I have two Canon 580ex's (well, one is my wife's but she lets me use it ), so I have manual controls. I will look into the Impact transmitters that you mentioned. I have seen a couple of these, but did not realize there were interference issues. Thanks for that input!I use an el cheapo Cactus V4 transmitter/receiver to fire flash units with my L1 and E-1 bodies. It unfortunately causes significant interference and image degradation with the G1 body (as well as with the Canon 5DII and some other cameras) so I can't use it with that. I'm planning to expand my lighting equipment with a set of the Impact transmitter/receivers that B&H Photo sells ... they are known to work with the G1 body well with no interference and are reasonably priced. A friend used Pocket Wizard units and another brand (name I forget) with his G1 and Nikon D200, they work well too. The PWs are somewhat pricey.
The slaved flash units I use run a gamut from old, cheap junk to the Olympus FL50 (of course, in manual output mode). They'd work fine with the Canon flash units too. BUT, I've found the lesser model Canon flash units (like the EX420 (?) I had) really only work best triggered by Canon cameras and control units ... the 420 had no manual adjustability at all when I tried to use it as a slaved flash: it always fires at full, unmetered output, which meant it was mostly useless to me.
If your Canon flash units have output power adjustability in manual mode, then they should work fine.
A good kit of three to four Vivitar 285HV flash units and RF receivers coupled with one transmitter on camera, along with a couple of stands and clamps to hold them, and some light modifiers, constitutes a great lighting kit suitable for professional work at a very reasonable price.
You might look at the radiopopper JrX. They are really close in price to the Impact ones from B&H and have a lot of nice controls, such as three biasing knobs for manual power control for three groups.
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