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It's a camera that just works really well; it feels far more refined and promises to be a much better all-rounder than its predecessor. It addresses the Mark II's most glaring weakness - its distinctly unimpressive autofocus system - in the most comprehensive fashion possible, while adding a huge array of interface tweaks and improvements on top. In essence it finally feels like a 'proper' professional camera, in a that way its slightly pedestrian forebears never quite managed.
The 5D and 5D Mark II were at heart quite simple cameras - the AF system was pretty simple, and the buttons did what Canon had written on them with customization options - but the Mark III is an entirely different beast. From the 61-point AF system and 6 fps shooting, through the silent shutter mode and expanded bracketing options, to the improved ergonomics - everything adds up to a camera that's a joy to use. The additional features such as in-camera HDR and side-by-side playback comparisons are the icing on the cake. We often say that the best cameras are those that get out of your way while shooting and don't impose significant constraints on the process, and the 5D Mark III fits into this category.
Of course the elephant in the room is the question of resolution; will the 5D Mark III's 22MP offer enough in comparison to the Nikon D800's 36MP to prevent users switching brands - especially given the D800's lower initial price? Canon appears to be confident that its users don't really need extra pixels anymore, and indeed don't want to put up with the processing and storage demands that come with them. It will be interesting to see whether that confidence is justified once the two cameras hit the shops - and we can't wait to get them into our studio to put them head-to-head in testing.
Certainly, existing D700 users will feel right at home with the largely familiar ergonomics, but the changes that Nikon has made (particularly the improved live view controls) make a noticeable difference to handling. Not everyone pondering the purchase of a D800 will consider video a priority, but if the feature interests you then rest assured that right now, the D800 offers pretty much the most advanced specification around, an extremely close second only to the D4.
In terms of its core functionality, the D800 incorporates an extraordinary feature set, taking some of the most interesting technology from the D4 and coupling it with the highest resolution 36x24mm sensor on the market. At 36.3MP the $3000 D800 makes the $8000 D3X look distinctly irrelevant, and is right up there with medium format digital equipment in terms of output size (and file size - 36.3MP .NEF files will take up approximately 76.5MB on a memory card). This makes it very attractive, potentially, to studio and landscape photographers who value resolution over speed. This is even more true of the D800E, which without a low-pass filter should in theory get very close to the sort of output that studio photographers would expect from much more costly medium format digital equipment.
Exactly how good the D800's image quality turns out to be is something that we're very keen to establish. On balance, increased pixel count generally counts as a good thing in everyday photography. The most important 'headline' benefits, all other things being equal, are that you get more detail in your images and greater scope for cropping. But there are potential downsides, too. The D3X, Nikon's current flagship, makes very high demands on lenses at 'only' 24MP and we wouldn't be surprised if some of the optics which D700 owners love so much don't shine quite so brightly at 100% on screen when bolted on front of a 36.3MP sensor...