Overall Impression
During the short time I was able to see the 5D MkIII, I was very impressed with the overall feel. The ISO sensitivity and noise levels stand out as some of the main features, but also of course the AF system. This is exactly the same as the 1D X, but is slower due to only having the single processor. For wildlife, I think it is ideal. It isn’t often that the motor drive is needed, I think I can count on both hands in two and a half years, the number of times that I have needed it on the 7D and if the AF matches the specs, then that will be a huge improvement, as will the IQ from the full frame sensor. The downside is the loss of reach from the crop factor, but I can keep the 7D to cover that problem. For landscapes, it will give me what I have now and probably a bit more too, especially if the DR is improved. If I was shooting primarily landscape photography, then it probably wouldn’t be worth the upgrade, but for wildlife, it will expand on what I can achieve, with improved AF and improved ability to shoot crepuscular subjects in low light.
For those who want full frame, but either can’t afford the 1D X or don’t need the extra features the 1D X has and shoot anything other than (or as well as) landscapes, then the 5D MkIII is the obvious upgrade path, but if you need the weather proofing bullet proof AF or any of the other features of the 1D X (such as the new anti-pixel blur), then that is the only option. Many who had ordered two 1D X’s, are now considering changing their pre-orders to one of each camera.
I’m sure there are some things I missed (we elected to skip the video bits) and there are other things that will come to light as more people see the camera and reps become more used to it, but hopefully this gives some insights into my first impression.