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 Canon EOS 6D match the marked ISOs within 1/6 stop accuracy, meaning ISO 100 indicated = ISO 100 measured.
Noise and Noise Reduction (JPEG)
The Canon EOS 6D does a good job of retaining detail as far as ISO 6400, when the noise reduction starts to have an increasingly visible impact on fine detail. At its default, 'Standard' setting, its noise levels are lower than its competition (and the EOS 5D Mark III), but that appears to mainly be down to its comparatively aggressive noise reduction. Turn the noise reduction down and the results are more directly comparable to its peers. Photographing test charts, of course only tells part of the story. Minimizing noise while retaining detail in organic textures is where you'll experience the 6D's noise performance in your own images. You can see noise comparisons with real world subjects on the high ISO comparison and image quality test pages of this review.
RAW noise (ACR 7.3, noise reduction set to zero)
The amount of noise that appears in ACR conversions is on a par with the Nikon D600, putting it a fraction below the EOS 5D Mark III. A small amount of this difference will be because this favors lower pixel-count sensors (if all else is the same), but the visual crops suggest the differences at the very highest ISO settings are not significant - it's going to be similarly difficult to get clean and detailed results with any of these cameras. For a closer look at how the 6D's sensor performs against its peers in lifting shadow detail at base ISO, please see the image quality test page of this review.