AK Grandpa wrote:
I've got a Tamron 150-600 G2. On a tripod, I'll turn the VC off . . .
Whether or not you should do that depends upon what you mean by "on a tripod".
The real questions should be...
1. Is there is any movement for the stabilization system to correct?
2. In the event that there is no movement what-so-ever, how will the stabilization system react?
3. Does the stabilization system have any effect on other camera functions?
There are techniques and situations when using a tripod that there is movement and stabilization can be helpful. For example, if using a gimbal mount "loose" to be able to track a moving subject, there's a lot of movement. Heck, with a large, long telephoto lens even a breeze can cause movements.... or the camera's mirror and shutter might cause vibrations.
Conversely, there also can be situations when there's no movement what-so-ever when using a lens on a monopod of even when hand-holding it, such as when resting the camera on a surface to help steady your shot.
So it's actually more about movement.... whether there is some or not.... than if the lens is "on a tripod".
The next concern is how any particular stabilization system reacts to lack of movement.
I don't know about the Tamron VC system in particular... haven't personally used any lenses with it. For fifteen years or so I have been using a lot of Canon lenses with their IS system (which might be different.... in fact probably is, considering patent protections). Most Canon lenses it's best to leave it turned on. Most of the 30 or so Canon IS lenses self-detect and turn off IS automatically, when it's not needed. All the various Canon lens user manuals I've looked at recommend turning it off "when using the lens on a tripod". However Canon USA's tech guru Chuck Westfall admits that in most cases this is just to save a little battery power. The stabilization system draws some power from the camera's main, rechargeable battery (but not very much, in my experience).
Among Canon lenses there are five that are widely recognized to have an IS that can go into sort of a feedback loop if left on when there is absolutely no movement. Those particular lenses' IS will actually cause motion blur in images, in that situation. You can see it happening, too.... a rapid shaking of the image in the viewfinder (not to be confused with a slower "image drift" that occurs with most stabilization at times, which won't have negative effect on most images).
If you forget to turn it off and the stabilization reacts that way, with Canon lenses and cameras, no harm done (other than possibly some blurred images). It doesn't damage the camera or lens in any way. So if it's seen happening, simply turn the IS off.
With my Canon IS lenses, I virtually never turn it off. Two models of lenses I use are the type that doesn't turn itself off. But those are lenses that lend themselves to handheld or, at most, monopod or gimbal use. Other lenses I use self-detect and handle it automatically. Same results as if I turn it off manually. In fact, on several lenses I have gaffer tape over the switches, to prevent accidentally turning it off.
You also might need to turn off stabilization in certain situations.... For example, shooting video or a precisely framed image, where even the slow "drift" might cause problems. (However, video shooters... which I'm not...tell me they appreciate stabilization a lot for handheld shots.)
Also, some stabilization systems are said to effect other camera functions.... Many Nikon shooters feel that VR slows down autofocus a little, and there's some evidence to support that. Personally I feel Canon IS does just the opposite, helps Canon AF acquire faster and track better. I really don't have any test data to support this... it's just based upon my fifteen years using IS lenses.
All these things may or may not apply to Tamron's VC.... or Sigma's OS... or Sony's OSS.... etc. No doubt all the stabilization systems are different in some ways, even though their basic intent and purpose is the same.
Look for Tamron-specific info (such as on their website or by sending them an email). And experiment with the lens to see for yourself how it works.
A side benefit of in-lens stabilization (as opposed to in-camera, sensor-based) is that it also stabilizes the image in the optical viewfinder. Particularly when working with longer focal length lenses, this can be helpful, much like stabilized binoculars. You don't get this with in-camera stabilization and optical viewfinders (but may see some similar assistance with an electronic viewfinder or in Live View).