Canon Speedlite 90EX Instruction Manual page 16

Positioning and Operation Range
M
The settings of the flash exposure compensation, high-speed sync (FP flash), FE lock, and
manual flash set on the master unit are wirelessly and automatically set on the slave unit.
Even with multiple slave units, all of them will be controlled in the same way via wireless.
16
Indoors
x
Outdoors
Transmission distance
Approx.
10 m
(32.8 ft.)
Approx.
8 m
(26.2 ft.)
Approx. 80°
Approx. 7 m
Approx. 5 m
(16.4 ft.)
(23.0 ft.)