Canon 580 EX II - Manual External Metering Mode
 

Has anyone tried using this mode? Normally I use my speedlites in ETTL, or full manual mode & meter the output.

However I want to use my 580EX on my Panasonic GX7 wandering around at a at party where I wont be able to meter the flash & obviously I can't use ETTL. From what I've read, it suggests that this mode is similar to the thyristor mode of yesteryear where the flash output is measured by a sensor on the flash unit & cut off when sufficient light has been delivered.

It works... in that the flash fires & the exposure is consistent... but its consistently ~2 stops under exposed. Ok I can create a work around by setting the camera at f2.8 ISO 200 & the speedlite to f5.6 ISO 200 to fool the speedlite into delivering 2 stops more light, but I don't understand why I'm getting this behavior. Interestingly I get the same -2 stops using this mode on my Canon which suggests its a speedlite issue.

Anyone any ideas?

I have this exact model also 550EX and whether alone or in combination, I always prefer to use manual.
This flash has a high GN but then as you say, and if in F based, auto sensor mode, sensor should work accordingly and not sure why 2 stop under. Unless you have set the flash in M mode at 1/4 power (Mode>M>Select>x2 minus). Check to see if power is on 1/1. If so then I am clueless.

Thanks for the reply... but I've tested it & changing the M power setting makes no difference whether its on 1/1 or 1/128.

Perhaps you can do me a favor & test yours to see if you get the same effect. To put in into Manual External Metering Mode you need to set custom function 5 = 3 & then set your f stop & ISO on the flash gun. I'd be interested to see what happens :)