Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 195)

Canon Speedlites Other Than the EL/EX Series
With an EZ/E/EG/ML/TL series Speedlite set to A-TTL or TTL autoflash mode, the flash
can be fired at full output only.
Set the camera's shooting mode to <
> or < > and adjust the aperture value
before shooting.
When using a Speedlite that has manual flash mode, shoot in the manual flash mode.
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