Canon EOS 5D Mark III Instruction Manual (en)

k Shooting Movies
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 dsf, and F modes
The ISO speed will be set automatically within ISO 100 - 12800.
Under [z2: ISO speed settings] (p.128), if you set the [ISO speed 
]’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.148), the ISO 
speed will be ISO 200 - 12800.
Under [z2: ISO speed settings], [Auto ISO range] and [Min. 
shutter spd.
] cannot be set (p.129, 130) 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/