Canon EOS-1 - Manual (Page 15)

I. CAMERA FEATURES AND OPERATION 15
photography when another subject or obstacle temporarily blocks the original subject.
The default setting is 0.5 seconds, but with this Custom Function it can be tweaked
faster or slower according to the photographer’s personal preference.
• C.Fn 20-0 is the camera’s standard setting. It programs the AF to remain at the most
recently focused distance for up to 0.5 seconds while an obstacle is blocking the
original subject.
• C.Fn 20-1 (“Slow”) programs the AF to remain at the most recently focused distance
for up to 1 second while an obstacle is blocking the original subject.
• C.Fn 20-2 (“Moderately Slow”) programs the AF remain at the most recently focused
distance for up to 0.75 seconds while an obstacle is blocking the original subject.
• C.Fn 20-3 (“Moderately Fast”) programs the AF to refocus on a new subject
approximately 0.25 seconds after the original subject is no longer being tracked.
• C.Fn 20-4 (“Fast”) programs the AF to refocus on a new subject approximately 0.125
seconds after the original subject is no longer being tracked.
In C.Fn 20-3 and 20-4, the lens focusing motor may seem more “jittery” than usual as it
constantly refocuses the lens. This is normal.
Custom Function 21 first appeared with the EOS-1D Mark II. It is the same as Personal
Function 13 from the EOS-1D and EOS-1Ds. The purpose of this setting is to control the
camera’s shutter release timing during continuous bursts in AI Servo AF. In all cases,
release timing for the first shot is controlled by the photographer, and the camera will
fire whether or not the subject is in focus. This is called “Release Priority,” as opposed
to “Focus Priority,” in which the camera does not fire until the AF system has confirmed
that the subject is in focus.
• C.Fn 21-0: Focus Priority within a prescribed time limit * for the second and
subsequent shots in a continuous burst with AI Servo AF.
* The shutter will release at the framing rate set by the photographer, as long as lens drive can be completed
within a prescribed time limit. The shutter releases at the end of the prescribed time limit, even if focus has
not been achieved.
• C.Fn 21-1: Release Priority for the second and subsequent shots in a continuous burst
with AI Servo AF.
Some photographers prefer C.Fn 21-1 because it ensures that the camera will fire at a
consistent framing rate, even though it means that the subject may not be accurately
focused. However, this is clearly a “use at your own risk” scenario, because there is no
way to be sure that your photos will be sharp.
Personal Function 14 disables focus detection (“hunting”) by the lens drive. This can
save time in low light or low contrast situations, especially when working with a USM
lens equipped with a distance scale. Such lenses can be focused manually even when
set to their AF mode.