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 RX1R match the marked ISOs within 1/6 stop accuracy, meaning ISO 100 indicated = ISO 100 measured.
Noise and Noise Reduction (JPEG)
The RX1R's JPEG produces similar amounts of noise to the SLT-A99 but with increasing divergence at high ISOs. Without the SLT's semi-transparent mirror, the RX1R's sensor is always receiving rather more light than the A99's, so is able to produce cleaner JPEGs (or, at least, not have to apply as much noise reduction to its files). Its noise reduction is a little more aggressive than the Nikon D600's, which explains how it appears to post better results in this test. Compared to the RX1, the RX1R gives slightly lower measured noise results at its very highest ISO sensitivity settings, although this is all but indiscernable when visually examining the camera's actual output and is within acceptable margin for error for this test.
The RX1R offers three noise reduction settings - off, low and normal. The 'off' setting still applies a fair dose of chroma noise reduction but leaves luminance noise relatively untouched, so you get a gritty but still sensibly-colored image. The Low and Normal settings are only very subtlety different in this test - possibly because it applies different amounts of noise reduction to different areas of the image (in an attempt to distinguish between smooth areas, detail regions and edges).
ACR Raw noise (ACR 8.1, noise reduction set to zero)
In raw mode the RX1R gives exactly the performance you'd expect - exactly the same as the RX1R, a touch ahead of the A99 and on a par with the Nikon D600.