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 Coolpix A match the marked ISOs within 1/6 stop accuracy, meaning ISO 200 indicated = ISO 200 measured.
Noise and Noise Reduction (JPEG)
The Coolpix follows the usual Nikon route of concentrating its efforts on suppressing chroma noise and allowing a little luminance noise to remain. This gives the impression of retaining detail longer than its more heavy-handed rivals, but it's a fairly similar approach to the one being taken by the Ricoh, so it's not particularly obvious, here.
Above ISO 6400 a little chroma noise starts to make itself felt in the greys but overall the image ends up looking better than the Ricoh's result at the same sensitivity.
Noise Reduction optionsThe Coolpix A's noise reduction options differ only in terms of intensity - there's no choice to be made between chroma noise reduction and saturation, or detail and luminance noise.
ACR noise (ACR 7.4, noise reduction set to zero)
Here we look at the Raw files processed through Adobe Camera Raw (in this case version 7.4). 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.
The Raw output is pretty clean and, interestingly, essentially identical to the Ricoh GR's results. They exhibit very similar noise and detail retention pattersn throughout the ISO range - lending credence to the idea that the two cameras are based around the same or, at least, very similar sensors.