Canon EOS 1D Mark III User Guide - Page 18

The 18 cross-type AF points other than the center AF point detect vertical lines at f/2.8 and horizontal lines at f/5.6. If the maximum aperture of the lens or lens/extender combination in use is f/2.8 or larger, high-precision, cross-type focusing is possible. Also, if the maximum aperture of the lens or lens/extender combination in use is f/5.6 or larger, horizontal-line sensitive AF is possible. The Assist AF sensors are horizontal-line sensitive at f/5.6. They have the same focusing detection performance as the EOS-1D Mark II's AF points (other than that camera's seven cross-type sensors). The EOS 5D's Assist AF points operate AF Sensor only during AI Servo AF when the camera is set for automatic AF point selection or the center AF point is manually selected and focusing point expansion is activated. However, with the EOS-1D Mark III, the Assist AF points now also operate during One-Shot AF when the camera is set for automatic AF point selection, or when any cross-type focusing point is manually selected and focusing point expansion is activated. The Assist AF points achieving focus also light up with the SI display. While the high-precision, cross-type AF points have substantial coverage in the Area AF, the center AF point can use cross-type focusing with f/4 and faster lenses and still can focus with f/8 lenses. The AF operation certainly befits an EOS-1D series camera. For lowlight focusing, to the EOS-1D Mark III is twice as sensitive as the EOS-1D Mark II N. The CMOS AF sensor's pixel sensitivity has been improved, thanks to improved pixel characteristics, an improved pixel fill factor due to finer semiconductor manufacturing processes, and optimized pixel size. As a result, the 1D Mark III's AF sensitivity has been improved to EV -1 through EV 18 (at 23°C/73°F, ISO 100). AF Unit As with previous EOS cameras, the TTL-SIR (Through-the-Lens Secondary Image Registration) focusing principle is employed. The light flux passes through the camera lens and passes through the half mirror at the center of the main mirror. The light flux proceeds downward by reflecting off the oblong, concave secondary mirror to a fully reflective mirror in the base of the camera's mirror chamber. Then it passes through an infrared- absorption glass, a secondary image-forming lens, and a cover glass before reaching the AF sensor. The AF unit uses materials having minimal swelling and moisture absorption characteris- tics, and the secondary image-forming lens is made of glass mold highly resistant to changes in size due to fluctuations in temperature and humidity. The AF unit is therefore very stable. Main mirror Main mirror Oblong, secondary mirror Reflective mirror Secondary image-forming lens Oblong, secondary mirror AF sensor Reflective mirror Infraredabsorption glass Secondary image-forming lens Cover glass AF sensor V. INCOMPARABLE EOS AUTOFOCUS, NOW EVEN BETTER 18