Editing Autofocus (section)

==Passive==
Passive AF systems determine correct focus by performing passive analysis of the image that is entering the optical system. They generally do not direct any energy, such as ultrasonic sound or infrared light waves, toward the subject. (However, an autofocus assist beam of usually infrared light is required when there is not enough light to take passive measurements.) Passive autofocusing can be achieved by phase detection or contrast measurement.

===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]]

===Contrast detection===
Contrast-detection autofocus is achieved by measuring [[contrast (vision)]] within a sensor field [[Through-the-lens metering|through the lens]]. The intensity difference between adjacent pixels of the sensor naturally increases with correct image focus. The optical system can thereby be adjusted until the maximal contrast is detected. In this method, AF does not involve actual distance measurement at all. This creates significant challenges when [[Video tracking|tracking moving subjects]], since a loss of contrast gives no indication of the direction of motion towards or away from the camera.

Contrast-detect autofocus is a common method in [[digital camera]]s that lack [[Shutter (photography)|shutter]]s and reflex mirrors.  Most [[digital single-lens reflex camera|DSLR]]s use this method (or a hybrid of both contrast and phase-detection autofocus) when focusing in their [[live-preview digital camera|live-view]] modes. A notable exception is Canon digital cameras with Dual Pixel CMOS AF. [[Mirrorless interchangeable-lens camera]]s typically used contrast-measurement autofocus, although phase detection has become the norm on most mirrorless cameras giving them significantly better AF tracking performance compared to contrast detection.

Contrast detection places different constraints on lens design when compared with phase detection. While phase detection requires the lens to move its focus point quickly and directly to a new position, contrast-detection autofocus instead employs lenses that can quickly sweep through the focal range, stopping precisely at the point where maximal contrast is detected. This means that lenses designed for phase detection often perform poorly on camera bodies that use contrast detection.

==={{anchor|Illuminator}}Assist lamp===
The assist light (also known as AF illuminator) "activates" passive autofocus systems in low-light and low-[[Contrast (vision)|contrast]] situations in some cameras. The lamp projects visible or [[infrared|IR]] light onto the subject, which the camera's autofocus system uses to achieve focus.

Many cameras and nearly all [[camera phone]]s{{efn|Counter-examples are the [[Nokia Lumia 1020]], the [[Samsung Galaxy S4 Zoom]] and the [[Samsung Galaxy K Zoom]].}} lack a dedicated autofocus assist lamp.  Instead, they use their built-in flash, illuminating the subject with bursts of light.  This aids the autofocus system in the same fashion as a dedicated assist light, but has the disadvantage of startling or annoying people.  Another disadvantage is that if the camera uses flash focus assist and is set to an operation mode that overrides the flash, it may also disable the focus assist.  Thus, autofocus may fail to acquire the subject.  

Similar [[Stroboscopic effect|stroboscopic]] flashing is sometimes used to reduce the [[red-eye effect]], but this is only intended to constrict the subject's eye pupils before the shot.

Some external [[flash gun]]s have integrated autofocus assist lamps that replace the stroboscopic on-camera flash.  Many of them are red  and less obtrusive.  Another way to assist contrast based AF systems in low light is to beam a laser pattern on to the subject.  The laser method is commercially called Hologram AF Laser and was used in [[Cyber-shot|Sony CyberShot]] cameras around the year 2003, including Sony's F707, [[Sony Cyber-shot DSC-F717|F717]] and [[Sony Cyber-shot DSC-F828|F828]] models.