This post is mostly to help others with any compatibility issues they may experience with the Yongnuo YN-E3-RT transmitter, and the YN600EX-RT speedlight. After some investigation I found the following that could be useful: First I should state I have both the original Canon 600EX-RT and the Yongnuo variant. I used both in my testing. The firmware of the YN-E3-RT is 1.14, the firmware of the YN600EX-RT is 1.8.1. The YN-E3-RT has a custom Function called "Legacy", which can be accessed by long-pressing the CFn button. The intend of this function is to allow the YN-E3-RT to be used for Non-Canon Cameras. I used this function to assess the different operation modes. Note that in the test cases below, only Manual mode was tested, as this is primarily a test to assess compatibility with other, non-Canon, cameras. Camera/ “Legacy”/ Canon 600EX-RT/ YN600EX-RT D810/ Off/ Yes/ No D810/ On/ Yes/ No M8.2/ Off/ Yes/ No M8.2/ On/ Yes/ No R-D1u/ Off/ Yes/ No R-D1u/ On/ Yes/ No X-Pro1/ Off/ Yes/ No X-Pro1/ On/ Yes/ No E-PL5/ Off/ Yes/ No E-PL5/ On/ Yes/ No 5D2/ Off/ Yes/ Yes 5D2/ On/ Yes/ No During this test I also noticed that the YN600EX-RTs had a rather long power recycle time. The YN-E3-RT has an option for setting the speedlite power output when pressing the “Test” button. The function seems to work as required when working in ETTL mode for both the Canon 600EX-RT as well as the YN600EX-RT. However, the function does not seem to work for the Canon 600EX-RT nor for the YN600EX-RT when working in Manual mode. It appears that when in manual mode, this function is disabled, and the test button assumes the power setting set on the controller for the relevant speedlite. Since the transmitter requires the user to push the “Test” button each time settings are changed (applies to both the Canon and the Yongnuo speedlites), this could cause the batteries to drain unnecessarily. Also note that since it only works in ETTL, this means you can will only have this option available with Canon cameras when shooting in ETTL. My conclusion is: 1) The “Legacy” Switch is not working at all. It does not have an effect when using the YN-EX-RT in conjunction with a Canon 600EX-RT. This is most likely an issue with the YN-E3-RT transmitter, and should be fairly easy to fix with a firmware update. 2) When used with a non-Canon camera, the YN-E3-RT only triggers Canon 600EX-RT speedlites. It does NOT trigger the YN600EX-RT speedlites. I can't say if this is an issue with the YN-E3-RT or the YN600EX-RT. Since the Canon 600EX-RT works, most likely it is related to the YN600EX-RT. It's possible that the YN600 expects transmissions that include Canon communication protocol, which may be included when used with a Canon camera, but obviously not with non-canon cameras. 3) The Test power option only works in ETTL, which means using a Canon Camera. In Manual mode the “Test” button will result in an output from the speedlites equivalent to the setting on the YN-E3-RT. This may be by design (I don't know how the ST-E3-RT from Canon works, it may do the same), and thinking about it this makes sense. In ETTL, you don't measure the output, but in manual you may want to use a lightmeter to measure the output, in which case the "Test" button should output exactly what you set. PS forgive the table formatting. This never works online and always screws up. the columns are separated by "/".