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). We found that measured ISO from the V1 is roughly 1/3EV higher than indicated - so ISO 100 = ISO 125 (approx). This holds true throughout the entire ISO range, but a discrepancy this small has little practical impact in real world use.
Noise and Noise Reduction (JPEG)
On close inspection, some noise is evident in the J1/V1's JPEG output even at base ISO. Impressively though, the cameras' noise suppression strikes a pleasing balance between noise removal and image detail, with the final result looking less 'processed' than we'd expect from some of Nikon's compact cameras. While noise levels start to noticeably impact image detail at ISO 800, it is only at ISO 1600 and higher that aggressive noise suppression begins to smear fine detail, but even ISO 3200 output is still usable for smaller prints and web use. The highest setting (ISO 6400) is usable for small prints and web use, but should definitely be reserved for emergency use only.
Despite the comparatively small size of the 1-system sensor, the J1/V1's JPEG output compares very well against results from the current crop of 12MP Micro Four Thirds cameras (although it cannot match the abilities of the newer 16MP sensor in Panasonic's G3/GX1). However it lags clearly behind the results obtainable from the current crop of APS-C sensor cameras.
The noise graph, shown here, doesn't tell the whole story and it is clear that noise reduction is keeping measured noise levels low. Switching noise reduction off results in more noise, both visible and measured, but noticeable better detail reproduction at higher ISO sensitivity settings.
RAW noise (ACR 6.6, noise reduction set to zero)
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 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.
Converting the Nikon J1/V1's images in Adobe Camera RAW 6.6 shows that even with ACR noise reduction turned off both cameras fare well against the APS-C and Micro Four Thirds sensor competition at the lowest ISO settings. Beginning at ISO 800 some NR is clearly being applied to the cameras' RAW files, even when both in-camera and ACR noise reduction options are turned off. Turn to the 'photographic tests' page of this review for some real-world examples of the effect that this has.
The measured noise graph shows that in-camera noise reduction also has an impact on the J1 and V1's raw files. Obviously this takes away a degree of control from the photographer, but fortunately results are good, and critical detail reproduction in raw mode is very similar with in-camera NR off and on.