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voila une reponse de canon USA

"Dear Nguyen Olivier:

Thank you for your inquiry regarding the flash system on the EOS 40D.
We value you as a Canon customer and appreciate the opportunity to
assist you.

The article you have read is correct in that when using a non-Canon
flash, the sync speed is decreased as opposed to the maximum sync speed
when using an EX-Series Canon Speedlite. We apologize for the
inconvenience.

Please let us know if we can be of any further assistance with the EOS
40D.

Thank you for choosing Canon.

Sincerely,

Michael
Technical Support Representative"


donc voila, au revoir le 40d pour ma part....(ou alors je devrais vendre mes flash sigma et prendre 2x580II)...