Zeidora said:
Custom function on Mt24? where? How?
On my 5DsR, I got to External Sppedlight control, Flash C.Fn setting, and I get a "This menu cannot be displayed. incompatible flash or flash's power is turned off." error message. The C.Fn button on the back of the MT24 only lights the rear LCD panel.
You have to go old school with the MT-24EX
Hold down on the LCD illumination button and it will begin the process.
Everything is explained in here (PDF warning): http://gdlp01.c-wss.com/gds/9/0300004139/04/mr14ex-mt24ex-im5-en-es-fr.pdf
Custom function 9 is the one you're looking for