Editing Autofocus (section)

===Phase detection===
[[File:Autofocus phase detection.svg|thumb|300px|Phase detection: In each figure (not to scale), the purple skyline represents the object to be focused on, the red and green lines represent light rays passing through apertures at the opposite sides of the lens, and the yellow rectangle represents sensor arrays (one for each aperture). Figures 1 to 4 represent conditions where the lens is focused (1) too near, (2) correctly, (3) too far and (4) much too far. The phase difference between the two skyline profiles can be used to determine in which direction and how much to move the lens to achieve optimal focus.]]

Phase detection (PD) is achieved by dividing the incoming light into pairs of images and comparing them. [[Through-the-lens metering|Through-the-lens]] secondary image registration (TTL SIR) passive phase detection is often used in film and digital [[Single-lens reflex camera|SLR cameras]]. The system uses a [[beam splitter]] (implemented as a small semi-transparent area of the main reflex mirror, coupled with a small secondary mirror) to direct light to an AF sensor at the bottom of the camera. Two micro-lenses capture the light rays coming from the opposite sides of the lens and divert it to the AF sensor, creating a simple [[Rangefinder camera|rangefinder]] with a base within the lens's diameter. The two images are then analysed for similar light intensity patterns (peaks and valleys) and the separation error is calculated in order to find whether the object is in front focus or back focus position. This gives the direction and an estimate of the required amount of focus-ring movement.<ref>{{cite web|url=http://www.nikon.com/about/technology/rd/core/software/caf/index.htm|title=Nikon - Technology - Predictive Focus Tracking System|access-date=2013-11-12|archive-url=https://web.archive.org/web/20131112195552/http://www.nikon.com/about/technology/rd/core/software/caf/index.htm|archive-date=2013-11-12|url-status=dead}}</ref>

PD AF in a continuously focusing mode (e.g. "AI Servo" for [[Canon EOS|Canon]], "AF-C" for [[Nikon#Digital single lens reflex cameras|Nikon]], [[Pentax cameras#Digital|Pentax]] and [[Sony#Photography and videography|Sony]]) is a [[closed-loop control]] process. PD AF in a focus-locking mode (e.g. "One-Shot" for [[Canon EOS|Canon]], "AF-S" for [[Nikon#Digital single lens reflex cameras|Nikon]] and [[Sony#Photography and videography|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.<ref>{{cite web|url=http://www.dpreview.com/articles/5402438893/busted-the-myth-of-open-loop-phase-detection-autofocus|title=Busted! The Myth of Open-loop Phase-detection Autofocus}}</ref>

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{{citation needed|date=March 2011}} 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]], [[Canon EOS-1D|1D]], [[Canon EOS 30D|30D]]/[[Canon EOS 40D|40D]], [[Pentax K-1]], [[Sony DSLR-A700]], [[Sony DSLR-A850|DSLR-A850]], [[Sony DSLR-A900|DSLR-A900]]) also have a few "high-precision" focus points with an additional set of prisms and sensors; they are only active with "[[Lens speed#Fast lenses|fast lenses]]" with certain geometrical [[Aperture|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.

[[Image:US pat 5589909 fig 2.png|none|thumb|320px|Phase detection system:
7 – Optical system for focus detection;
8 – Image sensor;
30 – Plane of the vicinity of the exit pupil of the optical system for photography;
31, 32 – Pair of regions;
70 – Window;
71 – Visual field mask;
72 – Condenser lens;
73, 74 – Pair of apertures;
75 – Aperture mask;
76, 77 – Pair of reconverging lenses;
80, 81 – Pair of light receiving sections]]