An illustrated chronological story about how Canon autofocus systems and viewfinders went from a single AF point (1987) to 191 AF points (2020).
This was the first Canon AF system with multiple AF positions. Canon called this new AF sensor design their Multi-BASIS technology. This new approach not only brought changes to the actual AF sensor but it also required a redesign of the optical assembly, adding two pairs of lenslets to the separator lens array and two additional mask openings. This new Multi-BASIS system offered lower working light capability and therefore provided faster, accurate focusing even in dim light.
Cameras using this 3-point AF system have a viewfinder that displays three AF frames in the center of the focusing screen. The viewfinder also displayed a partial metering circle.
This first Multi-BASIS chip was similar to previous BASIS chip designs. Package size was increased to 22.0 mm × 11.2 mm. The detector lines were arranged as shown in the illustration. The C-V-BASIS (center-vertical-BASIS) used two 42-bit and the C-H-BASIS (center-horizontal-BASIS) used two 32-bit detectors. Both L-V-BASIS and R-V-BASIS (left-vertical-BASIS and right-vertical-BASIS) each used two 38-bit detectors. Two additional pairs of separator lenses were used to cover these L-V and R-V detector lines.
The center AF point was a cross-type detector sensitive to vertical and horizontal contrast lines. Each of the outer AF points used a vertical pair of detectors sensitive to horizontal contrast lines. All sensors required a maximum aperture of f/5.6 to operate (standard-precision). There was no f/2.8 (high-precision) detector available.
The system had an AF working range of EV 0 - 18 at ISO 100.
The EOS D30 was the first Canon camera that used a CMOS (Complementary Metal Oxide Semiconductor) image sensor with a resolution of 3.1 megapixels. Read more about CMOS sensors here.
This system offered the same number and layout of AF points than the earlier 3-point AF system. However, this version was designed for APS-C format cameras.
Cameras using this 3-point AF system have a viewfinder that displays three AF frames in the center of the focusing screen. The viewfinder also displayed a partial metering circle.
This Multi-BASIS chip was similar to previous BASIS chip designs. Although housing multiple AF sensors, this chip was very compact by using the original 14.0 mm × 8.2 mm package that had also been used in the first single-point AF system. To achieve this, the left and right pairs of detectors have been brought closer towards the center. The C-V-BASIS used two 42-bit and the C-H-BASIS used two 32-bit detectors. The L-V-BASIS and R-V-BASIS used two 32-bit detectors each.
The center AF point was a cross-type detector sensitive to vertical and horizontal contrast lines. Each of the outer AF points used a vertical pair of detectors sensitive to horizontal contrast lines. All sensors required a maximum aperture of f/5.6 to operate (standard-precision). There was no f/2.8 (high-precision) detector available.
The system had an AF working range of EV 2 - 18 at ISO 100.