Canon EOS 5D Mark II - EOS 5D Mark II 21.1MP Full Frame CMOS Digital SLR..., Service Manual

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Technical Information
6
1.3 AF
１）AF system
The new AF system aims to improve the subject
detection and focusing precision at the center which is
used most often. It also aims to enhance subject tracking
performance. As shown in Fig. 010, the AF points are
concentrated at the center. The extreme left and right
AF points are positioned at the same place as the EOS-
1Ds Mark II's extreme left and right AF points. Subject
framing can thereby be given priority.
The AF sensor (Fig. 011) on the CMOS sensor has
nine AF points (plus 6 Assist AF points). They have
been newly developed to greatly improve focusing
performance at the center. The AF algorithm and AF
circuitry also have a new, dedicated design to attain
high-speed and high-precision AF.
⑴High-performance focusing at center
The following technologies have been incorporated
so that the frequently-used center AF point can focus
better:
・ Extreme defocus detection sensor
A subject way out of focus can be detected when
the center AF point sensitive to horizontal lines at
f/5.6 (Fig. 012) has a line twice as long. As a result,
the lens focus driving for detecting the subject is
shorter so that the AF is fast and easy.
・ f/2.8-sensitive sensor
With lenses brighter than f/2.8, the center AF
point uses the f/2.8 light flux to focus. The base
line of the f/2.8 and vertical line-sensitive sensor
is about twice as long as the f/5.6-sensitive sensor.
The focusing detection is thereby faster.＊
＊ Except with EF50mm f/2.5 MACRO and EF28-80mm f/2.8-4L
USM.
・ Four-line focusing with f/5.6-sensitive, cross-type
center AF point
The center AF point's f/5.6-sensitive, vertical/
horizontal line-sensitive sensors each have two lines
in a zigzag pattern for a total of four lines for cross-
type focusing at the center. It reduces irregular
focusing detection and increases the chances of
focusing difficult subjects.
If the lens is brighter than f/2.8, first the f/5.6-
sensitive, cross-type center sensor is used to focus.
When focus is almost achieved, it switches to the
f/2.8-sensitive sensor for high-precision focusing. It
is a two-step focusing control process.
EOS-1Ds focusing area
EOS 5D AF points
EOS 5D Assist AF points
EOS-1Ds AF points (for comparison)
Fig. 010 Position of AF points in
viewfinder
Fig. 011 AF sensor
f/2.8 / f/5.6-sensitive sensor
f/5.6-sensitive
major defocus
detection sensor
f/5.6-sensitive sensor
Fig. 012 AF sensor positions
Focusing area AF points x 9
7.5
3.6
1.
8
3.
2
Assist AF points
Fig. 013 Actual focusing area