While Canon made a lot of little--and important--tweaks to the camera's design, physically it feels fundamentally the same as its predecessor.
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The back controls have been moved around a little for more streamlined operation. These include a 7D-like button/switch for Live View and movie capture and the addition of a touch pad to the control dial for silent adjustments during movie shooting. The viewfinder now offers 100 percent coverage, and the camera incorporates the same 3.2-inch LCD that Canon's using in all its current high-end dSLRs.
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Dual card slots
Not only is it great that the camera has both CF (UDMA 7) and SDXC card slots, but it adds a nice twist to the more common myriad options for them: you can also configure to save say, large raw files to one and small to the other, or differently compressed JPEGs to each.
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The 5DM3 uses the same menu and control-configuration system as the 1D X, which in turn copies some of the 7D. I've always found this screen for configuring the various controls extremely well designed.
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Locking mode dial
Canon adds a locking mode dial to the camera, which is good, but it's the awkward center button that debuted in the 60D.
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Status LCD and top controls
The status LCD is the same, although it has a few more readouts to accommodate some of the new features, like silent burst shooting. There's also a tiny new programmable button on top. The other controls--metering, white balance, AF mode, drive mode, ISO, and flash compensation, as well as AF lock, AD lock, and focus-area selection--remain unchanged.
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Canon adds a headphone jack to its set of connectors. Unfortunately, the HDMI still outputs only with screen overlay.
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Redesigned battery grip
While both the camera and the optional vertical grip use the same batteries as the 5DM2, the grip has been slightly redesigned to add a jog controller for improved vertical handling. There's also a duplicate of the manual function button by the shutter.
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New flagship flash
The 580EXII is getting replaced by the Speedlite 600EX-RT, Canon's first flash with built-in wireless master capability (it supports RF or line-of-sight for backward compatibility). It supports up to five groups flashes in up to 16 channels, plus Canon claims it's got a 20 percent faster recycle time.
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Just the flash transmitter
If you just want the wireless transmitter with no flash attached, Canon has extracted the 600EX-RT's guts and created a transmitter-only product, the ST-E3 RT.
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Unlike the slim GPS module that the company announced with the 1D X, the 5DM3 is joined by a big hot-shoe model. It does have GPS logging capability, though, which allows you to track your movements between EXIF data location snapshots.