Actually IS is not ineffective or should I say inactive at fast shutter speeds. It will actually do a lot to ruin a shot. If turned on it is always working to detect camera motion or shake. At higher shutter speeds it will perceive a moving object you are shooting as camera shake and try to counter act it which will introduce blur to an image. So it will actually do the exact opposite of what it is supposed to do. If you read the manual for the lens it always says turn off IS when using a tripod or faster shutter speeds. IS is designed to be used at shutter speeds slower than one could normally hand hold without movement. For example, As a guideline, if normal handheld is 1 over the focal lenght on a 300 mm lens is should only be used below a shutter speed of 1/300. On a 50mm lens on speeds less than 1/50. This is why the OP is getting sharper pictures of moving objects by upping the shutter speed and turning off the IS

I can't imagine photographing wildlife (except possibly birds in flight) without using a tripod. The tripod needs to be sturdy. Too many people buy light weight tripods rather than getting ones that will properly support the weight of their lenses. I teach photography at a college and my advice to students is that their first accessory to their camera should be a sturdy tripod. They should imagine the largest lens they will ever use and buy a tripod for that lens. A person will end up buying fewer tripods that way. Take it from one who knows from experience.

I have recently been taking hand held eagle, wood stork, and egret pictures with medium telephoto (Nikon 55-300 and Panasonic 100-300) lenses. In most cases there is a little softness, not enough to totally ruin the picture, but not the sharpness I would like. I have experimented to try to determine if the problem is focus, camera shake, or inferior lens. Depending on the answer, the solution could be new camera, new lens, or always use tripod. I would hate to lay out the money only to find I have diagnosed incorrectly and there is no improvement.

I have concluded that the problem is camera shake (I have reasonably steady hands, but am 72, so not perfect). With cameras like the new Olympus advertising 5.5 "stops" of IS, it occurred to me why not just move the ISO 3 or 4 stops in my existing cameras and gain the resulting shutter speed. If my camera and lens IS is 2 or 3, maybe that would accomplish the same thing. There is no appreciable noise at 800 or even 1600, especially in sun light. I tried it and got much improved results. (I was already shooting at 1/effective focal length or more, so the above was at numbers like 1/3000.)

So my question is what am I missing? The above seems way too simple. Is there a direct correlation with shutter speed and image stabilization, or is something else going on? Any thoughts would be much appreciated.

So, what am I missing?