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). Note that these tests are based on the sRGB JPEG output of the cameras, in accordance with ISO 12232:2006, the standard used by camera manufacturers. In our tests we found that measured ISOs from the Nikon D610 match the marked ISOs within 1/6 stop accuracy, meaning ISO 100 indicated = ISO 100 measured.
Noise and Noise Reduction (JPEG)
ISO range noise comparison
Nikon's JPEG noise reduction settings include Off, Low, Normal and High. It's compared at default in the chart below to other full-frame cameras: the Canon EOS 6D, Sony Alpha 7 and the Nikon D800.
At the D610's default 'Normal' setting the JPEG noise performance is impressive throughout its 100-6400 ISO range. By the time it's using its ISO 12800 and 25600 extension settings, it's working pretty hard. Canon's EOS 6D appears to retain a touch more detail, but much of this is likely to be the slightly higher luminance noise level. Against the Sony Alpha 7, the D610 retains much more detail (the a7's content-aware noise reduction can be pretty heavy-handed).
ACR Raw noise (ACR 8.3 noise reduction set to zero)
Here we look at the Raw files processed through Adobe Camera Raw (in this case version 8.3). 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.
Just like the JPEG comparison, the D610's Raw file performance is also very impressive at higher ISOs. Chroma noise is suppressed to a greater degree than other full frame DSLRs and fine detail is preserved until ISO 6400. Above 6400 noise is significant, yet the D610 still delivers a usable degree of detail that makes it among the top performers in the full frame market.