Difference between revisions of "Autofocus"

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{{Glossary}}
 
{{Glossary}}
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|image_text= 1977 ad for [[Konica C35 AF]], first production autofocus<br/>camera, using [[Honeywell]] Vistronic system
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|image_text= 1977 advertisement for [[Konica C35 AF]], the first<br/> production autofocus camera, using a [[Honeywell]]<br/>Vistronic system<br/>
|image_by= Nesster
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'''Autofocus''' is a mechanism in an optical system which automatically changes the optics to focus an image.  In a camera, this is used to focus the subject image, through the lens, onto the [[focal plane]] - the film or digital sensor.
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'''Autofocus''' is a mechanism in an optical system which automatically changes the optics to focus an image.  In a camera, this is used to focus the subject image, through the lens, onto the [[focal plane]] - the film or digital sensor. A number of different methods are used for autofocus.
  
==Autofocus Methods==
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A number of different methods are used for autofocus:
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==Active systems==
===Active Systems===
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Active autofocus sends some form of energy towards the subject, then analyses the reflection:
Active Autofocus sends some form of energy towards the subject, then analyses the reflection:
 
 
* ''Sonar'' - ultrasonic sound is emitted by the camera; this is reflected from the subject back to the camera. The delay between the emitted and reflected sound is measured, giving a distance estimate, and the lens adjusted to this distance. Some [[Polaroid SX-70]] cameras used this system.
 
* ''Sonar'' - ultrasonic sound is emitted by the camera; this is reflected from the subject back to the camera. The delay between the emitted and reflected sound is measured, giving a distance estimate, and the lens adjusted to this distance. Some [[Polaroid SX-70]] cameras used this system.
* ''Infra-red beam'' - a pulsed infra-red light beam is emitted by the camera and reflected by the subject; the camera has an infra-red receiver set apart from the emitter. Adjusting the angles of emitter and receiver (in concert with moving the lens focus mechanism) and finding a maximum in the amount of light received gives a measure of the distance and a focused image. This method is common on [[compact camera|compact film cameras]].
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* ''Infra-red'' - a pulsed infra-red light beam is emitted by the camera and reflected by the subject; the camera has an infra-red receiver set apart from the emitter. Adjusting the angles of emitter and receiver (in concert with moving the lens focus mechanism) and finding a maximum in the amount of light received gives a measure of the distance and a focused image. This method is common on [[compact camera|compact film cameras]].
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===Passive Systems===
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==Passive systems==
Passive Autofocus analyses the image arriving at the camera, without transmitting anything towards the subject - except in some cases an "assistance" light is used to illuminate the subject when it is too dark for there to be enough image for focusing.
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Passive autofocus analyses the image arriving at the camera, without transmitting anything towards the subject - except in some cases an "assistance" light, used to illuminate the subject when it is too dark for there to be enough image for focusing.
 
* ''Image splitting'' - the image is divided into two parts, and these are analysed by an autofocus sensor. This effectively creates a rangefinder - working by comparing light peaks and their phases in the two images. ''SIR'' - Secondary Image Registration is one example of this method.
 
* ''Image splitting'' - the image is divided into two parts, and these are analysed by an autofocus sensor. This effectively creates a rangefinder - working by comparing light peaks and their phases in the two images. ''SIR'' - Secondary Image Registration is one example of this method.
 
* ''Contrast analysis'' - the contrast of the image is measured whilst adjusting the focus of the lens - the highest contrast is achieved when the image is in focus. This is more easily implemented in digital cameras, which already have a sensor and a processing system, and some video cameras.
 
* ''Contrast analysis'' - the contrast of the image is measured whilst adjusting the focus of the lens - the highest contrast is achieved when the image is in focus. This is more easily implemented in digital cameras, which already have a sensor and a processing system, and some video cameras.
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== Links ==
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* Octavian Baltag. [http://article.sciencepublishinggroup.com/pdf/10.11648.j.si.20150301.11.pdf History of Automatic Focusing Reflected by Patents.] Science Innovation. Vol. 3, No. 1, 2015, pp. 1-17.doi: 10.11648/j.si.20150301.11 (pdf)
  
 
[[Category: Focusing system]]
 
[[Category: Focusing system]]
 
[[Category: Autofocus|*]]
 
[[Category: Autofocus|*]]

Latest revision as of 20:28, 3 February 2016

Glossary Terms

Autofocus is a mechanism in an optical system which automatically changes the optics to focus an image. In a camera, this is used to focus the subject image, through the lens, onto the focal plane - the film or digital sensor. A number of different methods are used for autofocus.


Active systems

Active autofocus sends some form of energy towards the subject, then analyses the reflection:

  • Sonar - ultrasonic sound is emitted by the camera; this is reflected from the subject back to the camera. The delay between the emitted and reflected sound is measured, giving a distance estimate, and the lens adjusted to this distance. Some Polaroid SX-70 cameras used this system.
  • Infra-red - a pulsed infra-red light beam is emitted by the camera and reflected by the subject; the camera has an infra-red receiver set apart from the emitter. Adjusting the angles of emitter and receiver (in concert with moving the lens focus mechanism) and finding a maximum in the amount of light received gives a measure of the distance and a focused image. This method is common on compact film cameras.


Passive systems

Passive autofocus analyses the image arriving at the camera, without transmitting anything towards the subject - except in some cases an "assistance" light, used to illuminate the subject when it is too dark for there to be enough image for focusing.

  • Image splitting - the image is divided into two parts, and these are analysed by an autofocus sensor. This effectively creates a rangefinder - working by comparing light peaks and their phases in the two images. SIR - Secondary Image Registration is one example of this method.
  • Contrast analysis - the contrast of the image is measured whilst adjusting the focus of the lens - the highest contrast is achieved when the image is in focus. This is more easily implemented in digital cameras, which already have a sensor and a processing system, and some video cameras.


Links