Canon EOS Rebel T3 18-55mm IS II Kit Instruction Manual page 170

3 Setting the FlashN
Setting the External Speedlite Custom Functions
The Custom Functions displayed under [External flash C.Fn setting]
will vary depending on the Speedlite model.
170
Display the Custom Function.
1
Select [External flash C.Fn setting],
then press <0>.
Set the Custom Function.
2
Press the <U> key to select the
function number, then set the
function. The procedure is the same
as setting the camera's Custom
Functions (p.216).
To clear all the Custom Function
settings, select [Clear ext. flash C.Fn
set.] in step 1.