Canon EOS 5D Mark III Instruction Manual page 221

ISO speed in the A mode
In the A mode, the ISO speed will be set automatically within ISO

100 - 12800.
ISO speed in the d, s, f, and F modes
The ISO speed will be set automatically within ISO 100 - 12800.

Under [z2: ISO speed settings] (p.126), if you set the [ISO speed

range]'s [Maximum] setting to [25600/H] in the d, f, or F mode,
the automatic ISO range 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 [z3: Highlight tone priority] is set to [Enable] (p.146), the ISO

speed will be ISO 200 - 12800.
Under [z2: ISO speed settings], [Auto ISO range] and [Min.

shutter spd.] cannot be set (p.127, 128) for movie shooting. Also,
[ISO speed range] cannot be set in the s mode.
If [Minimum] is set to [L(50)] and [Maximum] to [H1(51200)] or
[H2(102400)] in [ISO speed range], and you switch from still photo shooting
to movie shooting, the minimum setting for automatic ISO range will be ISO
100 and the maximum will be H (ISO 25600, except in A and s shooting
modes). The ISO speed cannot be expanded to ISO 50 or ISO 51200/
102400.
k Shooting Movies
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