Iso Auto - Canon EOS 5D Mark III Instruction Manual

As H1 (equivalent to ISO 51200) and H2 (equivalent to 102400) are
expanded ISO speed settings, noise (dots of light, banding, etc.) and
irregular colors will be more noticeable, and the resolution lower than usual.
If you use a high ISO speed and flash to shoot a close subject,
overexposure may result.
If you shoot a movie when [Maximum: 25600] is set with [ISO speed range]
and ISO speed is set to ISO 16000/20000/25600, the ISO speed will switch to
ISO12800 (during movie shooting with manual exposure). Even if you switch
back to still photo shooting, the ISO speed will not revert to the original setting.
If you shoot a movie when 1. L (50) or 2. H1 (51200)/H2 (102400) is set, the ISO
speed setting will switch to 1. ISO 100 or 2. H (25600) respectively (during movie
shooting with manual exposure). Even if you switch back to still photo shooting,
the ISO speed will not revert to the original setting.
Under [z2: ISO speed settings], you can use [ISO speed range] to
expand the settable ISO speed range from ISO 50 (L) to ISO 102400 (H2) (p.128).
<h> can be displayed in the viewfinder when you set an expanded ISO
speed (L, H1, or H2) (p.324).

ISO Auto

Shooting Mode
A
d / s / f / a
F
With flash
*1: The actual ISO speed range depends on the [Minimum] and [Maximum]
settings set in [Auto ISO range].
*2: If fill flash will result in overexposure, ISO 100 or a higher ISO will be set.
*3: When using bounce flash with an external Speedlite in the <A> and <d>
modes, the ISO speed will be set automatically within ISO 400 - 1600.
If the ISO speed is set to "A" (Auto), the
actual ISO speed to be set will be displayed
when you press the shutter button halfway.
As indicated below, the ISO speed will be
set automatically to suit the shooting mode.
ISO Speed Setting
Automatically set within ISO 100 - 12800
Automatically set within ISO 100 - 25600*
Fixed at ISO 400*
1
Fixed at ISO 400*
1
i: Setting the ISO SpeedN
* *
2 3
1
127