This is our standard studio scene comparison shot taken from exactly the same tripod position. Lighting: daylight simulation, >98% CRI. Crops are 100%. Ambient temperature was approximately 22°C (~72°F).
Canon EOS-1D Mark IV vs. Nikon D3S
Canon EOS-1D Mark IV: Canon EF 50mm mm F1.4 lens, Manual Exposure, ISO 100 (base), JPEG Large/10, Manual WB, Default Picture Style (Standard), Self-Timer (with exposure delay)
Nikon D3S: Nikkor 85mm mm F1.8 lens, Manual Exposure, ISO 200 (base), JPEG Large/Fine, Manual WB, Default Picture Control (Standard), Self-Timer (with exposure delay), ADL off
Canon EOS-1D Mark IV
9.0MB JPEG (4896 x 3264)
5.1MB JPEG (4256 x 2832)
As we've becomes used to, the Canon's result is considerably sharper at default settings than the Nikon's but this is often simply down to the different approaches to sharpening taken by the manufacturers. This difference in sharpening emphasizes the resolution differences between the cameras, with the 1D Mark IV producing a more detailed image.