An illustrated chronological story about how Canon autofocus systems and viewfinders went from a single AF point (1987) to 191 AF points (2020).
The EOS 650 was the world's first EOS camera. For that reason, this single-point AF system was the first autofocus system ever used on Canon's EOS platform of DSLR cameras. It was the beginning of a race to increase phase detection autofocus performance for more than 30 years.
Both the Canon EOS 650 and the EOS 620 had a viewfinder that displayed one AF frame in the center of the focusing screen. The viewfinder also displayed a partial metering circle.
Canon named this innovative sensing device the BASIS (Base-Stored Image Sensor) chip. The BASIS chip consisted of support structures, leadframes, and the active sensor area embedded inside a clear-molded 14.0 mm × 8.2 mm package. This sensor had one set of horizontal detectors: The two black stripes were the actual light sensitive parts. The resolution of each stripe can be described as 47-bit, indicating that each detector line consisted of 47 tiny photoreceptor cells arranged next to each other. Some of the active area surrounding the detector lines was used for signal amplification and processing. The amplification circuitry allowed to use autofocus even in low light situations.
Due to the horizontal detector orientation, the AF point was sensitive to vertical contrast lines only. The detector used light that passed through the f/5.6 zone of the photo-taking lens. Therefore, autofocus was possible if the used lens offered a maximum aperture of f/5.6 or faster. Consequently, AF was not possible if the used lens was slower than f/5.6.
The system had an AF working range of EV 1 - 18 at ISO 100.
While this system still relied on a single AF point, the underlying AF sensor was more advanced. In addition to the pair of horizontal detectors (H-BASIS), this new system added a pair of vertical detectors (V-BASIS).
The Canon EOS 1 had a viewfinder that displayed one AF frame in the center of the focusing screen. The viewfinder also displayed a partial metering circle and a spot metering circle.
The overall sensor design was very similar to the previous one, where a clear 15.0 mm × 8.2 mm package was molded around the inner structures. This AF sensor was mounted to the phase detection unit in a vertical orientation.
One pair of vertical detectors was added to the device, creating the CT-BASIS (Cross-Type Base-Stored Image Sensor) chip. The two horizontal lines were 47-bit detector arrays and the two vertical ones were 29-bit detector arrays. The sensor pitch (distance between the individual photoreceptor cells) was 10% finer compared to the previous BASIS sensor.
Another innovation was the integration of two fine spot metering sensors (silicon photo cells) into the chip. This allowed the camera to perform very precise light readings around the center AF point. Another improvement was made by applying an antireflective film to the active area of the sensor. This prevented stray light from degrading the AF signal.
The cross-type sensor was sensitive to both vertical and horizontal contrast lines. The distance between vertical detectors was larger than that of horizontal detectors. This increased distance allowed high-precision autofocus detection on the vertical detector (sensitive to horizontal contrast lines). These high-precision vertical detectors required light from an outer area of the lenses exit pupil / aperture. For that reason, the vertical (high-precision, horizontal-sensitive) component of this sensor became active only when an f/2.8 or faster lens was attached to the camera. With lenses slower than f/2.8 but at least f/5.6, this autofocus system only offered standard-precision horizontal detectors (sensitive to vertical contrast lines). Autofocus was not possible with lenses slower than f/5.6.
The system had an AF working range of EV -1 - 18 at ISO 100.