I am planning to buy Yongnuo YN560III flash to use with my E500 camera. Is it safe to use it? Most of the online pages refer using it with Canon/Nikon. I want to be sure before I buy it. What radio triggers can I use with the above flash? I want to fire two such flashes with a single on-camera trigger — can it be done?
Good news and... Not quite bad news. First, this is a modern flash with a low trigger voltage, and there is no risk using it on any modern camera. So, good news.
Second: this will work with any generic radio trigger, from cheap to expensive. However, its headline feature is built-in radio triggering, and that works with Yongnou's own triggers, which they only make for Canon and Nikon.
I found a post on modifying a Canon trigger to work with Olympus, which seems a bit scary to me but may work for you.
An alternative might be the Cheetah Light V850, which also comes with an integrated (in this case, clips on the side of the flash) radio trigger system. It is completely generic, using only the standard center pin, but the cool thing is that you can set the power from the in-camera transmitter, so it's easy to make the manual adjustments — just test and terak until you have it right. You can fire multiple flashes, and actually have up to 16 sets with independently-set power levels (There's a dial to select groups quickly).
One downside is that this has a less-reliable 400mhz radio rather than the 2.4ghz band that Yongnou uses (I haven't had trouble in practice, but I haven't pushed it). Also it uses a rechargeable lithium battery, which yields amazing recharge times, but if it runs out, you can't just pop in drugstore AAs.
This flash is also sold as the Godox V850 and Neewer TT850 — read reviews at Fstoppers and FlashHavoc. The Cheetah Light brand is slightly more expensive, but you get a one-year warranty and US repair (rather than sending directly to China if there is a problem).
Or, you could of course go with the Yongnou you are looking at, or one of the other models Yongnou makes + Pocket Wizard triggers or whatever you like. I think, though, that if you are not using one of the big two camera systems, Yongnou isn't the best choice.
Yes, the YN-560 III is safe to use on modern digital cameras--the sync voltage is well below 10V.
However, the built-in slave receiver of the YN-560 III only works with four different units from Yongnuo, and only three of them will work on the four-thirds hotshoe. Avoid the RF-603 (Mark I). It's a transceiver unit that auto-senses—from a TTL pin signal—whether or not to switch into transmit (Tx) or receive (Rx) mode. That signal, unfortunately, doesn't work on any brand of flash/camera hotshoe other than Canon or Nikon, so the RF-603 triggers are a no-go without modification to short the correct signal to the correct pin for four-thirds. This is also why the 603 test button won't work unless the unit is on the camera hotshoe.
You can, however, use the RF-602 triggers (they use separate Tx and Rx units, so there's no swtiching issue). And Yongnuo heard all the complaints from the mirrorless and studio shooters with lightmeters, and the new RF-603II/RF-605 triggers now have an OFF/TX/TRX switch on them, which allows you to explicitly set a unit into transmitter mode. If you switch one unit to TX and put it on the camera hotshoe, it will work on four-thirds. But probably the best choice for an four-thirds or micro four-thirds shooter is to add a YN560-TX transmitter unit, which not only triggers the flash, but can also remotely set the YN‑560 III's manual power setting and zoom.
You can, of course, use any radio triggers you want by attaching a receiver unit to the flash's foot, but the convenience of the YN-560 III is that the radio receiver is built in.