Canon SPEEDLITE EL-1 Advanced User's Manual page 224

Linked Shooting
Standard exposure is not obtained. / Uneven exposure occurs.
If you fire multiple Speedlites simultaneously during linked shooting, the appropriate
exposure may not be obtained or uneven exposure may result. It is recommended to set
only one Speedlite to fire or to use a self-timer to space out the timing of firing.
The unit cannot be used as a receiver camera unit.
When used with a camera released up to 2011 or with EOS REBEL T100/4000D/3000D,
EOS REBEL T7/1500D/2000D, EOS REBEL T6/1300D or EOS REBEL T5/1200D, the
unit can be used only as "sender camera unit". The unit cannot be used as a "receiver
camera unit".
Optical Transmission Wireless Flash Shooting
The receiver unit does not fire or unexpectedly fires at full output.
Set the sender unit to <
( ).
Set the transmission channels of the sender unit and receiver unit to the same numbers
( ).
Make sure the receiver unit is within the transmission range of the sender unit (
Point the wireless sensor of the receiver unit toward the sender unit (
Position the receiver unit at a location with the clearest possible view of the sender unit.
If the sender unit and receiver unit are too close, the transmission may not take effect
properly.
When you use the built-in flash of the camera as a sender unit, raise the built-in flash of
the camera and configure the [Wireless functions] settings in the [Built-in flash
settings] of the camera.
The sender unit fires.
Even when sender flash firing is set to OFF, the sender unit fires a small flash to control
the receiver unit with optical transmission (
Pictures are overexposed.
When performing autoflash photography with three firing groups A, B, and C, do not fire
with firing group C pointing toward the main subject (
> and the receiver unit to <
).
).
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).
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