Custom Function (C.Fn)
From The Glossary of Digital Photography
A feature of Canon digital cameras. A two-digit number usually accompanies the Custom Function. Each C.Fn refers to a special feature on the camera that can be addressed and changed through the menus. For example, C.Fn 01 turns on long-exposure noise reduction. This allows each photographer to customize the camera's function in a way that suits his or her work.
