The 20D pop up will be sending out a pre-flash for colour balance setting. This pre-flash will be triggering your slaves. You need a newer type of slave trigger that copes with preflash.
I've not seen them sold in this country but I did see them list on a USA website. They are quiet expensive compared to the cheap old style ones.
Last edited by robski; 02-03-07 at 11:23.
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