Hello Roy, and welcome to the Camera Labs forum!
Body-based IS not working on this lovely Tamron 500mm mirror-lens (which I also own a copy of)?! That's a pity
But it conforms what I would suspect: that camera-based IS needs some focal-length information to calculate how much to compensate.
Perhaps you would be better off with the Tamron 200-500mm although sharpness of my copy
was not as good as other tests had promised
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
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