Hi Christine
I believe the Canon 20D multiplies the focal length of a lens by 1.5 or thereabouts due to the image sensor size, if this is correct then your 400mm f5.6 lens becomes a 600mm f5.6. If you now add a 1.4 converter it becomes an 840mm f8.0 lens. Very useable for wildlife etc.
Hope this is usefull.
nirofo.
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