In 1987, Canon released an autofocus (AF) SLR camera. Despite having just a single focus point in the center of the viewfinder, the AF-equipped camera was hailed for its revolutionary technology. In 1990 and 1992, the company launched cameras with three-point (central horizontalaxis direction) and five-point AF, respectively. Then, in 1998, Canon developed the EOS-3, an SLR camera with 45-point Area AF covering a wide range of 8 mm x 15 mm, putting the company in a class of its own for autofocus technology.

Autofocus automatically measures the distance between the camera and the subject, after which the lens automatically moves to the correct focus position. In the case of SLR cameras with five-point AF, five sets of line sensors are used to measure distance, guiding light that passes through the lens to the appropriate secondary image-formation lens for each AF point via the submirror. Since it was impossible to measure all 45 points by this method due to design constraints, Canon developed a new mechanism. Canon engineers made the submirror using an elliptical curved surface mirror, allowing light rays from 45 locations to be gathered by a single mirror and directed to two sets of secondary image-formation lenses. This new mechanism is based on the principle that light emanating from one elliptical focal point that is reflected from an elliptical surface must converge at a single point.

After light converges at the focal point, it once again passes through the secondary image-formation lens before being directed to the area AF sensor. The secondary image-formation lens, which comprises two sets of lenses equivalent to the left and right eyes of humans, as well as upper and lower "eyes" that humans do not have, form four parallax images on the area AF sensor. The distance from these images is calculated and, based on the value of the AF point selected by the photographer or automatically selected by the camera, the lens moves accordingly.

The area AF sensor was designed to conform to the 45-point Area AF. This sensor has 30 to 40 times more pixels than conventional AF line sensors, making possible quick and accurate detection of an image for each of the 45 points — a task that was impossible for previous sensors. Since 45-point area AF eliminates the need to position the subject at the center of the viewfinder, photographers can capture split-second photo opportunities with greater ease. The wide AF range enables photographers to successfully capture moving subjects, and is also well suited for sports photography.