One of the special features of the Canon S1iS is image stablization, a feature that Canon mentions twice on the camera: "Image Stablization" on the lens and "IS" in the product name. By default, it is turned on.
When should image stablization be turned off?
* When the camera is on a tripod. The purpose of IS is to let you hold the camera, taking telephoto pictures. Normally, the 10x telephoto zoom amplifies your shaking hands. Image stablization reduces that problem. When the camera is on a tripod, your hand-shaking is no longer a problem.
* When taking time lapse phototgraphs. You want the camera rock-steady when taking time lapse photos; otherwise, the resulting images jump around. When IS is turned on, the image stablizer does just that: the images move slightly from frame to frame.
* When the swimming image gives you motion sickness. The image stablizer causes the image in the viewfinder to swim around sometimes, creating a slight feeling of motion sickness -- equivalent to looking over the edge of a tall structure that moving in the wind.
* When the shutter speed is high. Camera shake is only a problem at lower shutter speeds. The rule-of-thumb is that the minimum shutter speed should be twice the focal length of the telephoto lens (converted to 35mm film size). The maximum (optical) zoom of our Canon S1iS is the 35mm-equivalent of 380mm; thus, the minimum shutter speed should be 380 x 2 = 1/760 sec to avoid telephoto-induced shake in the image. On bright sunny days, when the shutter speed is 1/1000 sec, turn off IS.
* When IS gets in the way. I have found two drawbacks to the image stablizer:
(1) when I am framing a shot, IS causes the image to move, and the resulting frame is not what I want.
(2) when I am trying to take a picture of a quickly-changing scene, IS can take too long to settle down, and the resulting picture is not what I want.
For these reasons, it makes sense to turn off IS.