The DxO Mark Score of Canon 1DX is not confirmed yet but take a look at Canon 1Dx vs 5D Mark III High ISO images comparison, the Canon 1Dx images showing better color and more details at High ISO compared to 5D Mark III.
If the lighting conditions you’re shooting in doesn’t require an ISO over 3200, both cameras will deliver almost identical quality images. However, if you need to shoot with an ISO 6400 and above (especially at 12,800 and above), the Canon EOS-1D X will deliver a superior, cleaner image. This does not mean the Canon EOS 5D Mark III cannot render deliverable images at ISO 12,800, it just means that the Canon EOS-1D X controls the noise in the shadow area much better in a high ISO. See it for yourself in the images below.
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.
Canon fixed the 5D mark III light leak issue with a simple black tape, LensRentals.com(bulk buyer) received the first shipment of fixed 5D Mark III bodies, they disassembled one to see changes and found that canon used a back tape under the LCD screen to resolve the light leak issue.
Canon 5D mark III Before and After Images 5D Mark III Before the Fix 5D Mark III After the Fix
A disappointment to Mark III owners hoping for some kind of better solution
More about Light Leak
See Cannon issued a canon product advisory to 5D Mark III owners , according to Canon Products whose sixth digit in the serial number is 1 or 2 are affected, For example, “xxxxx1xxxxxx” or “xxxxx2xxxxxx” ([x] represents any optional number.
Cannon issued a Product Advisory realted to 5D mark III light leak issue, according to Canon Products whose sixth digit in the serial number is 1 or 2 are affected, For example, “xxxxx1xxxxxx” or “xxxxx2xxxxxx” ([x] represents any optional number.
Canon says Under almost all shooting conditions (including dark environments) this phenomenon will not affect your captured images. However, if you would like Canon to inspect your camera, Canon will provide this service free of charge upon request beginning in mid-May.
Press Release
Canon Product Advisory to 5D Mark III Owners
To Users of the Canon EOS 5D Mark III Digital SLR Camera
Thank you for using Canon products.
Concerning the EOS 5D Mark III digital SLR camera, when the LCD panel illuminates in extremely dark environments, the displayed exposure value may change. Canon has concluded the investigation of this phenomenon, and this announcement informs you of our findings as described below.
Phenomenon
In extremely dark environments, if the LCD panel illuminates, the displayed exposure value may change. However, based on the results of extensive testing this change in exposure value will not noticeably affect the captured image.
Affected Product
Canon EOS 5D Mark III Digital SLR Camera
*Products whose sixth digit in the serial number is 1 or 2 are affected.
For example, “xxxxx1xxxxxx” or “xxxxx2xxxxxx” ([x] represents any optional number.)
Support
Under almost all shooting conditions (including dark environments) this phenomenon will not affect your captured images. However, if you would like Canon to inspect your camera, we will provide this service free of charge upon request beginning in mid-May. Please contact Canon using the information below to request service.
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
Contact Information for Inquiries
Canon Customer Support Center
Phone: 1-800-OK-CANON
1-800-652-2666
TDD: 1-866-251-3752
Email: carecenter@cits.canon.com
For additional support options: www.usa.canon.com/support
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.
A video Test done by Joe Marine, Nikon D800 sensor captures more light compared to 5D Mark II and 5D Mark III under low or High ISO settings, but both 5D Mark II and 5D Mark III have less noise at high ISO with bit flat details due to in-camera noise reduction filters.
Nikon D800 performance is excellent up to ISO 1600 the sensor captures more details compared to its competitors
At High ISO Both 5D mark III and II have less noise,
Personally I will prefer 5D mark III or II for video, Which camera do you prefer? – Nikon OR Canon – Share your thoughts with us
Video Details
5D Mark II vs. 5D Mark III vs. D800
Candlelight/Ambient Light Test
ISO 500-25,600
The only light in the room besides the candle was some ambient light from streetlights outside. The f-stop was kept consistent throughout. The Canon footage was converted to ProRes HQ using 5DtoRGB, with the Mark II set to 601 color matrix and the Mark III set to 709.
Nikon 85mm f/1.4 lens
Settings:
Default/Neutral Color Profiles
Canon ALL-I & Nikon KiPro ProRes 4:2:2 HQ
Noise Reduction Off
WB: 3200K
Shutter 50
F-stop stayed the same throughout – F/1.4
Music:
“John Stockton Slow Drag” by Chris Zabriskie (chriszabriskie.com)
Shutter 50 F/1.4
Buy 5D mark III from Amazon || Buy 5D Mark II from Amazon || Buy Nikon D800 from Amazon
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
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Cannon issued a Product Advisory realted to 5D mark III light leak issue, according to Canon Products whose sixth digit in the serial number is 1 or 2 are affected, For example, “xxxxx
DxOMark published the result of 
The LCD backlight in a dark room or sunlight / bright lights over top LCD of 
