Canon EOS 20D Service Manual page 83

(2) Focusing principle
As with previous EOS cameras, TTL-SIR (Through-The-Lens Secondary
Image Registration) is incorporated.
(3) Actual AF optics
Fig. 018 shows how the actual AF optics work. The light flux goes through
the camera lens, then the focusing light flux that passes through the half
mirror (40%) reaches the secondary mirror (flat and fully reflective) where it is
bent downward by 66
Fig. 019 shows the AF unit's basic configuration. While based on the EOS
10D's AF unit, it has the following improvements:
[1] Configured to minimize ghosting
Unwanted light is blocked by narrowing the field of view. The separator
uses a low-reflectance material.
[2] Highly resistant to temperature and humidity changes
The AF optics and framework have a low-line expansion coefficient and
low moisture absorption coefficient.
Main mirror
Field lens
AF mirror
Infrared cut filter
Secondary image-
formation lens
Fixed diaphragm
Fig. 018 Actual AF optics
3) AF mode
Like the EOS 10D, the EOS 20D has three AF modes: One-Shot AF, AI SERVO
AF, and AI Focus AF. In the Basic Zone modes, the optimum AF mode is set
automatically. In the Creative Zone modes, you can select any AF mode.
In the Sports mode (AI Servo AF set automatically) and modes where AI Focus
AF is set automatically, a low-volume beep sounds when focus is achieved in the
AI SERVO AF mode (beeper enabled).
Note: In the Creative Zone modes when AI SERVO AF is set, the beeper will not sound even when
focus is achieved.
Light-blocking cover
Secondary mirror
Light-blocking sheet
Field lens
Separator 2
Washer
AF sensor
11
Technical Information
Separator 1
AF plate
Secondary image-
Infrared cut filter
formation lens
AF mirror Fixed diaphragm
Fig. 019 AF unit configuration
Dust sheet
Sensor holder
AF sensor
AF holder