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 a7R match the marked ISOs within 1/6 stop accuracy, meaning ISO 100 indicated = ISO 100 measured.
Noise and Noise Reduction (JPEG)
The a7R's JPEG noise is in-step with the Nikon D800 and Canon EOS 5D Mark III through about ISO 3200. At that point, the two DSLRs start to break away, with the a7R remaining very low. If you look at the color patches, you'll see why: Sony is pouring on the noise reduction. The only time any obvious noise shows up on the a7R is in the blue patch at the top ISO of 25600.
The camera's context-aware noise reduction does a reasonable job of protecting detail in the crop of the Queen's head, up until ISO 800. There's then a drop in detail as noise reduction it ratcheted up, and there's a further drop in detail at ISO 12,800 - at which point it's all pretty smudged.
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 a7R manages to keep noise levels down throughout its ISO range compared to both the Nikon D800 and Canon EOS 5D Mark III. The fact that the D800 has more noise than the a7R hints that Sony may be applying noise reduction to the Raw images. If you look at the crops, you'll see that the a7R's color noise appears to be 'smudged', unlike on the D800, which has a grainier appearance.