Custom Functions are extra features and options that allow you to set up the speedlite to your own preferences. Access to these functions is usually through the speedlite menu although some functions can be set from the camera menu if both speedlite and camera are compatible.
Typical custom functions include:
Distance Indicator: measurements shown in feet or metres
Auto power off: When the speedlite is not used for approx. 90 seconds the power turns off to save energy. When this option is turned off the speedlite won’t power off until you switch it off.
AF-assist beam firing: Turn on or off the auto-focus assist beam.
Flash recycle with external power:
Beep: Turn on and off the flash ready beep.