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 5D Mark III match the marked ISOs within 1/6 stop accuracy, meaning ISO 100 indicated = ISO 100 measured.
Noise and Noise Reduction (JPEG)
If you look at the noise graphs below you can see that, at default settings, the Canon EOS 5D Mark III's measured noise remains pretty much on the same low level until you get to ISO 3200. Of course this can only be achieved by increasing noise reduction as you increase sensitivity. The Canon's JPEGs are clean and relatively noise-free but have visibly destructive noise reduction applied to them at all ISOs, and you can always get more detail out of an image by converting the raw files, even at ISO 50 or 100.
There is practically no benefit to switching noise reduction off at low ISOs, a difference between the 'Off' and 'Standard' settings only becomes visible at ISO 1600. That said, even the 'Off' setting doesn't mean that no noise reduction at all is applied. Instead there appears to be a baseline level of mainly chroma noise reduction that cannot be disabled. The noise reduction setting controls the amount of additional noise reduction on top of this baseline level.
At higher ISOs the NR 'Off' settings gives you some additional fine detail over the 'Standard' and higher settings but at very high ISOs, 25600 and above, the 'Standard' setting is the better options, as otherwise the noise levels are too intrusive.
Overall the Canon EOS 5D Mark III achieves very low levels of measured noise across the ISO range but this is paid for with a loss of fine detail at higher sensitivities. The two extended settings, ISO 51200 and 102400 should be reserved for emergency situations only, but up to ISO 25600 the camera's JPEGs are usable at least at smaller output sizes.
RAW noise (ACR 6.7, noise reduction set to zero)
The 5D Mark III's raw noise levels are, compared to its predecessor 5D Mark II, and its most direct rival, the Nikon D800, lower across the ISO range. Converting the the raw files and applying a customized noise reduction mix can get you excellent results, as we have demonstrated in the raw and image quality sections of this review.