EOS REBEL T5, EOS 1200D INSTRUCTION MANUAL

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Custom Functions are organized into four groups based on the function
type: C.Fn I: Exposure, C.Fn II: Image, C.Fn III: Autofocus/Drive, C.Fn
IV: Operation/Others.
3 Custom Function Settings N
C.Fn I: Exposure
C.Fn-1 Exposure level increments
0: 1/3-stop
1: 1/2-stop
Sets 1/2-stop increments for the shutter speed, aperture, exposure
compensation, AEB, flash exposure compensation, etc. This is
effective when you prefer to control the exposure in less fine
increments than 1/3-stop.
C.Fn-2 ISO expansion
0: Off
1: On
When you set the ISO speed, you will be able to set “H”
(equivalent to ISO 12800). Note that if [C.Fn-6: Highlight tone
priority] is set to [1: Enable], “H” cannot be set.
With setting 1, the exposure level will be displayed in the viewfinder and on
the LCD monitor as shown below.