image stabilization - Does OIS in some cases reduce the quality of the picture (softness, vignetting, loss of contrast...)?

I always take pictures with optical image stabilization enabled (except if I take long exposure shots with a tripod), but I was wondering if OIS could (in some cases) reduce the picture quality.

OIS works by moving one glass element inside the lens, so if I shake the camera while taking a picture the lens has one decentered glass element, which should cause some softness in the final picture I think, but that's something I never encountered.

Can an optical stabilization cause a reduction of the picture quality ? Is it something only visible on certain type of lenses or not at all?

Answer

With lens based IS, the movements of the IS element/group introduce mild misalignment of the lens. This is viewed as acceptable if the blur introduced by the less than 'perfect' optical alignment of the lens¹ induced by an IS movement is less than the blur that would otherwise be introduced by the motion of the lens/camera. The largest advantage of LBIS is that for narrow angles of view (long focal lengths) much more correction can be done than can be done by shifting a sensor that is limited by both the size of the image circle and the speed and distance at which in-camera servos can move the sensor while remaining relatively compact and efficient with regards to battery/energy consumption/cost.

_{¹ There's no such thing as a 'perfectly' aligned compound lens, even among non-IS prime lenses. There are always manufacturing tolerances to be considered.}