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There is a 2.0x tele converter EX DG from Sigma that should work
But I wouldn't expect autofocus to work at longer focal lengths as you get an effective aperture of f/11
And image quality deteriorates so much that it could be easier to crop/enlarge the original image (w/o converter) in postprocessing
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
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