Underexposure on 20D

I have a 20D with 18-85mm IS Lens and580EX

It seems that with a at the 18mm end all the shots seem underexposed like the flas output is too low. At mid to higher end zoom the light is fine, just too dark at the lower end.

Has anyone got any ideas

many thanks

Mike:p

Dark as in totally dark, or dark as in dark corners. If dark corners, try setting CFn 11 (on the flash) to '1' (disabled). There have been (early series) 580EX who didn't cover the wider lenses on crop factor bodys.
Otherwise: post some samples with EXIF data in 'em.