Canon EOS 90D Advanced User's Manual page 184

Wireless Flash Photography Using Optical Transmission
Autofl ash With One Receiver
If there is insuffi cient fl ash output from the sender, increase the ISO speed
o
( = 213).
Flash ratios from 8:1 to 1:1 correspond to 3:1 to 1:1 (in 1/2-stop increments)
o
when converted to number of stops of exposure.
184
You can adjust how shadows form on
subjects by changing the ratio of fl ash
output from a receiver and a sender.
1
Set [Wireless func.] to [0:3].
Set [Wireless func.] to [ 0 : 3 ] as

described in step 5 on = 183.
2
Set the ratio of fl ash output of the
external Speedlite to the built-in
fl ash.
Select [ 0 : 3 ] and set the fl ash ratio

within 8:1 to 1:1.