I. CAMERA FEATURESAND OPERATION 14
•C.Fn 18-0 isthe default setting. The camera can “register” (memorize) a user-specified
focusing pointor even automatic focusing point selection, then switch immediately to
the registered focusing pointby pressing the focusing point selector button and the
AssistButton.
•C.Fn 18-1 letsusers switch to the registered focusing point simply by pressing the
AssistButton.
•C.Fn 18-2 switchesto the registered focusing point only while the Assist Button is
pressed. Releasing the button causesthe camera to revert to the previously selected
focusing point. Thisis useful for switching quickly between user-selected and
pre-registered focusing points. For example, ifyou register the center focusing point
and then setthe camera for AFPS, you can switch between the two focusing point
selection methodsat will.
Custom Function 19programs the functions of the AF Stop buttons on a select group of
Image Stabilizer super-telephoto lensesincluding the EF 300mm f/2.8L IS USM, EF
400mm f/2.8LIS USM, EF 400mm f/4 DO IS USM, EF 500mm f/4L IS USM, and EF
600mm f/4LIS 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
buttonsis pressed. AF resumes when the AF Stop button is released. This isuseful for
sports or nature photographyif 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, AFoperation from the camera is disabled. This can be beneficial because it
letsyour left hand control AF operation, thereby freeing up your right hand to do other
thingslike selecting a new focusing point.
•In C.Fn 19-2, ifthe AF Stop button is pressed while the exposure metering system is
active, AELock takes effect in AE modes. This can be convenient when you wantto
focusand 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
instantlyswitches the camera to AFPS. This can be effective in AI Servo AF if it
becomesdifficult 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 ofthe AF Stop buttons
temporarilytoggles it to AI Servo AF, and vice versa. Thiscan be effective when
working with a subjectthat stops and moves frequently.
•In C.Fn 19-5, if the lens’ IS switch is on, the Image Stabilizer operates only when you
pressone of the AF Stop buttons. Autofocusing must be initiated from the camera
body. Thisoption is somewhat similar to C.Fn 19-1 because it lets you concentrate on
selecting focusing pointsand operating the shutter release with your right hand while
controlling ISwith your left hand.
Custom Function 20controls the degree of focusing “sensitivity” in AI Servo AF. Thisis
notthe same as AF calculation speed, which is always done as quickly aspossible.
Instead, whatis being adjusted is the length of time that the AF system is programmed
to stayat the most recently focused distance when the original subject is no longer
being tracked bythe active focusing point. This can frequently happen in sports