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 a7 match the marked ISOs within 1/6 stop accuracy, meaning ISO 100 indicated = ISO 100 measured.
Noise and Noise Reduction (JPEG)
Noise for JPEGs follows relatively the same path as the other full-frame cameras until you reach ISO 6400, at which point Sony's noise reduction really kicks in. If you view the samples, you'll see that the a7's patches are paved over with noise reduction, while the other three cameras leave a lot of noise behind in comparison.
Adobe Camera 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.
The a7 performs very much like the competition in Raw mode. One interesting thing to note is that the a7 is slightly noisier than the D600 at high sensitivities, despite the fact that they likely use the same sensor.