I am reasonably certain we have all run into situations where we simply can not hold a camera steady enough to avoid smeared images due to camera movement… that's why tripods were invented, right? But over the years we've seen lenses and camera sensors that offer real help in this regard. Optical image stabilization ties a gyroscope to a lot of software that measures camera movement and applies signal to motors controlling elements in a lens (or the camera sensor itself) that shift the image in real time in an inverse manner to minimize the motion the camera sensor sees. Most cameras/lenses with this technology claim a 3 to 4 f-stop improvement. This means being able to use a 8x slower shutter speed and stil get a capture that is not blurred by camera movement. So what's not to love… turn it on and leave it on, right?
Nope… And Here's Why
The first thing to consider is that if your camera is already steady enough for a sharp photo at the desired shutter speed, how could this mechanism possibly help? It can't. It's not perfect, and is designed to reduce the effects of camera movement, not eliminate it entirely. You don't want this thing whirring about if you don't need it… so when don't you need it?
When Using a Tripod
If your camera is on a tripod, turn OIS off. Failing to do so will usually result in unwanted movement of the lens shift element/sensor that actually causes motion blur where there would be none without it. While some cameras/lenses have algorithms to sense when things are steady, and go into a "tripod mode" automatically, why count on that decision making process when you can easily flip a switch and be certain it is off?
Here are two 100% crops from two identical photos shot at 200mm-f/6.3-1/4 sec, one with VR off, and the other with it on. Notice the degradation of the latter in both sharpness and contrast.
If your predominate source of light is from a strobe, turn OIS off. The amount of time the light from a strobe is flashed on to your subject is a tiny fraction of a second that is simply not long enough for typical hand held camera movement to be a factor. Of course if you are exposing for the ambient light in your background and using flash to fill the foreground, you may want OIS on if your goal is to keep that background sharp, and a slower shutter speed is needed to accomplish that.
High Shutter Speeds
When shooting sports (or any moving subject), high shutter speeds are required to 'freeze' that motion (OIS is only effective with camera movement and can't help with moving subjects). 1/1000th, 1/2000th, and even higher can be commonplace. You may very well find that with all but the longest of lenses, these shutter speeds will give you better results with OIS turned off.