In camera apps, if zoom is digital, how is (auto) focus achieved?

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? See Also: * Android.SE: In camera apps, is “zoom” a digital zoom, or an optical zoom? * Understanding camera AutoFocus * Wikipedia:Autofocus