I. CAMERA FEATURES AND OPERATION
14
• C.Fn 18-0 is the default setting. The camera can “register” (memorize) a user-specified
focusing point or even automatic focusing point selection, then switch immediately to
the registered focusing point by pressing the focusing point selector button and the
Assist Button.
• C.Fn 18-1 lets users switch to the registered focusing point simply by pressing the
Assist Button.
• C.Fn 18-2 switches to the registered focusing point only while the Assist Button is
pressed. Releasing the button causes the camera to revert to the previously selected
focusing point. This is useful for switching quickly between user-selected and
pre-registered focusing points. For example, if you register the center focusing point
and then set the camera for AFPS, you can switch between the two focusing point
selection methods at will.
Custom Function 19
programs the functions of the AF Stop buttons on a select group of
Image Stabilizer super-telephoto lenses including the EF 300mm f/2.8L IS USM, EF
400mm f/2.8L IS USM, EF 400mm f/4 DO IS USM, EF 500mm f/4L IS USM, and EF
600mm f/4L IS USM. It has no effect with other lenses.
• C.Fn 19-0 is the default setting. Autofocus is suspended when one of the AF Stop
buttons is pressed. AF resumes when the AF Stop button is released. This is useful for
sports or nature photography if an obstacle is temporarily blocking the main subject.
• In C.Fn 19-1, AF operates only when the AF Stop button is pressed. While the button is
pressed, AF operation from the camera is disabled. This can be beneficial because it
lets your left hand control AF operation, thereby freeing up your right hand to do other
things like selecting a new focusing point.
• In C.Fn 19-2, if the AF Stop button is pressed while the exposure metering system is
active, AE Lock takes effect in AE modes. This can be convenient when you want to
focus and meter separately. In this mode, AF is activated from the camera.
• In C.Fn 19-3, if the camera is set for MFPS, pressing one of the AF Stop buttons
instantly switches the camera to AFPS. This can be effective in AI Servo AF if it
becomes difficult to track a moving subject with a single focusing point.
• In C.Fn 19-4, if the camera is set for One-Shot AF, pressing one of the AF Stop buttons
temporarily toggles it to AI Servo AF, and vice versa. This can be effective when
working with a subject that stops and moves frequently.
• In C.Fn 19-5, if the lens’ IS switch is on, the Image Stabilizer operates only when you
press one of the AF Stop buttons. Autofocusing must be initiated from the camera
body. This option is somewhat similar to C.Fn 19-1 because it lets you concentrate on
selecting focusing points and operating the shutter release with your right hand while
controlling IS with your left hand.
Custom Function 20
controls the degree of focusing “sensitivity” in AI Servo AF. This is
not the same as AF calculation speed, which is always done as quickly as possible.
Instead, what is being adjusted is the length of time that the AF system is programmed
to stay at the most recently focused distance when the original subject is no longer
being tracked by the active focusing point. This can frequently happen in sports
