Read about the Camera ISO noise tests in the help section to learn more about the tests and how they are conducted. A key take-away from that page is that noise reduction is completely off unless otherwise specified.
The files required to be downloaded to make all of the links in this review function properly are very significant in size - please be patient while they load.
And again, this test gets us as close as possible to the image quality delivered directly by the sensor. And there are a lot of comparisons that can be made here. If you can't see a noise difference in a particular comparison, it is unlikely that you would see a difference in your actual images. Note also that you might see the differences more easily if you zoom in your browser's view (typically CTRL-+ with CTRL-0 to return).
An obvious observation that can be made is that the 1D X goes where no Canon DSLR has gone before (and has gone where we didn't even dream of in film days) - to ISO 204800. As usual, I wouldn't use the highest ISO setting in any of the recent DSLRs unless it was an emergency (perhaps with a gun held to my head). There once again is more noise than detail at the highest setting - there is no hope in reading all but the largest words in this example.
By ISO 51200, I can start reading some of the black text on this chart and even more is readable at 25600. I still would not want to use these settings for any project requiring high image quality - but they are indeed available for the situation that warrants their use. The 1D X does deliver noticeably cleaner images at these ultra-high ISO settings than any Canon DSLR has delivered to date. And noticeably less noise at the lower, more-used ISO settings as well.
Noise at ISO 12800 remains a touch strong for my taste, but ISO 6400 results are not too bad - closer to what the 1Ds III and 5D II delivered at ISO 3200. Even when up-rezzed to 5D III image dimensions (the "1DX>5DIII" results), the 1D X retains a noise advantage over these also-impressive DSLRs.
The 5D III competes most strongly with the 1D X as of 1D X review time. The 1D X has retains the low noise advantage even when up-rezzed to the same pixel dimensions - especially at the higher ISO settings. And the 5D III image is slightly sharper in that comparison (this difference is better seen below).
Noise reduction is a highly variable setting that can be applied to your taste with you software of choice. The "1D X NR" results show Canon DPP's default noise reduction enabled. Noise reduction is very destructive process (it does not always properly differentiate noise from subject), but the high ISO results look much better with noise reduction enabled.
Canon claims 2 stops of noise improvement in the 1D X over the already very good EOS 1D Mark IV. Unfortunately, you will need to shoot in JPEG format to see the full 2 stop advantage. The significant processing power in this camera and the algorithms it uses are able to deliver better in-camera noise reduction. I'm not planning to change my workflow from RAW to JPEG due to this better in-camera processing, but I definitely appreciate the improvements in the RAW image quality.
Below is another comparison example that includes fine details. These samples were taken from the same shot and processing as described above. Fine details better-show resolution and better-hide high ISO noise.
There are a lot of conclusions that can be made from the above comparisons, but ... the 1D X performance is looking very good from both resolution and noise standpoints. The 1D X wins the overall high ISO noise competition and the 5D III retains a modest resolution edge.
I don't like giving up resolution (from my 1Ds III bodies), but 18mp is still a lot to work with and more than most applications need - 3mp is not a great loss. So, even if I don't gain my wish-list line item resolution upgrade with the 1D X, I do not lose anything of big significance in this regard.
Back to the Canon EOS 1D X Review.