How Does Camera Focus Work

Optical system to focus on an automatically or manually selected point or expanse

For the flick of the same name, see Auto Focus.

Several greenish focus points/areas indicating where the autofocus has locked

I selected green focus point using pinpoint autofocus

An autofocus (or AF) optical system uses a sensor, a control system and a motor to focus on an automatically or manually selected indicate or surface area. An electronic rangefinder has a display instead of the motor; the adjustment of the optical system has to be washed manually until indication. Autofocus methods are distinguished as active, passive or hybrid types.

Autofocus systems rely on 1 or more sensors to make up one's mind right focus. Some AF systems rely on a unmarried sensor, while others use an assortment of sensors. Almost modern SLR cameras use through-the-lens optical sensors, with a separate sensor assortment providing light metering, although the latter can be programmed to prioritize its metering to the aforementioned expanse as one or more than of the AF sensors.

Through-the-lens optical autofocusing is usually speedier and more than precise than transmission focus with an ordinary viewfinder, although more than precise manual focus tin can be accomplished with special accessories such as focusing magnifiers. Autofocus accurateness inside ane/3 of the depth of field (DOF) at the widest aperture of the lens is mutual in professional AF SLR cameras.

Nearly multi-sensor AF cameras allow manual selection of the active sensor, and many offer automatic selection of the sensor using algorithms which attempt to discern the location of the subject. Some AF cameras are able to detect whether the subject is moving towards or abroad from the photographic camera, including speed and acceleration, and proceed focus — a function used mainly in sports and other action photography. Canon cameras call this AI servo; Nikon cameras call it "continuous focus".

The data collected from AF sensors is used to control an electromechanical arrangement that adjusts the focus of the optical system. A variation of autofocus is an electronic rangefinder, in which focus data are provided to the operator, but aligning of the optical system is still performed manually.

The speed of the AF arrangement is highly dependent on the widest aperture offered past the lens at the current focal length. F-stops of around f/2 to f/two.8 are generally considered best for focusing speed and accuracy. Faster lenses than this (e.g.: f/1.4 or f/1.eight) typically have very low depth of field, meaning that it takes longer to accomplish correct focus, despite the increased amount of light. Most consumer photographic camera systems volition only autofocus reliably with lenses that accept a widest aperture of at least f/5.6, whilst professional person models can frequently cope with a widest aperture of f/8, which is particularly useful for lenses used in conjunction with teleconverters.^{[ citation needed ]}

History [edit]

Between 1960 and 1973, Leitz (Leica)^[1] patented an array of autofocus and corresponding sensor technologies. At photokina 1976, Leica had presented a photographic camera based on their previous evolution, named Correfot, and in 1978 they displayed an SLR camera with fully operational autofocus.

The first mass-produced autofocus camera was the Konica C35 AF, a simple point and shoot model released in 1977. The Polaroid SX-70 Sonar OneStep was the first autofocus unmarried-lens reflex camera, released in 1978.

The Pentax ME-F, which used focus sensors in the camera body coupled with a motorized lens, became the first autofocus 35 mm SLR in 1981.

In 1983 Nikon released the F3AF, their outset autofocus camera, which was based on a like concept to the ME-F.

The Minolta 7000, released in 1985, was the first SLR with an integrated autofocus system, meaning both the AF sensors and the bulldoze motor were housed in the photographic camera body, every bit well equally an integrated movie advance winder — which was to go the standard configuration for SLR cameras from this manufacturer, and as well Nikon abandoned their F3AF system and integrated the autofocus-motor and sensors in the body.

Canon decided to discontinue their FD mount and switched to the completely electronic EF mount with motorised lenses in 1987.

Pentax was the first to innovate focusing distance measurement for SLR cameras with the FA and FA* series lenses from 1991. Their first One thousand_AF-mountain Pentax lenses with AF had been introduced in 1989.^[2]

In 1992, Nikon inverse back to lens integrated motors with their AF-I and AF-S range of lenses; today their entry-level DSLRs do not take a focus motor in the body due to the availability of motors in all new developed AF-Lenses.

Active [edit]

Active AF systems measure altitude to the subject independently of the optical system, and subsequently suit the optical organization for right focus.

At that place are various ways to measure distance, including ultrasonic sound waves and infrared calorie-free. In the first case, audio waves are emitted from the camera, and by measuring the delay in their reflection, distance to the subject field is calculated. Polaroid cameras including the Spectra and SX-70 were known for successfully applying this system. In the latter case, infrared light is usually used to triangulate the distance to the subject. Compact cameras including the Nikon 35TiQD and 28TiQD, the Canon AF35M, and the Contax T2 and T3, also equally early video cameras, used this organisation. A newer arroyo included in some consumer electronic devices, like mobile phones, is based on the fourth dimension-of-flying principle, which involves shining a laser or LED light to the subject and calculating the distance based on the fourth dimension information technology takes for the light to travel to the field of study and back. This technique is sometimes called laser autofocus, and is present in many mobile telephone models from several vendors. It is also present in industrial and medical^[three] devices.

An exception to the two-footstep arroyo is the mechanical autofocus provided in some enlargers, which accommodate the lens directly.

Passive [edit]

Passive AF systems determine correct focus past performing passive analysis of the image that is entering the optical system. They more often than not do not direct whatsoever 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 non enough light to take passive measurements.) Passive autofocusing can exist achieved past phase detection or contrast measurement.

Phase detection [edit]

Phase detection: In each effigy (non to scale), the area within the purple circumvolve represents the object to be focused on, the scarlet and green lines represent low-cal rays passing through apertures at the reverse sides of the lens, the yellowish rectangle represents sensor arrays (one for each discontinuity), and the graph represents the intensity profile as seen by each sensor array. Figures ane to iv represent conditions where the lens is focused (1) too near, (2) correctly, (3) too far and (iv) much as well far. The phase divergence between the two profiles can exist used to determine in which direction and how much to motion the lens to achieve optimal focus.

Phase detection (PD) is achieved past dividing the incoming calorie-free into pairs of images and comparing them. Through-the-lens secondary image registration (TTL SIR) passive stage detection is often used in pic and digital SLR cameras. The system uses a beam splitter (implemented as a minor semi-transparent area of the primary reflex mirror, coupled with a small secondary mirror) to direct light to an AF sensor at the bottom of the camera. Ii micro-lenses capture the light rays coming from the reverse sides of the lens and divert it to the AF sensor, creating a elementary rangefinder with a base within the lens's diameter. The two images are then analysed for similar calorie-free intensity patterns (peaks and valleys) and the separation fault is calculated in order to find whether the object is in front focus or back focus position. This gives the direction and an guess of the required amount of focus-ring movement.^[4]

PD AF in a continuously focusing mode (due east.g. "AI Servo" for Canon, "AF-C" for Nikon, Pentax and Sony) is a closed-loop control process. PD AF in a focus-locking way (e.g. "I-Shot" for Canon, "AF-Due south" for Nikon and Sony) is widely believed to be a "one measurement, 1 movement" open-loop control procedure, but focus is confirmed just when the AF sensor sees an in-focus subject. The only credible differences between the two modes are that a focus-locking manner halts on focus confirmation, and a continuously focusing mode has predictive elements to piece of work with moving targets, which suggests they are the aforementioned airtight-loop procedure.^[5]

Although AF sensors are typically one-dimensional photosensitive strips (simply a few pixels high and a few dozen broad), some modern cameras (Canon EOS-1V, Catechism EOS-1D, Nikon D2X) characteristic TTL surface area SIR^{[ citation needed ]} sensors that are rectangular in shape and provide ii-dimensional intensity patterns for a finer-grain analysis. Cantankerous-type focus points have a pair of sensors oriented at ninety° to one some other, although one sensor typically requires a larger aperture to operate than the other.

Some cameras (Minolta vii, 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 up of prisms and sensors; they are only agile with "fast lenses" with certain geometrical apertures (typically f-number two.8 and faster). Extended precision comes from the wider effective measurement base of the "range finder".

Phase detection system: vii – Optical organisation for focus detection; 8 – Prototype sensor; 30 – Plane of the vicinity of the exit student 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 low-cal receiving sections

Contrast detection [edit]

Contrast-detection autofocus is accomplished by measuring Contrast (vision) within a sensor field through the lens. The intensity difference between next pixels of the sensor naturally increases with correct paradigm focus. The optical arrangement tin can thereby be adjusted until the maximal contrast is detected. In this method, AF does not involve actual distance measurement at all. This creates meaning challenges when tracking moving subjects, since a loss of contrast gives no indication of the direction of movement towards or away from the camera.

Contrast-detect autofocus is a common method in digital cameras that lack shutters and reflex mirrors. Near DSLRs employ this method (or a hybrid of both contrast and stage-detection autofocus) when focusing in their alive-view modes. A notable exception is Catechism digital cameras with Dual Pixel CMOS AF. Mirrorless interchangeable-lens cameras typically used contrast-measurement autofocus, although phase detection has become the norm on most mirrorless cameras giving them significantly amend AF tracking performance compared to contrast detection.

Contrast detection places different constraints on lens design when compared with stage detection. While phase detection requires the lens to move its focus point rapidly and straight 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 stage detection frequently perform poorly on camera bodies that utilise contrast detection.

Help lamp [edit]

The assist light (likewise known every bit AF illuminator) "activates" passive autofocus systems in low-light and depression-contrast situations in some cameras. The lamp projects visible or IR light onto the subject, which the camera'southward autofocus organization uses to attain focus.

Many cameras and nearly all camera phones^[a] lack a dedicated autofocus assistance lamp. Instead, they use their built-in wink, illuminating the subject with bursts of light. This aids the autofocus organisation in the same way as a dedicated assist light, but has the disadvantage of startling or annoying people. Another disadvantage is that if the photographic camera uses flash focus assist and is set to an performance mode that overrides the wink, it may besides disable the focus assist. Thus, autofocus may fail to acquire the field of study.

Similar stroboscopic flashing is sometimes used to reduce the red-center effect, but this is only intended to constrict the subject'due south eye pupils before the shot.

Some external wink guns have integrated autofocus help lamps that supplant the stroboscopic on-photographic camera flash. Many of them are red and less obtrusive. Another way to help contrast based AF systems in low light is to axle a light amplification by stimulated emission of radiation design on to the field of study. The laser method is commercially called Hologram AF Laser and was used in Sony CyberShot cameras around the year 2003, including Sony's F707, F717 and F828 models.

Hybrid autofocus [edit]

In a hybrid autofocus system, focus is accomplished by combining two or more than methods, such as:

• Active and passive methods
• Phase detection and contrast measurement

The double effort is typically used to mutually compensate for the intrinsical weaknesses of the various methods in guild to increase the overall reliability and accurateness or to speed up AF part.

A rare example of an early hybrid system is the combination of an active IR or ultrasonic auto-focus system with a passive stage-detection system. An IR or ultrasonic system based on reflection will work regardless of the lite weather condition, merely can be easily fooled by obstacles like window glasses, and the accurateness is typically restricted to a rather limited number of steps. Phase-detection autofocus "sees" through window glasses without bug and is much more authentic, but it does not work in low-lite conditions or on surfaces without contrasts or with repeating patterns.

A very common example of combined usage is the phase-detection auto-focus organisation used in single-lens reflex cameras since the 1985s. The passive phase-detection auto-focus needs some contrast to work with, making information technology hard to utilize in low-light scenarios or on fifty-fifty surfaces. An AF illuminator will illuminate the scene and project contrast patterns onto even surfaces, then that phase-detection automobile-focus can work under these conditions also.

A newer form of a hybrid system is the combination of passive phase-detection motorcar-focus and passive dissimilarity auto-focus, sometimes assisted past active methods, as both methods need some visible contrast to work with. Under their operational conditions, stage-detection machine-focusing is very fast, since the measurement method provides both information, the amount of outset and the direction, so that the focusing motor can motion the lens right into (or close to) focus without additional measurements. Additional measurements on the fly, however, tin can improve accuracy or help keep track of moving objects. However, the accuracy of phase-detection motorcar-focus depends on its effective measurement ground. If the measurement ground is large, measurements are very accurate, merely can only work with lenses with a large geometrical discontinuity (e.yard. 1:2.8 or larger). Even with high contrasty objects, phase-detection AF cannot piece of work at all with lenses slower than its constructive measurement footing. In order to work with most lenses, the effective measurement ground is typically gear up to between ane:5.6 and 1:half dozen.7, then that AF continues to work with slow lenses (at least for as long as they are not stopped down). This, however, reduces the intrinsical accuracy of the autofocus organization, even if fast lenses are used. Since the constructive measurement basis is an optical belongings of the actual implementation, it cannot be changed easily. Very few cameras provide multi-PD-AF systems with several switchable measurement bases depending on the lens used in society to allow normal automobile-focusing with most lenses, and more than accurate focusing with fast lenses. Contrast AF does non have this inherit pattern limitation on accuracy as it merely needs a minimal object contrast to work with. Once this is available, information technology tin can work with loftier accuracy regardless of the speed of a lens; in fact, for as long every bit this condition is met, it can fifty-fifty work with the lens stopped downwardly. Also, since contrast AF continues to work in stopped-downward mode rather than only in open-aperture style, it is allowed to aperture-based focus shift errors phase-detection AF systems endure since they cannot work in stopped-down mode. Thereby, dissimilarity AF makes capricious fine-focus adjustments past the user unnecessary. Also, dissimilarity AF is allowed to focusing errors due to surfaces with repeating patterns and they can work over the whole frame, not just near the center of the frame, as phase-detection AF does. The down-side, however, is that contrast AF is a closed-loop iterative process of shifting the focus back and forth in rapid succession. Compared to stage-detection AF, contrast AF is slow, since the speed of the focus iteration process is mechanically limited and this measurement method does not provide any directional data. Combining both measurement methods, the phase-detection AF tin assist a contrast AF system to be fast and authentic at the same fourth dimension, to compensate discontinuity-based focus-shift errors, and to go on to piece of work with lenses stopped downwardly, equally, for example, in stopped-downward measuring or video mode.

Recent developments towards mirrorless cameras seek to integrate the phase-detection AF sensors into the image sensor itself. Typically, these phase-detection sensors are not as accurate as the more sophisticated stand-alone sensors, merely since the fine focusing is now carried out through contrast focusing, the phase-detection AF sensors are only need to provide fibroid directional data in order to speed up the contrast car-focusing process.

In July, 2010, Fujifilm announced a compact camera, the F300EXR, which included a hybrid autofocus system consisting of both stage-detection and dissimilarity-based elements. The sensors implementing the phase-detection AF in this photographic camera are integrated into the photographic camera's Super CCD EXR.^[vi] Currently information technology is used past Fujifilm FinePix Series,^[seven] Fujifilm X100S, Ricoh, Nikon one series, Canon EOS 650D/Insubordinate T4i and Samsung NX300.

Comparing of agile and passive systems [edit]

Active systems will typically not focus through windows, since sound waves and infrared light are reflected by the glass. With passive systems this volition generally not be a trouble, unless the window is stained. Accuracy of active autofocus systems is often considerably less than that of passive systems.

Active systems may also fail to focus a subject that is very close to the camera (e.g., macro photography).

Passive systems may not discover focus when the contrast is low, notably on large single-colored surfaces (walls, bluish sky, etc.) or in low-light conditions. Passive systems are dependent on a certain caste of illumination to the bailiwick (whether natural or otherwise), while active systems may focus correctly even in total darkness when necessary. Some cameras and external flash units have a special low-level illumination mode (usually orange/red lite) which can be activated during auto-focus operation to allow the camera to focus.

• 

  Active autofocus system via infrared - Canon AF35M (1979)

• 

  Early passive autofocus system integrated in the lens with Pentax ME-F (1981)

• 

  Modern (2014) autofocus single lens reflex camera

Trap focus [edit]

A method variously referred to equally trap focus, focus trap, or take hold of-in-focus uses autofocus to accept a shot when a subject area moves into the focal plane (at the relevant focal point); this can be used to get a focused shot of a rapidly moving object, peculiarly in sports or wild fauna photography, or alternatively to set up a "trap" so that a shot tin automatically exist taken without a person present. This is done past using AF to find but not set focus – using manual focus to set focus (or switching to manual afterwards focus has been ready) but and then using focus priority to detect focus and but release the shutter when an object is in focus. The technique works past choosing the focus adjustment (turning AF off), so setting the shooting mode to "Single" (AF-S), or more specifically focus priority, then depressing the shutter – when the subject moves into focus, the AF detects this (though it does non change the focus), and a shot is taken.^{[viii] [9] [x]}

The first SLR to implement trap focusing was the Yashica 230 AF. Trap focus is also possible on some Pentax (due east.1000. G-ten and K-five), Nikon, and Canon EOS cameras. The EOS 1D can do it using software on an fastened computer, whereas cameras like the EOS 40D and 7D have a custom office (III-i and III-4 respectively) which tin terminate the camera trying to focus after it fails.^{[ citation needed ]} On EOS cameras without 18-carat trap focus, a hack called "almost trap focus" can exist used, which achieves some of the effects of trap focus.^[eleven] Past using the custom firmware Magic Lantern, some Canon DSLRs can perform trap focus.

AI Servo [edit]

AI Servo is an autofocus mode plant on Canon SLR cameras, and in other brands such equally Nikon, Sony, and Pentax, under the proper noun "continuous focus" (AF-C).^[12] Besides referred to every bit focus tracking, information technology is used to track a subject as it moves around the frame, or toward and away from the camera. When in apply, the lens will constantly maintain its focus on the subject, hence it is commonly used for sports and action photography. AI refers to bogus intelligence: algorithms that constantly predict where a subject is near to be based on its speed and acceleration information from the autofocus sensor.

Focus motors [edit]

Mod autofocus is done through one of two mechanisms; either a motor in the camera body and gears in the lens ("spiral bulldoze") or through electronic manual of the drive educational activity through contacts in the mount plate to a motor in the lens. Lens-based motors can be of a number of dissimilar types, simply are oftentimes ultrasonic motors or stepper motors.

Some camera bodies, including all Canon EOS bodies and the more upkeep-oriented among Nikon's DX models, do not include an autofocus motor and therefore cannot autofocus with lenses that lack an inbuilt motor. Some lenses, such every bit Pentax' DA* designated models, although unremarkably using an inbuilt motor, can fall dorsum to screwdrive operation when the photographic camera torso does not back up the necessary contact pins.

Notes [edit]

1. ^ Counter-examples are the Nokia Lumia 1020, the Samsung Galaxy S4 Zoom and the Samsung Galaxy G Zoom.

See likewise [edit]

Wikimedia Eatables has media related to Autofocus.

• Circular polarizer, the only polarizer to piece of work with some SLR autofocusers
• Stock-still-focus lens
• List of Nikon compatible lenses with integrated autofocus-motor
• Transmission focus override
• Light-field camera, a camera that enables focusing in a postprocessing step

References [edit]

1. ^ "Southward System: Autofocus – Leica Fotografie International". Archived from the original on 2009-06-21. Retrieved 2009-05-15 .
2. ^ "Milestones - Asahi Optical Historical Guild". Retrieved 2021-08-29 .
3. ^ Fricke, Dierk; Denker, Evgeniia; Heratizadeh, Annice; Werfel, Thomas; Wollweber, Merve; Roth, Bernhard (28 May 2019). "Non-Contact Dermatoscope with Ultra-Vivid Lite Source and Liquid Lens-Based Autofocus Function". Applied Sciences. nine (eleven): 2177. doi:x.3390/app9112177.
4. ^ "Nikon - Technology - Predictive Focus Tracking System". Archived from the original on 2013-11-12. Retrieved 2013-11-12 .
5. ^ "Busted! The Myth of Open up-loop Phase-detection Autofocus".
6. ^ Fujifilm Launches Powerhouse 15X Long Zoom Bespeak and shoot Digital Camera: The FinePix F300EXR, Fujifilm, Usa
7. ^ "Fujifilm launches FinePix HS50EXR and HS35EXR loftier-cease superzooms". Retrieved June viii, 2013.
8. ^ Trap Focus for Nikon Users, by Kenneth William Caleno, January 28, 2009
9. ^ How to shoot sport, Ken Rockwell, 2006
10. ^ Focus Trap or Catch In Focus, April four, 2009
11. ^ EOS Documentation Project: Well-nigh Trap Focus Archived August 18, 2010, at the Wayback Machine, past Julian Loke
12. ^ "Manufactures tagged "larn": Digital Photography Review".

External links [edit]

• interactive Wink demo showing how stage detection autofocus works.
• interactive Flash demo showing how contrast detection autofocus works.
• How Stuff Works – Autofocus
• Canon EOS DSLR Autofocus Explained

Source: https://en.wikipedia.org/wiki/Autofocus

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