Though I've had the opportunity to use the 1D4 for nearly two weeks now, sadly I've been ridden like a pony at work leaving little time to do much else. Well until now. The 1D4 is an impressive camera and there are already several reviews out there. Some comparing it to the Nikon D3s, others to the 1Ds Mark III. I won't rehash a lot of whats already been said. I am more interested in what this body can offer for my photography.
The foremost thought on most people's minds is what the auto focus is like. I didn't experience any of the more severe issues with the 1D Mark III. The most notable thing about the AF on the Mk 3 series in my mind is that it has more of a tendency to jump to the background when the AF point leaves the subject (even for a moment) and refuses to come back onto the subject.
The 1D Mark IV tends to jump away less and when it does, if you bring the AF back onto your subject, it returns quickly. I photographed this Ibis hand holding my 800mm lens and it was challenging to keep the AF point on the on the bird's head. At 10 FPS, it would move off the bird and onto the background as the AF point drifted (due entirely to my weak arms), but would almost instantly snap back onto the bird when I got the AF point back on target. This was not something I could get the Mark III bodies to do even with experimenting with all kinds of AF custom functions.
The other big question is image quality, especially at high ISOs. The image quality is decent. On a per pixel level, noise levels are generally comparable to the 1Ds Mark III, though the 1D Mark IV seems to have less noise in the deep shadows. This may not seem impressive as the 1Ds Mark III is over 2 year old technology, however keep in mind that the pixel density of the 1D Mark IV is higher, so progress has been made. The 1D Mark IV is definitely a better choice over the 1Ds Mark III for focal length limited photography.
The current champion of focal length limited photography is the 7D and one question I was very curious about is figuring out at what focal length the noise levels of the 7D negate its ability to resolve details.
The 1D Mark 4 files even at ISO 1600 require little noise reduction in post processing and take sharpening very well. In contrast the 7D does require some work at and above ISO 800 to get the very best out of it.
My own quick testing indicates that the 7D is still king of resolving detail for focal length limited photography up to ISO 400. At ISO 800, its possible to get more detail with the 7D with work (especially if the image is well exposed or exposed to the right). At ISO 1600 and beyond I'm not sure the 7D offers much more in terms of resolved detail and the 1D Mark IV files post process a lot easier.
This last image is only one in this post taken with the 7D, it was taken at ISO 400 but was a little under exposed so had to be pushed by about half a stop. Even this results in noise which comes out in the smooth areas when sharpening and makes it more pronounced than a similar image I shot with the 1D Mark IV at ISO 1600 (the first one).
I have a few more thoughts regarding auto focus performance in extreme low light, but thats a subject for another post :)