That setting mainly applies to speedlites that don't have an LCD Screen to enable/ disable AF Assist Beam Firing. It can also be used to completely disable the AF Assist Beam from ALL speedlites. Some speedlites rely on the camera menu to set certain functions that cannot be physically set on the speedlite itself. For instance my old 420EX lacks an LCD screen to enable or disable AF Assist Beam Firing, 2nd Curtain Sync & Flash Exposure Compensation (FEC). Instead it relies on the camera body to set those functions. Since you have no way to physically set those physically on the speedlite. So the speedlite follows the AF Assist Beam setting in the camera. Because there is no way to set it on the speedlite itself. Speedlites with an LCD screen will also follow the camera body's setting when set to DISABLE. When the camera's setting is set to ENABLE but the speedlite's setting is set to DISABLE. The speedlite will NOT project an AF Assist Beam. The speedlite's setting overrides the camera body's setting. The only exception to the rule is the EOS R Series (EXCEPT EOS R100). Since the IR AF Assist Beam is INCOMPATIBLE with mirrorless cameras. Instead the speedlite will fire continuously (Intermittent Flash Firing Mode) to assist the camera's AF system instead. Regardless if the speedlite has a choice between IR & Firing Continuously for AF Assist.
The Speedlite 420EX has partially compatibility to set rear curtain sync from the camera instead of the speedlite. As long as the HSS switch is set to OFF. If not the HSS switch overrides the ability to set 1st/ 2nd Curtain Sync from the camera body. There's no way to set that function from the speedlite.
Notice the lack of an LCD screen to set the speedlite's settings.
What you’re seeing is the red eye reduction lamp. This is only found on APS-C cameras not Full Frame. The light usually appears to be yellow. There isn’t a red eye reduction setting on Full Frame cameras. Certain parts of the camera menu are not grayed out when a speedlie is not attached. The camera will tell you the speedlite is off or incompatible. But when the IR AF Assist Beam is emitted it’s a very distinct red grid pattern. The AF Assist Beam is not used when the camera is set to Servo AF. So if you see what looks like an AF Assist Beam from the camera. It’s more than likely not one at all. The AF Assist Beam is disabled when Servo AF is being used. It would have to be continuously on since there is no focus lock in that mode. On APS-C DSLRs the AF Assist Beam can only come from the built in flash firing intermittently.
