Before we start, just a quick reminder of 6D Major specification – Newly announced Canon 6D features new 20.2-megapixel full-frame CMOS sensor and DIGIC 5+ Image Processor, 11-Point AF with Center Cross-Type Point, 63-Zone Dual Layer Metering Sensor, Extended ISO Range of 50-102400, Up to 4.5 Full Resolution FPS and Built-In HDR and Multiple Exposure Modes.
Building upon the qualities that made the EOS 5D Mark II camera so successful, the EOS 6D bridges the gap for budget-minded photographers, videographers and cinematographers who are eager to step up into the world of full-frame imaging. but how this camera stand in front of 5D Mark III, let’s see the specification comparison below
It’s no doubt that Canon 5D Mark III have a very advance AF system compared to 6D,
Canon 6D and 5D Mark III share same ISO range, The AF system of Canon 5D Mark III is advance compared to Canon 6D but the core specification of both the camera says
Canon 6D AF Working Range : Center AF Point: EV -3 to 18 Canon 5D Mark III AF Working Range: EV -2 to 18
AF working range also depend on type of lenses you use, but under test condition the Canon 6D (center) AF point will perform better under low-light environment compared to 5D Mark III.
Conclusion
Canon 5D Mark III have more shutter range of 1/8000, Fast continuous shooting speed with bit more megapixel, better LCD screen and a 100% viewfinder.
Canon 6D will cost you approx $ 1.3K less than Canon 5D Mark III, features Full Frame sensor and same ISO range as 5D Mark III with DIGI5+ image processor and built in WiFi with GPS.
The decision is your’s but being a nature photographer I will prefer 6D to save few bucks for new lenses.
Comparison between Canon 5D mark III and Black Magic Cinema Camera is bit odd, since BBCC have sensor smaller than the M4/3 format, but the small sensor camera performed surprisingly good during the test, the results are difficult to digest for Canon.
DxOMark published the result of Canon 5D Mark III sensor, the overall score of 5D mark III sensor is 81, before jumping to any decision please remember that the test score of DxOMark is based on Sensor RAW output, the low-light score of Canon 5D Mark III is 2293 ISO, Canon EOS 5D Mark III nearly ½ Stop better in low light conditions compared to Canon 5D Mark II (1,815 ISO).
The dynamic range score is almost same as 5D Mark II, the 5D mark III shows its improved performance behavior at high ISO test only.
Canon 5D Mark III vs Nikon D800
On sensor war Nikon D800 is a clear winner, the overall score, dynamic range, portrait color depth and low-light score is better than the Canon 5D Mark III.
The LCD backlight in a dark room or sunlight / bright lights over top LCD of 5D mark III causes the exposure to change, to test it you have to put a body cap and also cover the viewfinder, Then place the camera in “P” mode at ISO800 and press the LCD backlight button.
OR put your 5D mark III under the bright-sunlight
Canon today announced a product advisory for acknowledging the light leak issue,
To Users of the Canon EOS 5D Mark III Digital SLR Camera
Thank you for using Canon products.
The phenomenon described below has been confirmed when using the Canon EOS 5D Mark III Digital SLR Camera.
Canon is now examining the countermeasures and once the countermeasures are decided, we will post the information on our Web site.
Phenomenon
In extremely dark environments, if the LCD panel illuminates, the displayed exposure value may change as a result of the AE sensor’s detection of light from the LCD panel.
Affected Product Canon EOS 5D Mark III Digital SLR Camera
Support Once the preparations are complete, we will be making an announcement on our Web site.
This information is for residents of the United States and Puerto Rico only. If you do not reside in the USA or Puerto Rico, please contact the Canon Customer Support Center in your region.
Please register the EOS 5D Mark III. By registering, we will be able to notify you via email when service updates are available. If you already registered, please ensure you are opted-in to receive the notification.
Thank you,
Customer Support Operations
Canon U.S.A., Inc
Low Light: ISO 6400 Video Samples – Canon 5D Mark III
Quick low light with the Canon 5D Mark 3 in low light. Every shot in this video was filmed with a shutter speed of 50, F/2.8 and at ISO 6400.
Please notice: This was just a quick test. A longer low light short film wll come out in the next few weeks!
Canon 5D Mark III: Timelapse Test
Quick time lapse test with the new Canon 5D Mk3. The special motion blur look was created by using ND filters to shoot with a longer exposure than usual in daylight.
We changed the contrast and colors a little bit in Adobe After Effects because it looked too flat and the weather was really sad and grey…
2K video resolution: 2048×853
Lenses: Canon 85mm F/1.8, 15mm F/2.8
+ one more Canon 5D Mark III: Photo Shoot
Behind the scenes at a sexy photo shoot under low light conditions with the brand new Canon 5D Mark III.
Canon 5D III vs. Canon 5D II vs. Nikon D4 High ISO test images, the Canon 5D mark III images have remarkably less noise compared to canon 5D mark II, Nikon D4 preserves more image details at high ISO compared to 5D mark III.
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.
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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Before we start, just a quick reminder of 6D Major specification – Newly announced Canon 6D features new 20.2-megapixel full-frame CMOS sensor and DIGIC 5+ Image Processor, 11-Point AF with Center Cross-Type Point, 63-Zone Dual Layer Metering Sensor, Extended ISO Range of 50-102400, Up to 4.5 Full Resolution FPS and Built-In HDR and Multiple Exposure Modes.
DxOMark published the result of 
The LCD backlight in a dark room or sunlight / bright lights over top LCD of 
