Editing Autofocus (section)

==Hybrid autofocus==
{{More citations needed section|date=January 2018}} 
In a hybrid autofocus system, focus is achieved by combining two or more 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 order to increase the overall reliability and accuracy or to speed up AF function.

A rare example of an early hybrid system is the combination of an active [[Infrared|IR]] or ultrasonic auto-focus system with a passive phase-detection system. An IR or ultrasonic system based on reflection will work regardless of the light conditions, but can be easily fooled by obstacles like window glasses, and the accuracy is typically restricted to a rather limited number of steps. Phase-detection autofocus "sees" through window glasses without problems and is much more accurate, but it does not work in low-light conditions or on surfaces without contrasts or with repeating patterns.

A very common example of combined usage is the phase-detection auto-focus system used in [[Single-lens reflex camera|single-lens reflex cameras]] since the 1985s. The passive phase-detection auto-focus needs some contrast to work with, making it difficult to use in low-light scenarios or on even surfaces. An [[AF illuminator]] will illuminate the scene and project contrast patterns onto even surfaces, so that phase-detection auto-focus can work under these conditions as well.

A newer form of a hybrid system is the combination of passive phase-detection auto-focus and passive contrast auto-focus, sometimes assisted by active methods, as both methods need some visible contrast to work with. Under their operational conditions, phase-detection auto-focusing is very fast, since the measurement method provides both information, the amount of offset and the direction, so that the focusing motor can move the lens right into (or close to) focus without additional measurements. Additional measurements on the fly, however, can improve accuracy or help keep track of moving objects. However, the accuracy of phase-detection auto-focus depends on its effective measurement basis. If the measurement basis is large, measurements are very accurate, but can only work with lenses with a large geometrical [[aperture]] (e.g. 1:2.8 or larger). Even with high contrasty objects, phase-detection AF cannot work at all with lenses slower than its effective measurement basis. In order to work with most lenses, the effective measurement basis is typically set to between 1:5.6 and 1:6.7, so 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 system, even if fast lenses are used. Since the effective measurement basis is an optical property 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 order to allow normal auto-focusing with most lenses, and more accurate focusing with fast lenses.
Contrast AF does not have this inherent design limitation on accuracy as it only needs a minimal object contrast to work with. Once this is available, it can work with high accuracy regardless of the speed of a lens; in fact, for as long as this condition is met, it can even work with the lens stopped down. Also, since contrast AF continues to work in stopped-down mode rather than only in open-aperture mode, it is immune to [[Spherical aberration|aperture-based focus shift errors]] phase-detection AF systems suffer since they cannot work in stopped-down mode. Thereby, contrast AF makes arbitrary fine-focus adjustments by the user unnecessary. Also, contrast AF is immune 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 phase-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 information. Combining both measurement methods, the phase-detection AF can assist a contrast AF system to be fast and accurate at the same time, to compensate aperture-based focus-shift errors, and to continue to work with lenses stopped down, as, for example, in stopped-down measuring or video mode.

Recent developments towards [[Mirrorless camera|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, but since the fine focusing is now carried out through contrast focusing, the phase-detection AF sensors are only need to provide coarse directional information in order to speed up the contrast auto-focusing process.

In July, 2010, [[Fujifilm]] announced a compact camera, the F300EXR, which included a hybrid autofocus system consisting of both phase-detection and contrast-based elements.  The sensors implementing the phase-detection AF in this camera are integrated into the camera's [[Super CCD]] EXR.<ref>[http://www.fujifilmusa.com/press/news/display_news?newsID=879875 Fujifilm Launches Powerhouse 15X Long Zoom Point and shoot Digital Camera: The FinePix F300EXR] {{Webarchive|url=https://web.archive.org/web/20100727174907/http://www.fujifilmusa.com/press/news/display_news?newsID=879875 |date=2010-07-27 }}, Fujifilm, USA</ref>  Currently it is used by [[Fujifilm FinePix]] Series,<ref>{{cite web |url=http://www.dpreview.com/news/2013/01/07/Fujifilm-finepix-hs50-exr-hs35 |title=Fujifilm launches FinePix HS50EXR and HS35EXR high-end superzooms |access-date=June 8, 2013}}</ref> [[Fujifilm X100|Fujifilm X100S]], [[Ricoh]], [[Nikon 1 series]], [[Canon EOS 650D|Canon EOS 650D/Rebel T4i]] and [[Samsung NX300]].