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In almost all Android devices, zoom is digital -- there just isn't a mechanism for changing the length of the barrel and gathering more info.

So how is focus achieved?

If you're not physically changing the lens properties, or extending the barrel length, you've only got dilation of the aperture as the last focus method. Which is mechanical. Unless you've got a fixed focus system -- which most android phones do not seem to have.

The android-spec for the autoFocus() method isn't much help - although it says it must be supported by the hardware, although most phones seem to have it.

So if it is hardware -- how? I'm not hearing any little motors whirring around in there.... And is there really even any room for a mechanism in these things?

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2 Answers

up vote 5 down vote accepted

The focus function is achieved by moving the lens back or forth so it is actually a mechanical thing.

You can try it yourself. Download ZXings Barcode Scanner and fire it up. You'll hear a click every time it tries to refocus you can also see the lens moving back and forth.

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Loooong delay prior to accept. I just didn't hear the auto-focus sound until recently. –  Michael Paulukonis Nov 26 '10 at 12:32
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Often there is no focus at all. These cameras often make up for a complete lack of focus by having a large depth of field, which is exactly what is done with those "disposable" film cameras.

Digital zoom is a simple scaling of the input image, so there's no need to adjust zoom when digital zoom is applied, for the same reason there's no need to apply focus when scaling an image in photoshop. Focus is only needed for optical zooms because optical zoom changes the focal length of the camera.

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My Incredible has digital zom, but it also has (Auto) focus. I know it's not wide depth of field, because whatever is out-of-focus is variable. –  Michael Paulukonis Nov 8 '10 at 12:05
    
Digital zom? Does your Android have digital zom? nope! –  Michael Paulukonis Nov 26 '10 at 12:33
    
@Michael: I can't afford an Android, so I have absolutely no idea. You'll notice that nothing in my answer is inherently android specific -- I'm just generalizing from cell phone cameras I've seen that don't actually have hardware (optical) zooms. If your camera has optical zoom, then it would need some kind of focus. Otherwise there's no reason it couldn't get away with the high depth-of-field trick used in the "instamatic" cameras which don't have focus hardware at all. This is why I used the word "often" above -- I don't have specific hardware to play with. –  Billy ONeal Nov 26 '10 at 19:03
    
And what I said about digital zooms is true -- there's no change in focus required on a digital zoom. I have no idea whether or not android as a system uses digital zoom or not. –  Billy ONeal Nov 26 '10 at 19:04
    
Sorry, I was riffing on my mis-spelling; Most androids [as of late 2010] do have auto-focus instead of fixed focus, so I went w/ @Octavian's answer –  Michael Paulukonis Nov 26 '10 at 20:06
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