Canon EOS 5D Mark III Instruction Manual (en)

k Shooting Movies
ISO speed during manual-exposure shooting
With [Auto] (A), the ISO speed will be set automatically within ISO 
100 - 12800.
You can set the ISO speed manually within ISO 100 - 12800 in 1/3-
stop increments. Under [z2: ISO speed settings], if you set [ISO 
speed range
]’s [Maximum] setting to [25600/H], the maximum ISO 
speed for manual ISO speed setting will be expanded to H 
(equivalent to 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), you can 
set the ISO speed within ISO 200 - 12800 (depending on the [ISO 
speed range
] setting).
Under [z2: ISO speed settings], [Auto ISO range] and [Min. 
shutter spd.
] cannot be set (p.129, 130) for movie shooting.
Since shooting a movie at ISO 16000/20000/25600 may result in much 
noise, it is designated as an expanded ISO speed (displayed as [H]).
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 manual ISO range 
will be ISO 100 and the maximum will be H (ISO 25600). The ISO speed 
cannot be expanded to ISO 50 or ISO 51200/102400.
Changing the shutter speed or aperture during movie shooting is not 
recommended since the changes in the exposure will be recorded.
When shooting a movie of a moving subject, a shutter speed of 1/30 sec. 
to 1/125 sec. is recommended. The faster the shutter speed, the less 
smooth the subject’s movement will look.
If you change the shutter speed while shooting under fluorescent or LED 
lighting, image flicker may be recorded.
If ISO Auto is set, you can press the <A> button to lock the ISO speed.
When you press the <A> button and then recompose the picture, you 
can see the exposure level difference on the exposure level indicator 
(p.22, 227) compared to when you first press the <A> button.
By pressing the <
B> button, you can display the histogram.