Canon EOS 6D Instruction Manual page 206

k Shooting Movies
ISO speed in Basic Zone modes
The ISO speed will be set automatically within ISO 100 - 12800.
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ISO speed in Creative Zone modes
The ISO speed will be set automatically within ISO 100 - 12800.
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Under [z3: ISO speed settings], if you set [ISO speed range]'s

[Maximum] setting to [25600/H] (p.109), the maximum ISO speed
for the automatic ISO speed setting will be expanded to H
(equivalent to ISO 25600). Be aware that when you set [Maximum]
to [25600], the maximum ISO speed will not be expanded and
remains ISO 12800.
If [z4: Highlight tone priority] is set to [Enable] (p.130), the ISO
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speed will be ISO 200 - 12800.
Under [z3: ISO speed settings], [Auto ISO range] or [Min.
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shutter spd.] cannot be set (p.110, 111) for movie shooting.
For [ISO speed range], if [Minimum] is set to [L (50)] and [Maximum] to
[H1 (51200)] or [H2 (102400)], and you switch from still photo shooting to
movie shooting, the minimum setting for the automatic ISO range will be
ISO 100 and the maximum will be H (equivalent to ISO 25600). The ISO
speed cannot be expanded to ISO 50 or ISO 51200/102400.
Using an EX-series Speedlite (Sold Separately)
Equipped with an LED Light
With autoexposure (mode other than a) movie shooting, the camera
will automatically turn on the Speedlite's LED light under low-light
conditions. For details, refer to the EX-series Speedlite's instruction
manual.
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