Canon Speedlite ST-E3-RT Mode D'emploi page 32

Using Fully Automatic Wireless Flash
7: FE Lock
FE (Flash Exposure) lock locks the correct flash exposure setting for
any part of the scene.
Perform FE lock by operating the camera. For the operations, see the
camera and flash's instruction manual.
If a correct exposure cannot be obtained when FE lock is performed, <Q>
blinks in the viewfinder. Move the slave unit closer to the subject, open
the aperture, and perform FE lock again. You can also increase the ISO
speed when using a digital camera.
If the target subject is too small in the camera's viewfinder, FE lock might
not be very effective.
About Master Units
You can use two or more master units (master units + slave units =
maximum of 16 units). By preparing multiple cameras with master units
attached, you can shoot by changing cameras while keeping the same
lighting (slave units).
Note that when using two or more master units, the color of the <D>
lamp varies depending on the order in which the power was turned on.
The first master (main master) is green and the second and subsequent
masters (sub-masters) are orange.
If the <D> lamp is red, the connection has not been established. After
checking the transmission channel and wireless radio ID, turn the power of
each master unit off, and turn it on.
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