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.
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.
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
