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like it says in their article, "The 1D Mark IV sensor is 1.66 times smaller than the Nikon D3s."
The D3s is ranked number 4 in overall total sensor performance score, out of all cameras tested. The 1D Mark IV ranks 16th in overall total sensor performance, out of all cameras tested.
As they say in terms of low light High ISO performance, "The Nikon D3s achieves an exceptional result with a score of 3,253 and takes the lead for DxOMark Low-light ISO rankings. This result is obtained thanks to a slightly better SNR and a better Dynamic Range in comparison with the D3. The Canon EOS 1D Mark IV ranks 12th with a score of 1,320. So for high ISO, and after normalization, the difference between these two sensors is more than 1 f-Stop. These two cameras are the first to provide a real ISO 100,000 setting, and here are the results:"
In the low-light ISO scores, the Canon 1D Mark IV scores 7.8 in color sensitivity, while the Nikon scores 10.4 in screen mode. In print mode, the Nikon maintains the lead, scoring 11.4 against the Canon's 9.3.
For a good visual comparison of the superior low-light ISO performance of the full-frame sensor of the 5D mark II over the 1D Mark IV, and for a good visual comparison of the D3s's superior low-light ISO performance, check out this 3-camera comparison. The images give one a pretty good idea of how the D3s looks, with its 3,253 score, followed by the 1D IV in the middle with its scores of 1,320, and the 5D Mark II on the right hand side with its score of 1,815. Overall, the comparisons show what most people understand: a full-frame sensor performs better at elevated ISO settings than a camera with a smaller-than-full-frame sensor.
Canon EOS 1D Mark IV vs. Nikon D3s: ISO Comparison | neutralday
And DScience, as Canon USA's Chuck Westfall said in an interview with CNet, when Canon introduced the 1D series body, it was designed to compete with camera that had smaller, APS-C sensors, so at the time the 1.3x FOV of the 1D series APS-H sensor made perfect sense.