Autofocus
Revision as of 06:18, 20 February 2015 by Hanskerensky (talk | contribs) (/photos/nesster/5538964661 changed rights from nc to cc after checking flickr image)
Glossary Terms
1977 ad for Konica C35 AF, first production autofocus camera, using 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.
Autofocus Methods
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 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 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 is 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.