It is also called, depending on your camera manufacturer, Image Stabilisation or Vibration Reduction. Irrespective of the name, the idea is the same – if you are hand-holding the camera, there will be some movement of the camera while the shutter is open, giving some amount of blur to the resulting image. SR/IS/VR attempts to compensate for this movement to give a sharper image. We do not need to consider how it does this, except to mention that in some cases the technology is built into the lens and in others it is in the camera body.
Now, if there is plenty of light, the shutter is only open for a very short time, insufficient to have any significant shake captured. Also, the shorter the lens focal length, the less any shake will be magnified, so we are really concerned with lower light and longer lens situations where the camera is being hand-held. At this stage it is worth recalling the “rule” for hand-holding – take the focal length of the lens as a fraction of a second as the limit of exposure time. For a 50mm lens, don’t hand hold for more than 1/50th of a second. For a 300mm lens, the limit would be 1/300th of a second. For those of us with APS-C size sensors the limit is increased by a half, so the 1/50th becomes 1/75th, and the 1/300th becomes 1/450th. While most times there is enough light to allow something faster than 1/75th, even quite bright days with a mid-range aperture will struggle to allow faster than 1/450th.
So, it is a dull day, you are using a 300mm lens, APS-C sensor, you want f11 to get a reasonable depth of field, and the suggested shutter speed is 1/100th. The rule says don’t hand hold. You could, indeed you should, put the camera on a tripod, but maybe, despite being an avid reader of our tips you have forgotten to bring it. You could put the camera on a convenient wall, if there is a convenient wall, and use the built-in timer, and hope that in 12 seconds the subject won’t have moved. Or you could turn on your shake reduction, which will give you about 3 stops. From 1/450th, one stop gives 1/225th, another gives 1/112th, and the third gives about 1/60th. We only need 1/100th, so SR/IS/VR saves the day!
One of the nice things about SR/IS/VR is that, most times, it does not do any harm, so rather than worrying about it, you can just leave it on all the time. However, there are a couple of situations where it should not be used. One is when the camera is on a tripod, so unless you have a recent Canon camera, turn it off when using a tripod. (Recent Canons automatically shut it off when on a tripod). The other principal case is in really low light situations, such as night shooting, but then you’ll be using a tripod anyway.
The tip, then: when you take the camera off the tripod, turn SR/IS/VR back on. When you put the camera on the tripod, turn it off. And if you use a Canon, turn it on and forget about it.