Well, actually, there are instances when you don't need it.
One instance is when you're shutter speed is fast. There is a rule that says you should shoot at a speed that is equal to the inverse of the focal length of your lens. That is, say you have a 50mm f/1.8 lens, then you should not shoot below 1/50 of a second; anything slower and you should use image stabilization (IS for Canon cameras, VR - vibration reduction - for Nikon cameras) or else camera shake will be apparent in your photos. (You can also increase the ISO value of your camera to retain the minimum shutter speed required.)
Things get a little complicated when you start using long lenses, like a 100mm f/2.8L macro lens. See, the long lenses are, well, longer, so they also tend to be heavier. Thus, holding them becomes more difficult and camera shake will be more visible in your shots. So, for the example here, you should only shoot without IS if your speed is faster than 1/100s. Just imagine shooting with a Canon 400mm lens!
However, when your shutter speed is faster than the inverse of the focal length of your lens, say 1/200s while using a 24-105 f/4L lens, then you can turn off your IS so as to save some battery power.
But, there is another occasion when you can turn off IS; actually, in this instance that I am about to tell you, you should turn IS off: when your camera is on a tripod.
I actually discovered this through an accident. I thought that my aperture setting was set at the biggest opening of my lens because I was getting some "bokeh". So I set my aperture to something bigger, like f/16, and the "bokeh" became worse! I really couldn't tell from the small screen at the back of the camera so I loaded the photos in my computer. It turned out the problem was camera shake, not bokeh. And I realized that, when I made the aperture smaller (I was shooting in aperture priority, Av), the camera shake became worse because of the longer shutter speed.
So, I turned the IS off, and everything was right in the world again.
Now, I am focusing here on Canon lenses and Nikon lenses since IS (or VR) for both cameras are built in the lenses. For Sony cameras, however, IS is built in the body so I don't know if those cameras will be affected by the same phenomenon. Sony owners, you can chime in at the comments section below. Also, I noticed that for point-and-shoot cameras, I could not see any difference whether the shot was taken with the camera on a tripod or off.
So, when do you turn off your IS? When you are shooting with a very fast shutter speed and when your camera is on a tripod.
|IS turned on. Look at the Lowepro logo all blurred.|
|IS turned off. Now we have a very good photo.|
|With a point-and-shoot, I really can't tell the difference... IS turned off.|
|With a point-and-shoot, I really can't tell the difference... IS turned on.|
Shots of the Lowepro DMC-Z made with a Canon 5D Mark II. Want the best bag for your DSLR? Consider my favorite bag, the Lowepro Versapack 200AW. My review of the bag can be found here and here.
Shots of the iPhone made with a Canon SX230 HS. My review of the SX230 can be found here. A very nice case for the SX230 HS can be found here.