I have a Sigma 18-200mm zoom with Optical Stabilization, and I wonder...how can I tell if it's working? It makes no sound other than focusing. I'm not having any issues with image sharpness, so it seems to be, but is there a way to be sure?
I use both Sigma and Nikkor lenses, some of each with image stabilization functions. Whenever I receive new equipment of any kind, I thoroughly test it in order to learn about it and to see if it works as advertised or reviewed. These image stabilization functions (different names by each lens manufacturer)are quite silent, so far as my ears can tell. To test this function, use a simple, but consistent scientific method. Take a series of images, one series with IS turned on, and one with it off, all hand held. Important in this test is to shoot the same subject at the same settings in the same lighting, for example, 8 shots with IS on and 8 shots with IS off, leaving all other functions alone. You can do this indoors or outdoors, but make sure the f-stop and shutter speed do not vary (try using Manual camera settings). If you are outdoors, make sure clouds and wind are not changing your subject as you shoot. Then, in the computer, enlarge the images and compare them carefully. You should see a difference. Of course, IS is primarily designed to help prevent blurring by movement when your shutter speeds need to be slower, so try a number of series tests with the shutter speed dropping from 1/60th of a second and slower. Good luck.
One method is to hold the camera as if you are ready to take a shot with Auto, VR/OS turned on hold the shutter button down half way and watch the subject to see the difference in the movement when you toggle the shutter button... you should see more move movement with shutter released and almost no movement with shutter half way.