So how does Auto-Focus work?

[Chairman7w]

I mean, I know the meaning of it (focuses automatically), but how does it work from a physics standpoint? Is there a little radar? Sonar? Infra-red?

How in the WORLD does it do that?

 

[Dao]

I believe there is a contrast based autofocus. So maybe that's why it doesn't work too well in low light situation.

 

[Dao]

After I did a search, I find this.

Understanding Camera Autofocus

 

[bhop]

There's also this
HowStuffWorks "How Autofocus Cameras Work"

 

[KmH]

Ya gotta just love the I N T E R N E T.

 

[Garbz]

Neither of those apply to DSLRs.

Contrast based autofocus is slow, clugy and relies on image processing (see the slow complaint). This is the autofocus mode that cameras revert to when you enable live view, and also the AF mode used by all point and shoots.

Edit: Found it include an animation on Nikon's site. http://www.nikon.com/about/technology/core/software/caf/index.htm This is how the predictive focus works on Nikon cameras, but the principles of phase detection are the same for all DSLRs.