Canon MARK IV User Manual (en)

Custom Functions are organized in four groups based on the function 
type: C.Fn I: Exposure, C.Fn II: Image/Flash exp (exposure)/Disp 
(Display), C.Fn III: Autofocus/Drive, and C.Fn IV: Operation/Others.
Note that some Custom Function numbers differ from the 
EOS-1D Mark III’s Custom Functions.
Custom Function Settings
C.Fn I: Exposure
C.Fn I -1     Exposure level increments
0: 1/3-stop set 1/3-stop compensation
1: 1-stop set
1/3-stop compensation
Sets full-stop increments for the shutter speed and aperture.
2: 1/2-stop set 1/2-stop compensation
Sets 1/2-stop increments for the shutter speed, aperture, and exposure 
C.Fn I -2     ISO speed setting increments
0: 1/3-stop
1: 1-stop
C.Fn I -3     Set ISO speed range
Disable: The settable ISO speed range will be 100 - 12800.
Enable:  The settable ISO speed will range from the highest ISO speed 
to the lowest ISO speed set with [Register].
Register: The highest ISO speed can be registered within 100 to H3 
(102400), and the lowest ISO speed can be registered within L 
(50) to H2 (51200). After entering the settings, select [Apply].
If 2 is set, the viewfinder and the top LCD panel display for 1/2-stop 
increments will change as follows: “
→  “
If you want to set the ISO speed range expansion in the same way as 
other EOS cameras, set the upper limit to [H1], [H2], or [H3], and the 
lower limit to [L].
Even if C.Fn I -3 is set to L as the lower limit, L cannot be selected during 
manual exposure movie shooting (ISO 50).