wrote in message
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Kibo informs me that "Don" stated that:

Folks

I have been trying to use the Tamron SP AF TeleConvertor 300F-CA 2X with
my
400 F5.6 L series lens but the results are dismal. I obviously was aware
I
would have to manual focus but I haven't been able to get a sharp shot or
even a keeper for that matter yet. Any one has any ideas or similar
experience or would care to comment and assist me if I am doing something
wrong. I have tried from F11 to F14 on tripod etc.

I also have a 10D, & I've found it impossible to get a good manual focus
in general, due to fuzziness of the & lack of focussing aids on the
viewfinder. I'm guessing that you're running into the same problem.

No. His problem is that he is severely compromising a decent lens by using
a 2X extender. This is to be expected.
Even with Canon's own L-grade 2X, you are going to lose significant
sharpness.
The 1.4x Canon is a MUCH better performer than any 2X, and may be a worthy
alternative.