text: PD AF in a continuously focusing mode (e.g. "AI Servo" for Canon, "AF-C" for Nikon, Pentax and Sony) is a closed-loop control process. PD AF in a focus-locking mode (e.g. "One-Shot" for Canon, "AF-S" for Nikon and Sony) is widely believed to be a "one measurement, one movement" open-loop control process, but focus is confirmed only when the AF sensor sees an in-focus subject. The only apparent differences between the two modes are that a focus-locking mode halts on focus confirmation, and a continuously focusing mode has predictive elements to work with moving targets, which suggests they are the same closed-loop process.
  Although AF sensors are typically one-dimensional photosensitive strips (only a few pixels high and a few dozen wide), some modern cameras (Canon EOS-1V, Canon EOS-1D, Nikon D2X) feature TTL area SIR sensors that are rectangular in shape and provide two-dimensional intensity patterns for a finer-grain analysis.  Cross-type focus points have a pair of sensors oriented at 90° to one another, although one sensor typically requires a larger aperture to operate than the other.
  Some cameras (Minolta 7, Canon EOS-1V, 1D, 30D/40D, Pentax K-1, Sony DSLR-A700, DSLR-A850, DSLR-A900) also have a few "high-precision" focus points with an additional set of prisms and sensors; they are only active with "fast lenses" with certain geometrical apertures (typically f-number 2.8 and faster). Extended precision comes from the wider effective measurement base of the "range finder".
  Some modern sensors (for example one in Librem 5) include about 2% phase detection pixels on the chip. With suitable software support, that enables phase detection auto focus.