Does IS help with slow moving subjects?

Well, I am right in so far as I go...

Your point about potentially increased subject motion blur from use of AS/IS could be further expanded by considering the rare occasions when camera shake actually cancels subject movement to some degree...

.. that is, when image stabilisation is NOT engaged, and the inadvertent shake happens to be in the SAME DIRECTION as the subject movement... and therefore cancels it out to a greater or lesser degree whilst blurring the background more.... (that is, after all, what the act of panning does, in controlled fashion.)

So, it may be argued that engaging AS/IS actually ENSURES that the subject movement that just MIGHT have been fortuitously reduced by a lucky twitch from the cameraman...

.... instead gets rendered in its FULL burry glory!

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Regards,
Baz
"Ahh... But the thing is, these guys were no ORDINARY time travellers!"