Canon EOS 70D Instruction Manual page 121

i: Setting the ISO SpeedN
If [z4: Highlight tone priority] is set to [Enable], ISO 100/125/160 and
"H" (equivalent to ISO 25600) cannot be set (p.145).
Shooting in high temperatures may result in images that look grainier.
Long exposures can also cause irregular colors in the image.
When you shoot at high ISO speeds, noise (such as dots of light and
banding) may become noticeable.
When shooting in conditions that produce an extreme amount of noise,
such as a combination of high ISO speed, high temperature and long
exposure, images may not be recorded properly.
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.
If you use a high ISO speed and flash to shoot a close subject,
overexposure may result.
If you shoot a movie while "H" (equivalent to ISO 25600) is set, it will
switch to ISO 12800 (with movie manual exposure shooting). Even if you
switch back to still photo shooting, the ISO speed will not revert to the
original setting.
Under [z3: ISO speed settings], you can use [ISO speed range] to
expand the settable ISO speed range up to ISO 25600 (H) (p.123).
<z> can be displayed in the viewfinder when you set the "H" expanded
ISO speed (p.376).
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