One figure hidden away in every SLR's spec is the size of the viewfinder (often in a format that makes comparison between competing models impossible). The size of the viewfinder is a key factor in usability - the bigger it is, the easier it is to frame and focus your shots, and the more enjoyable and involving process it is.
Because of the way viewfinders are measured (using a fixed lens, rather than a lens of equivalent magnification), you also need to take the sensor size into account, so the numbers in the diagram below are the manufacturer's specified magnifications divided by the respective 'crop factors'.
The EOS 5D Mark III's viewfinder size has, with its 100% coverage, slightly increased in size over the 5D Mark II's (98%), although magnification stays the same. It's not quite on the same level as the EOS-1D X, but close, and substantially bigger than the viewfinders in APS-C DSLRs such as the Canon EOS 7D. It offers 100% coverage of the final image, with no crop - what you see is what you get.
The viewfinder in the 5D Mark III has been improved over it predecessor, now offering an approximately 100% coverage, rather than 98%. Magnification remains the same at 0.71X but the viewing angle has been increased and the eye point raised to 21mm.
Set below the prism itself is a translucent LCD that can be used to show different grid lines and to give a clearer understanding of which AF points are active. The focus screen is now fixed, though, so there's no equivalent to the 'S' type screen for more-accurate focusing with fast lenses.
The other advantage to the viewfinder's electronic overlay is the addition of a customizable warning. Depending on how you like to shoot, you can configure the camera to show a warning exclamation mark (!) in a small triangle at the bottom right of the viewfinder. It can be configured to signify one of the following shooting parameters is active:
Auto Lighting Optimizer
At the heart of the 5D Mark III is its newly-developed 22MP 24x36mm CMOS sensor. It offers a standard ISO range of 100-25600, expandable down to ISO 50 (with the loss of a stop of highlight range) and right up to 102,400.
The 5D Mark III gains the customizable M-Fn button first seen on the EOS 7D, that's now standard on high-end EOS models. This can be used to activate a range of functions, including the electronic level display in the viewfinder.
The mode dial now has a central lock button, and the previous 'green square' full auto and 'Creative Auto' positions are consolidated to a single 'Auto+' mode (as seen previously on the EOS 600D).
Underneath it is a 7D-style power switch, that's now completely separated from the rear dial-lock switch.
The IR remote control receiver is now on the front of the handgrip, where it's easier to activate when working from behind the camera.
The red lamp beside it is the visual indicator for the self-timer. The 5D Mark III has no built-in AF illuminator.
Below these is the large depth of field preview button, placed for operation by the third finger of your right hand. It's now much easier to reach when using large lenses or shooting in portrait format.
As usual there's a hotshoe on top of the pentaprism, that accepts Canon's EX series flash units such as the newly-announced Speedlite 600EX-RT.
Like previous 5D-series cameras, the Mark III doesn't have a built-in flash and therefore can't control external flash units remotely - you need to use the ST-E2 or new ST-E3-RT transmitters (or a controller hotshoe flash).
The 5D Mark III gains dual SD and CF card slots. It allows the same file management options as 1D-series cameras, so you can duplicate all files to both cards, or record JPEGs to one and RAWs to the other, for example. You can alternatively set the camera to auto-switch to the second card when the first is full.
You can also use Eye-Fi cards in the SD slot.
The comprehensive bank of connectors adds a headphone jacket for monitoring audio when recording video. Aside from this there are USB and HDMI connectors, a stereo microphone socket, PC studio flash and the familiar E3-type remote control socket.
The Q button brings up a context-sensitive Quick Control screen, e.g. to change shooting parameters.
The rear control dial gains a new trick - becoming a touch-sensitive 4-way controller when shooting video (with the 'buttons' placed on the inner rim of the dial). This allows use of the Q menu without jogging the camera, and because it doesn't click, it shouldn't disrupt your soundtrack.
The microphone is repositioned on the camera's shoulder, and as on the Mark II it is monaural.
The 5D Mark III uses the same 7.2V, 1800mAh LP-E6 battery as its predecessor. As with most Canon SLRs, it's housed in the handgrip. The compartment door is sprung, and has foam sealing around it to help prevent water ingress.
The compartment is also sufficiently far from the tripod socket for the battery to be changeable on many heads.
The tripod socket is positioned in-line with the lens axis, and is surrounded by a decent-size rubber pad for a quick release plate.
New accessories - Radio-Controlled Wireless flash and grip
Alongside the 5D Mark III Canon has also introduced a range of accessories, most notably a new wireless flash system that's based on radio, as opposed to infrared communication. Spearheaded by the weather sealed Speedlite 600EX-RT and the WT-E3-RT wireless transmitter, the switch to radio control greatly increases the system's operational range to 30m, removes the requirement for unobstructed line-of-sight communication between Speedlites and the controller, and increases the number of units that can be used to 15 flashes in 5 groups. There's also a hot-shoe-mountable GPS unit, the GP-E2, and a WFT-E7 Wi-Fi transmitter.
Rounding off the story is a new vertical grip for the 5D Mark III, which has an almost-complete set of replicate controls for portrait-format shooting, including the all-important joystick for AF point selection (only a DOF preview button is missing). It can hold a pair of LP-E6 batteries for double the battery life, or run off a cassette full of AAs.
Hands-on Preview Video*
*This video was originally published as part of our Canon EOS 5D Mark III preview