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.
There are many MB of files required to be downloaded to make all of the mouseover and mouseclick links in this review function properly - please be patient while they load. Also note that my 7D delivered exposures that are 1/3 stop darker than my compared-to bodies at the identical exposure settings in this test - thus the +1/3 EV examples presented here.
Differences in noise between the 1D III, 1D IV and 5D II samples up through ISO 6400 are minor and insignificant to my eyes - all 3 DSLRs produce impressive results. Thus, higher resolution wins at final output. The 1Ds IV maintains its good results while providing far more resolution than the 1D III without an increase in sensor size. The 5D II has the sensor size advantage over the other two cameras.
The 1D IV results are noticeably cleaner (and sharper) than the 7D results even at lower ISO settings.
At ISO 12800 (a setting not available on the 1D III), we get into my avoid-if-possible range for the 1D IV. ISO 12800 noise is noticeable, but images are usable. The 1D IV results appear to be very slightly cleaner than the 5D II at this setting and at ISO 25600, even slightly cleaner. The 5D II retains the resolution advantage of course. 1D IV image detail is being affected significantly at ISO 25600.
Canon EOS 1D Mark IV images taken at ISO 51200 with no noise reduction added are, to be clear, ugly at 100%. Adding some noise reduction and reducing the image size significantly gives you something useable. ISO 102400 appears to me to be a marketing invention more than an engineering feat - images at this ISO setting are a disaster at 100%. Significant noise reduction and downsizing will help, but ...
To be fair, Canon refers to ISO 25600, 51200 and 102400 as extended settings H1, H2 and H3. You will want to avoid them if possible, but images can still indeed be made at the High settings. Photographers without these settings available will be putting their cameras away while you continue shooting into the darker lighting conditions.
The "w/ NR" noise reduction samples show that while noise in high ISO images can indeed be reduced, detail is sacrificed in the process. The in-camera-suggested noise reduction amounts are a good starting point and add a stop or so to the usable ISO setting depending on your taste and needs. I generally use a small amount of noise reduction in my high ISO images, but I typically lean toward saving detail over cleaner images.
Here is another comparison example that includes fine details that better-hide high ISO noise.
More resolution brings out more details in the fabric and, aside from the 7D results, the big difference between DSLRs I see in this comparison relates to the resolution of the compared cameras' sensors. More resolution shows more detail.
The 7D's smaller (physical size) sensor has more megapixels than the 1D IV, but it delivers a less sharp image at an identical sharpness setting of "1" ("2" works better) and shows more noise than the larger sensor cameras it is being compared to.
If, even after evaluating the differences, you cannot see a clear low-noise winner in these comparisons, the differences in noise are probably not going to be of significance to you in real world shooting.
Same with resolution. And in this case, the Canon EOS 1D Mark IV shows a very nice resolution increase over the 1D III while retaining very good sharpness even at this low sharpness setting.
Back to the Canon EOS 1D Mark IV Review.