A bit bigger
It's a bit larger and heavier than the 5DM2, as well as the D800, but it doesn't fee too heavy and it's well balanced. It's a little more comfortable to grip than the 5DM2, thanks to a "stickier" texture. While the body isn't as tough as the 1D series, it has a little more dust-and-weather sealing than its predecessor.
Modernized interface
As you'd expect, the 5DM3 incorporates some of the more modern aspects of Canon's dSLR designs, including a quick-access menu and easily acessible movie recording.
Dual card slots
The ability to take both a CF card and an SD card has (thankfully) become an essential feature in this class of camera.
More configurable
Another welcome change from the 5DM2 is how much more physically configurable the camera is. In addition to this programmable function button, you can assign another function to the depth-of-field button (moved to the right side for operation with your right-hand ring finger).
Mode dial and power switch
Though I'm sure a lot of people will welcome the lockable mode dial, it's got the design I dislike, which debuted in the 60D with the pushbutton lock in the middle of the dial. I also don't like the new location of the power switch; I don't know whether it gets moved when I put the camera in my bag or take it out, but whenever I take it out it's set to "on."
Ports and connections
There are no new connections on the 5DM3, though the AV Out jack is now a dedicated headphone jack.
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