Canon EOS-3 Instructions Manual page 111

This makes it easier to change the aperture during studio sessions with
studio flash units. Also, when AEB is used in the manual exposure mode,
the shutter speed can be fixed while only the aperture is shifted for AEB.
If you are using a super telephoto lens with more than one EOS-3 body,
you can still set the aperture with the camera(s) not attached to the lens.
This function is mainly for pro photographer assistants.
Same as setting 2.
• To use this Custom Function together with Custom Function CF-11,
see page 123.
Remarks
Custom Function Settings
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