Canon EOS R10 User Manual page 206

Caution
Image noise (dots of light or banding) and irregular colors may increase and
apparent resolution may decrease at H (equivalent to ISO 51200), because this is
an expanded ISO speed.
Setting [ : Highlight tone priority] to [Enable] or [Enhanced] ( ) will prevent
you from selecting ISO 100/125/160 or H (equivalent to ISO 51200).
When shooting with a high ISO speed, high temperature, long exposure, or multiple
exposure, image noise (graininess, dots of light, banding, etc.), irregular colors, or
color shift 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.
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