Canon SPEEDLITE EL-1 Advanced User's Manual page 158

Caution
Setting the focus mode switches of the lenses attached to the receiver camera unit
to < > and taking the picture with manual focusing is recommended. If focus
cannot be achieved with autofocus, linked shooting is not possible with the
corresponding receiver camera unit.
There is a short time lag between the release of the receiver camera unit and the
release timing of the sender camera unit. Perfectly simultaneous shooting is not
possible.
When set to P.Fn-03-1 (
multiple flashes are fired simultaneously during linked shooting, the proper
exposure may not be obtained or inconsistent exposure may occur.
When [Flash firing] in [External Speedlite control] or [Flash control] is set to
[Disable] ( ) on the sender camera, linked shooting cannot be performed.
When set to P.Fn-03-0 (
view image, set [Soft LV shooting] or [Silent LV shoot.] in the menu of the sender
camera unit to [Disable]. If [Mode 1] or [Mode 2] is set, the receiver camera units
may not be released depending on the camera model used.
The transmission distance may be shorter depending on the conditions such as the
positioning of the flashes, the surrounding environment, and weather conditions.
The linked shooting function is similar to linked shooting on the WFT series of
wireless file transmitters. However, linked shooting cannot be performed in
combination with the WFT series file transmitters. Moreover, the release time lag
differs from linked shooting performed using the WFT series.
Caution
Linked Shooting Using Live View Function
When set to P.Fn-03-0 (
sender cameras, linked shooting in the Live View mode is not possible.
Perform linked shooting after switching Live View shooting to viewfinder shooting or
set the Speedlite to P.Fn-03-1.
EOS REBEL T6S/760D, EOS REBEL T6i/750D, EOS REBEL T5i/700D,
EOS REBEL T4i/650D, EOS REBEL T3i/600D, EOS REBEL T2i/550D,
EOS REBEL T1i/500D, EOS REBEL XSi/450D, EOS REBEL XS/1000D
), the flash can be fired during linked shooting, but if
), if linked shooting is not possible while viewing the live
), if the following cameras are combined and set as
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