On Camera Fill Light

[PIC]

Has anyone used Canon 10D with 420 EX as fill w/ STO FEN diffuser and to fire 1 - 3 key monolights ?
I would be interested in what settings are used for camera and flash.
I use Bowens 500WS esprit 2 and often see 2 stops underexposed when camera 10d and 420 flash is in auto. But not always {???}
Have not tried manual settings or flash meter yet.
Any thoughts ?
Eric

[kevinb]

Do you have a preflash triggering the mono lights before the image capture? Auto mode on the camera and the flash is likely to be inconsistant when used with additional lights that aren't communicating with the camera's processor.
Disable any redeye reduction, use manual settings, meter each light seperately to calculate desired fill ratio's and use a shutter speed a little slower than max sync speed.

[PIC]

Thats what I was thinking. Not sure why there is a preflash w auto mode - do not use red eye reduction.
Some one suggested covering 4 of the 5 pins at bottom of flash.
Eric