveals a more complete story, as the D600 shows a more natural, less 'processed' look, avoiding more obvious image artifacts. Fine detail is retained impressively well up through ISO 12,800. It is only at its maximum ISO of 25,600 that noise suppression significantly degrades image quality compared to ISO 6400 output, though the files are still eminently usable for even medium-sized print output.
RAW noise (ACR 7.3 beta, noise reduction set to zero)
As we saw in the JPEG comparison, the camera's raw file performance is very impressive, particularly at higher ISO sensitivities. Chroma noise is suppressed to a greater degree than most other full frame DSLRs yet even fine details are well preserved up until ISO 6400. Above ISO 6400, noise patterns become significantly more prominent. The D600 still manages, however, to maintain a degree of image detail that ranks it among the top performers in the full frame DSLR market.