From time to time I ponder buying a 2X converter to use with my 500mm f/4 Canon lens and 20D. Two things hold me back: loss of autofocus, and concerns about image quality.
I really need the autofocus - these useless damn modern viewfinders are too dim and small to focus manually without a proper split image focus aid, which the 20D doesn't have. I suppose I'd get the Canon one even so, as I'm certainly very happy with the results that my Canon 1.4 converter delivers with the 500. It isn't as incredibly sharp as the bare 500 alone, but it often comes remarkably close, particularly if the light is good. As a general rule, I leave the 1.4 on the 500 all the time, only removing it when the light is problematic or when I expect to be close enough not to need it.
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