Canon Speedlite 90EX Instruction Manual page 22

Position the slave unit using the mini stand supplied with the slave unit (the one with a

tripod screw hole).
For slave units with the horizontal bounce function, point the sensor of the slave unit

toward the master unit.
Indoors, the wireless signal can also bounce off the wall so there is more leeway in

positioning the slave unit(s).
If the sensor on the slave unit is not facing the master unit, the slave unit may not fire.

After positioning the slave unit(s), be sure to test the wireless flash operation before

shooting.
To avoid interference with the transmission, do not place any obstacles between the

master unit and slave unit.
20
Indoors
x
Outdoors
M
Transmission distance Approx. 5 m
Approx.
10 m
(32.8 ft.)
Approx.
8 m
(26.2 ft.)
Approx. 80°
Approx. 7 m
(16.4 ft.) (23.0 ft.)