So today I went to use my pop up flash on the 810 and it wouldn't fire. I tried the external flash and it would fire either. The custom setting e3 was on TTL. I went thru the setting as well as I could and could see no setting that I thought would keep flash from working. The only other thing I knew to try was to do a factory reset. The flash immediately began working after the reset. But now I have to go thru all my setting and reset them. A small pain. If it ever happens again, what setting could it have been that would have kept the flash from working?