I think many of you must wonder why you should switch off Image stabilization (IS) when using a tripod? IS was meant to suppress vibrations right? And using it on a tripod might get you really, really sharp pictures?
So here is the secret behind it:
Your camera has two types of IS. Digital and optical. Digital IS is something like digital zoom in the way it affects images. It isn't as good as the true, mechanical IS (which uses lenses). There are various ways of implementing digital IS. The most basic and efficient method behind it is to monitor certain points on the screen and as your hand moves in tiny vibrations, it matches the sensor data aquisition rate to that of the frequency/pattern of vibration. That misses most of the displaced frames and gives out a stream of "aligned frames" which aren't perfectly aligned but further proprietary algorithms make it look well aligned. In any case, your photos aren't really "untouched" coming out of a digital IS system.
Now the optical or "physical" IS system. The optical IS employs a magnetic coil around a lens that keeps the lens "floating". When you enable the optical IS, your camera releases the mechanism and it physically moves parts inside your lens barrel to compensate for small, fast vibrations. It is most effective for low magnitude, constant and fast vibrations. Sudden jerks can still shift the internal parts too much and give you a blurred photo. That's why you should use the shutter button with care. No sudden pressing.
What happens when you use the tripod? Suppose now the vibrations are not there at all and the IS mechanism is still active. The magnetic field in the coils causes something of a "resonance" due to mutual induction and feedback. It behaves just like a speaker and starts producing little vibrations. Not noticeable, but they are there. They cause small pixel level blurs and make the whole picture "soft". Bird photos can turn out especially bad.
What you can think of all this is: The optical IS uses constant vibrations when it is enabled. If your hand is shaking and the lens is being shaken by the IS system, your photos turn out steady. Now if the camera is rock steady and IS is enabled, the vibrations can't cancel anything and end up shaking the lens that is already steady.
Sometimes people might just use the IS when the camera is kept on a rock. It is still not a good idea. IS is related to camera shake only!
Not to the fact that it is on a tripod or whatever else.
It all comes down to:
If your camera is "rock steady", switch off IS.
Suppose you are shooting in a windy location, the tripod isn't steady...no use turning on the IS there either. That is because of the fact that wind causes vibrations of a very low frequency and high magnitude. IS cannot deal with it. You can though, using a bob hanging from your tripod to cancel out wind effects. (Use only if wind is severe. Make sure the camera strap isn't fluttering around). People sometimes hang their little point-and-shoot or whatever they find instead of a bob. But again, it would make it worse. A sphere is best in the sense that it isn't sensitive to wind direction.
Another point worth mentioning is that the IS is very inefficient when you're pointing the camera straight up. The IS works best when the lens barrel is horizontal. The "panning IS" might work better with a vertical lens barrel. But better just use a tripod if a low shutter speed is needed and the subject is right above or below you.
NOTE: Everything mentioned above is in a very general manner and true for every system. Thought some new IS systems have the ability to detect a steady camera body. But either way, the above stuff can only make your images better.
Some of my photos with IS (camera horizontal and shutter speed still quite low - but no time to set up tripod):
Images without IS (when camera was kept on a rock, etc or subjects were directly above or below and shutter speed was too low):
The boat photo above was challenging because I shot it from a little boat. My own boat was rocking away like crazy and my height and weight at 6' 2" and 190lbs is too much for a little boat meant for two people. That too in the ocean! I couldn't even stand. So I used my extended arm as IS and shot some 30 photos on burst mode. Only this one had a horizontal water surface! (Thank God films are history. I'd have drowned with a boat full of used films anyway. I shot 640 photos in 2 hours that day.)
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