I mounted 600EX-rt on Fujifilm xt-1 with 35mm lens, and put the sppedlite in EXT-M mode, just to see what would happen. It works almost like ETTL with some restrictions, exposure compensation is also possible (by setting the camera and the speedlite to different ISO speed and/or apeture combinations). Bounsing off the wall also seem to work fine. I was shooting subjects at different distances to make sure that the exposure is more or less correct. I suspect that EXT-A will also work. I must revisit xt1, it has been laying around for too long, after first testing it a year ago, and opting for 6D instead (because of the image quality).