How works a phase detection autofocus?

[Panama Hat & Camera]

I am an amateur photographer of film cameras trying to learn a little about digital photography. This is the reason for my technical questions.
How works a phase detection autofocus? What is the difference between phase detection autofocus and contrast detection autofocus?
Greetings.
Antonio

[Manfred M]

If you have ever used a manual rangefinder camera like a Leica M series camera, you are essentially using the same technique as phase detection. You have a beam-splitter that creates two images and you focus by trying to align the two images so that they are right on top of each other. The focus mechanism is linked with the the camera's focus ring and a mirror s rotated as you turn the ring. By measuring the angle that the mirror turns and knowing the offset between the viewfinder and the rangefinder window, simple trigonometry determines the distance of the object that is being brought into focus.

Phase detection in a DLSR performs exactly in the same way; there is a beam-splitter that creates two independent images and when the two are aligned, the camera circuitry detects this and you have focus,

[Andrew1]

http://en.wikipedia.org/wiki/Autofocus

[Snarkbyte]

Click on the links below for animated illustrations explaining the two autofocus methods. Hope this helps

Click here for Phase Detection

Click here for Contrast Detection

[Panama Hat & Camera]

Manfred, Andrew and Al,
Thank you very much!
Antonio

[Dave Humphries]

Al,

Thanks, those two apps are great to see it working

Also just trying SynthCam on my iPod Touch

[jcuknz]

Contrast detection is just that and only if you have a uniform tone in the selected area will it not work. I have deliberately tried to stop my camera focusing and it was very hard, even in a low street light situation. State of the art CD can be as good as PD in many situations and suprised the dpreviewer when checking my lens. [Lumix 014140] CD used not to be that wonderful in the past so PD was 'the way' to go