Canon EOS 5DS Instruction Manual page 159

As H (equivalent to ISO 12800) is an expanded ISO speed setting, noise
(dots of light, banding, etc.) and irregular colors will be more noticeable,
and the resolution lower than usual.
As L (equivalent to ISO 50) is an expanded ISO speed setting, the
dynamic range will be somewhat narrower than usual.
If [z3: Highlight tone priority] is set to [Enable] (p.187), L (equivalent
to ISO 50), ISO 100/125/160, and H (equivalent to ISO 12800) cannot be
selected.
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.
If you use a high ISO speed and flash to shoot a close subject,
overexposure may result.
Under [z2: ISO speed settings], you can use [ISO speed range] to
expand the settable ISO speed range from L (equivalent to ISO 50) to H
(equivalent to ISO 12800) (p.161).
i: Setting the ISO SpeedN
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