As some already pointed out, you have to use the canon EOS camera body to set the aperture before mounting an EF mount lens to the GH1. This is a pain in the butt and people only do this when they have a collection of specialty lenses, such as those tilt shift lenses. Conversely, you cannot mount a FD lens on the EOS body. I believe there are adapters but these adapters contains optical components in them and will degrade IQ. Thus you do not have to use FD lenses with the GH1. Zuiko OM lenses are good alternatives as they are smaller with outstanding IQ and just as inexpensive.
