Focusing Principle; Actual Focusing Optics; Af Mode And Af Point Selection; Focusing Computation - Canon EOS 5D Service Manual

⑵Focusing principle

It is the same as the TTL-SIR (Through-The-Lens Secondary Imaged Registration)
system used in previous EOS cameras.

⑶Actual focusing optics

Fig. 016 shows the actual focusing optics. The focusing light flux passes through
the camera lens and half-silvered mirror (40%), then it is reflected downward by the
secondary mirror (flat, fully reflective). It passes through the field-of-view aperture, field
lens, AF mirror (fully reflective), infrared-blocking filter, fixed aperture, secondary image-
formation lens, and reaches the focusing sensor.
Fig. 017 shows the AF unit's basic construction. The unit is designed to create no
ghosting. It also uses materials which can withstand changes in the temperature and
humidity, for more stable AF performance.
Note 1: Ghosting countermeasures: The field-of-view aperture eliminates stray light, and the separators use a
low-reflectance material.
Note 2: Temperature/humidity countermeasures: The focusing optics and chassis use materials having a low-line
expansion coefficient and low absorption coefficient.
Main mirror
AF sensor
Secondary image-
Infrared cut filter
formation lens
Fixed diaphragm
Fig. 016 Actual AF optics
３）AF mode and AF point selection
The One-Shot AF, AI SERVO AF, and AI Focus AF modes are provided. In the Full Auto
mode, AI SERVO AF is set automatically. In other shooting modes, the user can set any of
the three AF modes. Specifications for AF point selection are the same as with the EOS 20D.
４）Focusing computation
The AF-dedicated, 32-bit RISC (main clock speed 32MHz) microcomputer enables high-
speed computing. Also, the parallel processing of the AF sequence (SI display and metering
at the same time as the lens driving and mirror reflex) helps to attain the same AF speed as
the EOS 20D.
Secondary
mirror
Field lens
AF sensor
Sensor holder
Secondary image-formation lens
AF mirror
8
Technical Information
Light-shielding cover
Light-shielding seat
Field lens
AF holder
Fixed diaphragm
Infrared cut filter
Separator 2
Separator 1
Fig. 017 AF unit construction
AF plate
AF mirror