Canon EOS-1D X Mark III Advanced User's Manual page 173

ISO Speed Range When Set Manually
You can set the manual ISO speed setting range (minimum and maximum limits). You can
set the minimum limit within L (equivalent to ISO 50) to H2 (equivalent to ISO 409600), and
the maximum limit within ISO 100 to H3 (equivalent to ISO 819200).
1.
Select [
2.
Select [ISO speed range].
3.
Set [Minimum].
: ISO speed settings].
Select the [Minimum] box, then press <
Select the ISO speed, then press < >.
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