The AC57 routes the battery pack cable to a position that reduces EMI and increases the range and reliability of your 580EX or 580 EX II / AC7 combination. pack when using a 580EX or 580EX II Speedlite in an AC7 RF Hard Shield.
Here's how it works:
1. Plug the cord of your Canon compact battery pack into the AC57 Power Adapter.
2. Plug the AC57 Power Adapter into the side of your 580EX or 580EX II Speedlite.
3. Slide your 580EX or 580EX II Speedlite into the hot shoe of an AC7 RF Hard Shield and lock in place.
4. Start shooting.