Canon 5D mark III comes with new sensor, improved 61 point AF system same as 1D-X and better video mode. Nikon D800 is a megapixel monster with limited ISO range and slow burst mode compared to new 5D mark II.
Megapixel: Both have same Fullframe sensor but Nikon D800 features 36.3 Megapixel sensor and 5D mark III comes with 22.3 Megapixel sensor, with large individual 6.25 µm pixels and canon 5D mark III has a much-improved S/N ratio resulting in better images from the base ISO.
AF system: Canon 5D mark III features better AF system with 61 point AF (up to 41 cross-type points) but limited up to f/5.6, the Nikon D800 has working AF at f/8 and comes with 51 point AF system .
ISO range: Nikon D800 you get limited standard ISO range up to 6400 only, canon 5D mark III offers standard ISO range up to 25600.
Continuous shooting: The canon new DIGIC+ image processor Working with two 4-channel A/D converter front-end processing circuits, and delivering speeds of up to 6.0 fps, Nikon D800 Continuous shooting speed is limited to 4 fps only.
Video: Canon offers both All-I and IPB compression, supports H.264/MPEG-4 AVC High Profile, and automatically splits files greater than 4GB (FAT specifications) for extended recording without interruption, No details available about Nikon D800 uncompressed HDMI output, Nikon D800 Max Video length is limited to 20 minutes only.
Conclusion:
Canon 5D Mark III recommended for videographers, professional action and sports photographers, photojournalist, Studio, commercial and still life photographers.
Nikon D800 is recommended for Studio, commercial and still life photographers.
Buy/ Pre-order Canon 5D mark III at Amazon – B&H
Buy Pre-order Nikon D800 at Amazon – B&H
Update @ March 24, 2012
High ISO Sensitivity test done at dcfever.com (chines website)
settings – Canon EOS 5D Mark III: EF 100mm f/2.8L Macro IS USM, f/8.0, AWB (Jpeg), the default of NR, the IS Off,
the Nikon D800, tripod shooting : AF-S VR Micro-Nikkor 105mm f/2.8 G IF-ED,, f/8.0, AWB (Jpeg), the default of NR, the VR Off tripod shooting
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Canon 5D mark III vs Canon 5D mark II