Difference between revisions of "Autofocus"
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{{Glossary}} | {{Glossary}} | ||
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|image_source= http://www.flickr.com/photos/nesster/5538964661/in/pool-camerawiki | |image_source= http://www.flickr.com/photos/nesster/5538964661/in/pool-camerawiki | ||
|image= http://farm6.static.flickr.com/5215/5538964661_9e0e791802.jpg | |image= http://farm6.static.flickr.com/5215/5538964661_9e0e791802.jpg | ||
| − | |image_align= | + | |image_align= |
| − | |image_text= 1977 | + | |image_text= 1977 advertisement for [[Konica C35 AF]], the first<br/> production autofocus camera, using a [[Honeywell]]<br/>Vistronic system<br/> |
|scan_by= Nesster | |scan_by= Nesster | ||
|image_rights= creative commons | |image_rights= creative commons | ||
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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. | + | </div> |
| + | '''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. | ||
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| − | + | ==Active systems== | |
| − | + | Active autofocus sends some form of energy towards the subject, then analyses the reflection: | |
| − | Active | ||
* ''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 | + | * ''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 | + | ==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. | * ''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 == | == Links == | ||
Latest revision as of 20:28, 3 February 2016
Glossary Terms
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| 1977 advertisement for Konica C35 AF, the first production autofocus camera, using a Honeywell Vistronic system scanned by Nesster (Image rights) |
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
- Octavian Baltag. 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)
