Sorry to be back. I'm just reporting my results, because I feel I had to contribute some input and not simply ask for other people's knowledge.
Now.. I've been around recently, bothering you people all about how to shoot fast moving animals with a red one, avoiding stutter... My specific issue was : how to shoot a golden eagle diplay flight, meaning high speed aerobatics, stooping, , etc., when either some background might be present (not totally out of focus), and/or framing has to be adjusted suddenly in order not to loose the subject because of unpredictable moves and very fast changes in direction, requiring quick reframing. No albatros/pelican/vulture gliding, no swan taking off, no linear move, but rather something like a cheetah after an antilope desperately trying to escape.
Shots had to go from wide to as close as consistent with keeping the eagle in the frame. I had hoped I'd be able to avoid fps higher than 50fps, because I wanted the 3k resolution, and because corresponding higher shutter speeds would require very good light conditions.
I got many replies to my posts, public or private, all interesting and valuable. Some suggesting 3k, 50 fps, shutter speed higher or lower than 180°, some suggesting I had to go 2K. Many implying my video background (50i/60i) had to be sort of emulated in order to get corresponding results.
So, things definitely had to wind up like me, my red one, 6 battery packs, tripod and 300m 2.8 plus water and food, all atop that huge cliff in the middle of nowhere. Actually, eagles were on my side that day, allowing me to repeat the actual shot and try different combinations. And the answer is : you simply can't shoot that kind of shot 4K, and you can't 3K either. As simple as that... I hear some say :*«* of course, just what I said... what else could you expect*?». Ok... Let's assume I'm just speaking to people that know quite a lot about wildlife, and maybe not quite so much about shooting techniques.
To these, it's enough to say that 50 fps won't suffice, shutter speed being either less than 1/100e in order to increase motion blur, or 180°, or up to 1/200e , whatever you might be willing to try. 3K image is just gorgeous, no question, but there are times when the relative speed of the subject/camera will inevitably cause some stuttering, occasionally ruining the shot. Playing slo-mo does not change a thing, of course, except making your suffering longer when you sit watching the dailies..
No surprise to many, then. However, more surprising may be, you won't be able to shoot the same eagle (vulture, bird, whatever) simply soaring/circling 3K, unless the very moment it's facing you, or unless you avoid any background (a totally blurred, or a very soft one, like water may be, will do, though), and unless you avoid any brisk reframing of the subject. Even against the sky and with no distinct background, if you happen to try to make up for being a bit back or ahead of your subject, your image will stutter, because the relative speed of the camera/subject move will exceed the acceptable limit. Naturally, if you should wish to convey the idea of the speed and power of the eagle and try to shoot very wide, camera steady, not following , just letting the bird fly across your frame.. then stutter will be terrible. Anyway, remember your camera moves will have to be absolutely smooth and match your subject's in such a way it will appear almost still in the frame.
What works perfectly if you go 50 or 60i in video, won't work with 50p. The only solution must be to shoot 2K, 120fps or so. I tried various output formats, for instance 10 bits 50i, only making the issue worse.
Of course, golden eagle is just an example, just happening to be my subject here. But there are a lot of wildlife/sports subjects that are equally or more demanding.
I'm reporting because anyone shooting wildlife, especially those with a video background, will face the issue, and will be in need of that one single, very simple answer : forget about 4K/3K, go 2K.. or wait for Epic X.
Red one is absolutely great for many things, and especially when it comes to wides, landscapes, reasonably still subjects. However, IMHO, when Epic X comes up with a reasonable booting time, with 125 fps 4K, 800 iso, it will be a totally superb, absolutely perfect tool for wildlife shooting.