Compatibility With Ef-S Lenses; Af System - Canon EOS 20D Service Manual

1.3 Compatibility with EF-S lenses

Like the EOS DIGITAL REBEL/EOS 300D DIGITAL,
the camera is compatible with EF-S lenses. EF-S lens
compatibility is enabled by the mirror swing-up
mechanism that moves the
mirror to the back as the
mirror goes up. The lens
mount periphery is also
lower.
The lens mount index
(white) for EF-S lenses has
been moved to the stainless
steel mount surface to make it easier to be seen.

1.4 AF system

A newly-developed, 9-point, CMOS sensor serves as the AF sensor. It is the first
time that an f/2.8-compatible sensor is incorporated in a camera other than the
EOS-1 and EOS-3 series of cameras. It enables higher AF precision.
The AF circuitry and AF algorithm are based on the EOS 10D's, and they
improve the overall AF performance.
1) AF system objectives
•
AF detection precision
(1) f/2.8-compatible center AF point
When you use a lens brighter than f/2.8, focusing is detected with the
f/2.8 light flux. The base line length of the f/2.8-compatible, horizontal AF
point at the center is about twice as long as the f/5.6-compatible sensor's
for higher focusing precision. (except with EF50mm f/2.5 MACRO and
EF28-80mm f/2.8-4L USM)
(2) Two-line focusing with f/5.6-compatible center vertical sensor
With two lines in a zigzag pattern for focusing, the focus detection becomes
more consistent.
(3) Higher precision of f/5.6-compatible peripheral vertical sensor
For better focusing precision, the base line length of the diagonal and left
and right AF points is 30% longer than the EOS 10D's.
•
Improved focusing performance in low light
Compared with the EOS 10D, the performance is better by EV 1 (EV -0.5 to
18).
•
Larger area of focus
(1) AF points are at the diagonal position (golden section).
(2) Nine AF points provide a better chance of covering the subject.
EF-S lens
mount index
EF lens
mount index
Fig. 012 Lens mount index
9
Technical Information
EOS 10D
EOS 20D
Lens rear
edge
Fig. 013 Swing-up mechanism