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John Mills wrote:
580, 430 and 420.
Ah, that's the problem. Hypersync is mainly for use with studio strobes that have long flash durations. With Speedlites you can't get good results with Hypersync because the flash duration is too short (thus the name SPEEDlite). Also, the lower the power, the shorter the duration...just the opposite of most studio strobes. (The Einstein and other IGBT/thyristor-controlled strobes are the exception; they're like Speedlites.)
For Speedlites you need to use High Speed Sync, not Hypersync. I don't know about the 420EX, but the 580EX and 430EX both have that feature.