Canon EOS 6D Instruction Manual page 171

Canon Speedlites Other Than the 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 <a> (manual exposure) or
<f> (aperture-priority AE) and adjust the aperture setting before
shooting.
When using a Speedlite that has manual flash mode, shoot in the
manual flash mode.
Using Non-Canon Flash Units
Sync Speed
The camera can synchronize with non-Canon compact flash units at
1/180 sec. and slower speeds. With large studio flash units, be sure to
test the flash synchronization before shooting with the sync speed set
within approx. 1/60 sec. to 1/30 sec., since the flash duration of such
units is longer than that of compact flash units and vary depending on
the models.
Cautions for Live View shooting
If you use a non-Canon flash unit with Live View shooting, set [A2:
Silent LV shoot.] to [Disable] (p.190). The flash will not fire if it is set to
[Mode 1] or [Mode 2].
If the camera is used with a flash unit or flash accessory dedicated to
another camera brand, the camera may not operate properly and
malfunction may result.
Do not attach a high-voltage flash unit on the camera's hot shoe. It may
not fire.
D Flash Photography
171