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How does focusing work on mobile phone cameras?

Aren't their optics simple enough to not have any moving parts whatsoever?

If so, how come most smartphones have an auto-focus, and with various Android releases it always wastes huge amount of time and causes huge shutter lags, plus sometimes you end up with pictures of objects at focal infinity that clearly appear out-of-focus?

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    \$\begingroup\$ Related: How does the iPhone Auto Focus work? \$\endgroup\$
    – Caleb
    Jan 7, 2013 at 19:15
  • \$\begingroup\$ Cool question. I had always assumed it was an electromagnetic stage like in a CD player. \$\endgroup\$
    – Phil
    Mar 4, 2013 at 16:11

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How does focusing work on mobile phone cameras?

Tiny AF motor, in the case of iPhone. Probably the same for other phones with adjustable focus cameras. I've read about the possibility of using liquid lenses and other whizzy technology, but I'm not aware of any devices on the market that use them.

Aren't their optics simple enough to not have any moving parts whatsoever?

Many phones do have cameras with such tiny apertures that they have nearly infinite depth of field, so focusing isn't necessary. But starting with the iPhone 4, iPhones and iPads have had adjustable focus. Same for other brands.

If so, how come most smartphones have an auto-focus, and with various Android releases it always wastes huge amount of time and causes huge shutter lags, plus sometimes you end up with pictures of objects at focal infinity that clearly appear out-of-focus?

Tradeoffs, limitations of current technology, and user error:

  • Tradeoffs: it'd probably be possible to design a phone with a really fast and accurate AF system and no shutter lag, but it might not be a very good phone (or game machine, surrogate for your computer, etc.)

  • Limitations: part of the lag in both AF and shutter may be due to the capabilities of the cameras built into these devices. The distance that the lens can move to focus must be very small -- perhaps that necessitates slower movements to find focus.

  • User error: there's no reason that a distant object should be in focus if you're telling the camera to focus on something close. Like any camera, you've got to learn to control the one in your phone appropriately. Many smartphones use taps on the screen to choose the area to focus on and adjust exposure for. Learn when the shutter fires -- is it when you touch the button, or when you lift your finger?

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  • \$\begingroup\$ In non-daylight conditions, the shutter on my Galaxy Nexus fires 3 seconds after I touch the shutter button or lift the finger (how do I determine which one?). That's usually 3 seconds after I already touch to focus, or after I've already taken a picture and haven't physically moved the phone at all prior to taking another one. If your answer emphasises user error, and doesn't include inadequate engineering, design, testing and QA, I won't +1! :-) \$\endgroup\$
    – cnst
    Jan 7, 2013 at 20:12
  • \$\begingroup\$ @cnst I'm not at all saying that a 3 second lag is user error. Crappy implementation may well deserve to be a 4th point above, but I was trying to stick to reasons not related to the quality of any specific product. If your question is "why does my Nexus 3 work poorly?", that's something different. \$\endgroup\$
    – Caleb
    Jan 7, 2013 at 20:20
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    \$\begingroup\$ @cnst Inadequate compared to what? The cameras in the iPhone 5 cost $18 (for both), I expect other top-of-the-line smartphones are similar. Even in an age of mass production there are limits to how much engineering $18 will buy :) \$\endgroup\$
    – j-g-faustus
    Jan 7, 2013 at 20:36
  • \$\begingroup\$ @j-g-faustus, the specific problem with Galaxy Nexus is poor software engineering and lack of regression testing; the phone is supposed to have zero shutter lag according to all the docs (both the hardware and the initial 4.0 software), but OTA updates to 4.1 and 4.2 seem to have broken it, and never fixed it back (e.g. see android.stackexchange.com/questions/37032/…). \$\endgroup\$
    – cnst
    Jan 7, 2013 at 20:59
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Regarding "always wastes huge amount of time", this is not unlike many compact cameras using the contrast-based AF, the focusing algorithm has to blindly try various lens positions and compare the results, because it has no other way to know what the correct focus distance could be.

Some smartphones, having specialized focusing hardware, can also use faster methods, that often give near zero shutter lag:

More about auto focusing methods: How does autofocus work?

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  • \$\begingroup\$ no, my Canon PowerShot SD1000 wastes no time on autofocus -- it's default focus is on infinity, and i effectively was getting the zero shutter lag on it; if i need microfocus, there's a simple setting for that -- use it only when needed, provides for much-much better results than the broken autofocus in Nexus and PowerShot 300HS that only ensures that everything is delayed by a whole lot \$\endgroup\$
    – cnst
    Oct 24, 2014 at 5:27

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