Thank you very much Stephen for your comprehensive reply. I would be prepared to put up with the manual focus because the particular subjects I wish to photograph are fairly static on the far side of the river. I refer to herons, kingfishers (not that static) and a little owl which displays its self all day, every day, at the front of a hole in a tree. Once, I owned a Sigma 50/500 and I never got a sharp shot from it even though I always used a tripod. I did read, most of these lenses are extremely good, but there were one or two duds in the basket. I think I had one of the duds. Could a camera on my spotting scope be the answer? I have an IXUS400 which has an infinity setting on it. I may be able to find an adaptor. Incidently, this is a cracking camera I can produce extremely sharp A4s from it.
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Originally Posted by Stephen Fox
Many auto-focus systems do not perform when the effective maximum aperture is beyond f5.6. I do not recommend the use of a teleconverter on a lens with maximum aperture already at f5.6 such as Canon 100-400 f4-f5.6.
If a lens has maximum aperture f2.8 then both 1.4X (1 stop) and 2X (2 stops) can be used but many find acceptable or imperceivable degradation of quality using 1.4X but 2X it becomes more marked and it a debate within itself.
A teleconverter will exaggerate lens imperfections and should only be used on the front of the finest glass, there is no substitute for a 500 or 600mm.
How about a Sigma 50-500mm 'bigma' for greater reach only a little more cost than the best teleconvertors I am sure quality should be as good if not better and 100/400 with TC.
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