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AF System Type
The performance of any automatic focusing system depends primarily on its ability to recognize the subject to be photographed. The EOS 1 uses a CT-TTL-SIR (“Cross-Type, Through-The-Lens, Secondary Image Registration”) phase detection mechanism for this purpose.
Cross-Type refers to the shape of the focusing area. Unlike other focusing sensors that detect focusing information in only one dimension, the EOS 1’s Cross-Type sensor can read focusing information in both horizontal and vertical sections of the viewfinder image. (However, the vertical component of the EOS 1’s AF sensor is only available for use with Canon EF Lenses that have maximum apertures of f2.8 or greater.) This feature enhances the camera’s ability to recognize difficult AF subjects, by increasing the area of the image to be evaluated and by enabling horizontal subject contrast to be detected.
Secondary Image Registration means that the autofocusing system uses a “secondary image” which is created by an auxiliary optical system that diverts part of the light beam entering the lens to a point inside the camera that is geometrically equivalent to the film plane. This system enables the viewfinder image to remain uniformly bright, while at the same time, passing sufficient information to the camera’s AF detection system.
Phase Detection means that the AF system obtains its information by dividing the incoming light into pairs of images that can be compared to each other. This sort of AF system is classified as a “passive” type, because it has no moving parts and emits no energy toward the subject. Instead, it analyzes the image presented to it.