Canon EOS White Paper page 11

EV 18. Finally, the wider focusing area with nine AF points and the diagonal configura-
tion of AF points in and around the center afford improved subject focus tracking.
Focusing speed is on a par with that of the EOS 30D because the Digital Rebel XTi
shares its high-speed 32-bit RISC microcomputer. The algorithm for AI SERVO AF contin-
uous shooting has been fine-tuned and optimized for 3 frames per second.
In the Creative Zone modes, the following AF modes can be selected with the AF mode
selection screen: One-Shot AF (when focus is achieved, AF operation stops and locks),
Predictive AI SERVO AF (tracks subject movement and focuses continuously until the start
of exposure) and AI Focus AF (automatic switching between One-Shot and Predictive AI
Servo AF). In the Basic Zone modes, the optimum AF mode is selected automatically, a
convenience for the less technically inclined, certainly a part of the XTi's demographic. In
the Sports, Flash Off, and Basic Zone's Full Auto modes, the beeper sounds softly (when
the beeper is enabled) when focus is achieved with AI SERVO AF. In the Creative Zone
modes with AI SERVO AF, the beeper does not sound when focus is achieved. The AF
mode can be set even while the lens focus mode switch is set to MF. (When focus is
achieved, the focus confirmation icon and superimposed display lights up.) However,
"MF" will be displayed on the LCD screen when it displays the camera settings
The predictive AF calculation speed is the same as that of the Digital Rebel XT. With an
EF300mm f/2.8L IS USM lens, the camera can focus-track a moving subject approaching
at 31 mph/50 kph up to about 32.8 feet/10 meters away. As with the EOS 30D, the
lens drive keeps still when focusing a still subject with AI SERVO AF. If the subject
begins to move, the camera can start to focus-track the subject immediately because
focus detection is conducted continuously. As in the case of the EOS 30D, if you press
the shutter button completely in one quick stroke (rather than pressing halfway first),
the lens will focus, if possible, before the picture is taken. If focusing is not possible,
the picture will be taken anyway to give priority to taking the picture. This extreme
instance of shooting priority is especially useful should something occur that is so
remarkable that even less than perfect focus will not spoil the record of the moment.
As with the Digital Rebel XT, the XTi's AF-assist beam is a series of stroboscopic flashes.
It is not emitted in the Landscape, Sports or Flash OFF modes. In a Creative Zone mode,
it fires automatically when the built-in flash has been popped up manually. Firing can
be enabled/disabled with C.Fn-5. Emission time is 250 ms or less. The working range
is approximately 13.1 feet/4 meters at the center and approximately 11.5 feet/3.5
III. FEATURE DISCUSSION
Because the Digital Rebel XTi uses the same AF point
selection algorithm as the EOS 30D, there is a high prob-
ability that the correct AF point will be selected automati-
cally. The selection procedure is the same as with the
Digital Rebel XT. With the Digital Rebel XTi, you can select
the AF point manually on the LCD screen's large and easy-
to-read display.
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