Ray, i'm not sure where your 1/4fl comes from. 1/fl for a long hand-held lens without IS may be a bit optimistic, but with a sequence of multiple images there's a pretty good probability of some keepers. the shutter speed required for action has nothing to do with the focal length - only the speed of motion of the subject which for wildlife is 1/500 to 1/1000 at most (except some small birds and amimals, and of course hummingbirds). in reasonable light, the problem of handholding the 400 + extender is more about getting focus on the subject than shutter speed and ISO
The issue was raised recently in the review of the Nikon D800. The claim was made by Michael, and supported by others, that in order to benefit from the unusually high pixel density of such a camera, one may need to use a faster shutter than one is used to using with lower resolution cameras.
1/FL even with VR was considered to be inadequate to get that extra resolution the D800 is capable of. I claimed that it wasn't inadequate, and took a number of test shots, hand-held, using my D7000 with 24-120/F4 zoom to demonstrate my point. But I admit the test was flawed because I used a zoom lens instead of a first rate prime. I don't have any Nikkor primes.
No-one mentioned this, not even Slobodan. A bit slack really.
The pixel density of the 7D is equivalent to that of a 46mp full-frame sensor, a bit higher than that of the D800, so such reasoning would be even more relevant to the 7D.
Whilst it's true that there is always a certain degree of variability in the sharpness of results, due to the variability of camera shake, a shutter speed of 1/FL with the older versions of VR and IS, seemed about right to ensure a sharp image most of the time. With the newer versions of image stabilization, such as VRII, that claim up to a 3 stop advantage, one may get away with a shutter speed of 2/FL. I don't know. I haven't got any such lenses.
However, I agree that taking multiple shots in continuous mode will increase your chances of getting a sharp image, whatever the shutter speed.
As regards subject movement versus camera movement, I haven't carried out any tests. One might assume if the shutter speed required to freeze subject movement is faster than 1/FL then that will also take care of any camera shake, if the lens has IS or VR.
If the lens doesn't have IS, it may be a different kettle of fish. We have two competing sources of movement. If such movements are in opposite directions at the precise time the exposure is taken, we may get a blurred shot.
the 100-400, 400 5.6 and over-priced 400 DO are overdue for up-grade - maybe because Nikon still has no equivalent for any of these lenses
Absolutely! I would like Nikon to take the plunge and design a first-rate 200-400/F5.6 which is significantly sharper at F5.6 than at F8 and costs no more, or little more, than its 80-400/F5.6 which is a bit behind the Canon 100-400/F5.6.