Canon EOS 20D Service Manual page 84

Technical Information
4) Focusing computation
The AF speed is on par with the EOS 10D thanks to the high-performance, 32-
bit RISC microcomputer (main clock 32 MHz, A/D conversion 4 µs, minimum
command execution of 0.03 µs) and an improved AF sequence (AF processing
front loading, SI illumination, metering executed in parallel with the lens drive
and mirror swing-up) to match.
5) One-Shot AF
The high-speed computing and lens drive processing and the latest high-speed
AF algorithm employed since the EOS REBEL Ti/300V enable a faster One-Shot
AF speed than the EOS 10D despite having nine AF points.
6) AI SERVO AF
(1) Predictive AF computation
Predictive AF can focus track a
subject approaching at 50 kph up
to 8 meters away with an EF
300mm f/2.8L IS USM.
As with the EOS 10D, the EOS
20D's predictive AF computation
uses statistical prediction that
incorporates the focusing data of
past focusing operations. Since it
can repeat more focusing
operations in a short length of
time, the predictive AF control can
effectively operate from the first
shot even for a subject moving
erratically. Also, even if the subject
movement changes right before the
shot is taken, the predictive AF
control will have a good chance of
catching it.
(2) For still subjects
Even when AI SERVO AF is used to focus a still subject, the focus control is
stable enough so that the lens drive does not move minutely. If the subject
moves suddenly, the camera will be able to track it immediately.
(3) Complete pressing of shutter button
As with the EOS-1D Mark II, when focusing is possible, the lens drive is
executed based on the focusing result right before the shutter release. The
shot is then taken.
100
(m)
20
10
8
1
5 10
Fig. 020 Predictive AF performance
12
50 100 500
Subject's speed (kph)