Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 427)

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3 Custom Function Settings
Same exposure for new aperture C.Fn2
If the < a > mode (manual exposure shooting) is set and the ISO
speed is set manually (except when set to ISO Auto), the open
aperture value may change to a higher number (a smaller aperture) if
you perform any of the following: 1. Change the lens, 2. Attach an
Extender, or 3. Use a zoom lens whose open aperture value
(f/number) changes. If you then shoot while the maximum aperture is
set, the image will be underexposed by the amount the maximum
aperture f/number changes to a higher number. However, by changing
the ISO speed or shutter speed (Tv) automatically, you can obtain the
same exposure that would be obtained without performing 1, 2, or 3.
OFF: Disable
Automatic changes in settings to maintain the specified exposure
will not be applied. The ISO speed, shutter speed, and aperture
already set will be used for shooting. If you perform 1, 2, or 3 and
the maximum aperture becomes slower, adjust the ISO speed and
shutter speed before you shoot.
ISO: ISO speed
If you perform 1, 2, or 3, a higher ISO speed will be set
automatically to compensate for the number of stops the maximum
aperture has become slower. The same exposure that would be
obtained without performing 1, 2, or 3 is thereby obtained. The ISO
speed will change automatically within the range set with [
Range
for stills ].
ISO/Tv: ISO speed/Shutter speed
If you perform 1, 2, or 3, a higher ISO speed will be set
automatically to compensate for the number of stops the maximum
aperture has become slower. If the ISO speed reaches the upper
limit of the range set with [
Range for stills ], a slower shutter
speed will be set automatically. The same exposure that would be
obtained without performing 1, 2, or 3 is thereby obtained. The
shutter speed will change automatically within the range set with
[
8 3: Set shutter speed range ].