Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 172)

Caution
Image noise (dots of light or banding) and irregular colors may increase and
apparent resolution may decrease at H1, H2, or H3 (equivalent to ISO 204800,
409600, and 819200, respectively), because these are expanded ISO speeds.
Because L (equivalent to ISO 50) is an expanded ISO speed setting, the dynamic
range will be somewhat narrower compared to the standard setting.
Setting the ISO speed to L (equivalent to ISO 50) may result in loss of gradation in
highlights of your shots, even if gradation is visible in Live View image highlights.
Setting [
: Highlight tone priority ] to [ Enable ] [ Enhanced ] ( ) will prevent you
from selecting L (equivalent to ISO 50), ISO 100/125/160, H1, H2, or H3
(equivalent to ISO 204800, 409600, and 819200, respectively).
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.
Note
Low ISO speeds reduce image noise but may increase the risk of camera/subject
shake or reduce the area in focus (shallower depth of field), in some shooting
conditions.
High ISO speeds enable low-light shooting, a larger area in focus (deeper depth of
field), and longer flash range but may increase image noise.
Can also be set on the [
ISO speed ] screen in [ : ISO speed settings ].
Shutter sounds after the camera shoots may vary depending on the ISO speed
setting.
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