I currently have a Yongnuo YN560-TX trigger and a Yongnuo YN-560 Mark III. I would like to get a iTTL flash that also works with remote triggering.
How compatible are are different flash brands (Neewer, Yongnuo, etc.) when it comes to things like setting zoom, etc. ?
Generally when it comes to manual-flashes-with-remote-power-control, only flashes and triggers in the same brand and triggering system will be compatible enough to allow you remote control over power and zoom. A built-in receiver in the flash has to match the radio system, and only a built-in receiver can remotely control power/zoom on a single-pin manual-only flash (because that pin can only communicate the sync signal). You can only have remote power/zoom control with an external trigger over a TTL-capable flash--if an add-on TTL trigger for it exists in the system.
And with Yongnuo gear, mixing TTL and manual-only gear is highly problematic. Yongnuo's three separate triggering systems (560/60x, 622, and RT) are all incompatible with each other. The only way to mix TTL and manual triggering is by stacking triggers on top of each other. And power/zoom control typically cannot be maintained, unless everything is in manual mode.
For example, you could purchase a YN-685, and you could control its group, power, and zoom with a YN-560-TX, but you wouldn't be able to use it remotely in TTL mode or get HSS with it, which are the two features you probably bought it for. You would have HSS/TTL with the YN-685 on the camera hotshoe, but then you have no way to use the YN-560-TX. You could have TTL and HSS remotely if you used a YN-622-TX transmitter, but the 622-TX cannot trigger/control your YN-560III.
Godox, RadioPopper, PocketWizard, and Phottix have triggers/flashes where the TTL/manual flashes do interoperate together and are compatible. Phottix, RadioPopper, and PocketWizard gear, however, is more expensive (but has a better reputation) with prices closer to OEM gear at the TTL end of the gear spectrum. Godox gear can be lower-cost, but the new X1 trigger system is still having some teething issues, and you may run into some early-adopter bugs and the chances of newer models working better in the near future (just like with Yongnuo gear back in the day).
But, for example, you could get a Godox TT600--the US$70 Godox equivalent to your YN-560III. And a $50 X1T-n transmitter, and remotely control its power and even get HSS. You can then add a $120 TT685 to the mix, which is a TTL flash, with all the TTL bells and whistles and you get HSS and remote power control of both flashes from the X1T.
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