Canon EOS Rebel T4i 18-135mm IS STM Lens Kit Instruction Manual page 91

When you shoot at high ISO speeds, noise (such as dots of light and
banding) may become noticeable.
If you use a high ISO speed and flash to shoot a close subject,
overexposure may result.
With ISO 12800 or "H" (equivalent to ISO 25600), the maximum burst for
continuous shooting will greatly decrease.
As "H" (equivalent to ISO 25600) is an expanded ISO speed setting,
noise (such as dots of light and banding) and irregular colors will be more
noticeable, and the resolution will be lower than usual.
As the maximum ISO speed that can be set differs between still photo
shooting and movie shooting (manual exposure), the ISO speed you set
may change when you switch from still photo shooting to movie shooting.
Even if you switch back to still photo shooting, the ISO speed will not
revert to the original setting.
The maximum ISO speed settable varies depending on the setting for [2:
ISO expansion] under [54: Custom Functions (C.Fn)].
• When [0: Off] is set: If you set ISO 12800 during still photo shooting
and then switch to movie shooting, ISO speed will be changed to ISO
6400.
• When [1: On] is set: If you set ISO 12800/H (equivalent to ISO 25600)
during still photo shooting and then switch to movie shooting, ISO
speed will be changed to H (equivalent to ISO 12800).
ISO [AUTO]
i: Changing the ISO SpeedN
If the ISO speed is set to [AUTO], the
actual ISO speed to be set will be
displayed when you press the shutter
button halfway. As indicated on the next
page, the ISO speed will be set
automatically to suit the shooting mode.
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