Canon EOS 90D DSLR Camera with 18-135mm Lens - Manual (Page 268)

268
AF-Assist Beam Settings
o If an external Speedlite’s [AF-assist beam fi ring] Custom Function is set to
[Disable] , the AF-assist beam will not be emitted.
o The AF-assist beam of an attached EX series Speedlite equipped with an LED
light will fi re as needed from the Speedlite’s LED light in Live View shooting
when you specify [Enable] or [Enable external fl ash only] .