Canon’s manual for the 580EXII flash says:
Cautions for firing continuous flashes
To avoid overheating and degrading the flash head, do not fire more than 20 continuous flashes. After 20 continuous flashes, allow a rest time of at least 10 min.
If you fire more than 20 continuous flashes and then fire more flashes in short intervals, the inner overheating prevention function may be activated to make the recycling time about 8 to 20 sec. If this occurs, allow a rest time of about 15 min. and the flash will then return to normal.
Canon says: “continuous”. What’s continuous? 20 flashes in a row? Distance, power, power source… There’s not definite answer, but it is close to full power.
The 580EXII is a sealed unit. It’s “weather resistant”, meaning that it can operate in a light rain for a short period of time. It’s not waterproof! That’s because the 580EXII is a sealed unit, the heat cannot dissipate easily. The 580EXII has a heat sensor that will disable the flash when it gets too hot. This is a new “feature” of the 580EXII, the 580EX did not have the overheating prevention function, and many have died.
At great expense and risk, aka stupidity, I decided to test it myself!
- In manual mode, full power with fully charged Sanyo Eneloop batteries, it took 26 flashes to reach the rest time. My rest time is not very accurate, but it took close to 23 minutes!
- In manual mode, ½ power with fully charged Sanyo Eneloop batteries, after 50 flashes, I couldn’t reach the thermal shutdown.
So it’s somewhere in between. Now with e-TTLII, bouncing of the light, recharge time depending on the batteries… Who knows how many flash does it take to reach the thermal shutdown?
The thermal shutdown is not only a Canon “feature” but also applies to Nikon and many other brands.
What are the alternatives? There are some Metz flashes, the “potato” heads, the big handle with the flash on top but as far as I know, they are not fully dedicated. There’s Quantum with their Trio, but it’s big, heavy and expensive when you add the battery pack and the accessories. The last time I looked at it, the Quantum Trio with the Slim powerpack was around $1,500. Now, I would need 2 of them…