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While many shooters are happy to leave focus selection entirely up their camera, experienced shooters know that you can get more creative and predictable results by making that decision yourself. And with a smartphone camera it couldn't be simpler to do.
Using the built-in camera app on either an Android device (running 4.0 or later) or a recent iPhone, just tap the screen to specify where the camera should acquire focus. An autofocus square will appear for a moment onscreen at the location you tapped. And you can see that region of the image suddenly snap into focus.
It's important to realize though, that as well as focus, you're probably also telling the camera where to take its light reading from. So don't be surprised if you see a difference in the brightness between images taken with focus set on different points. You can see this demonstrated in the examples below. Click through for full size versions that make focus differences easier to judge.
You should know that if the scene objects are more than a couple of feet away from your cellphone your main subject will probably be in focus wherever you set the focussing point. Smartphone cameras have a very wide area of simultaneous focus (known as depth of field). In fact, you may find that this "tap to focus" technique actually comes in handy more often as a tool to quickly set exposure (scene brightness).
This technique can also be helpful if your camera cannot seem to lock focus on a low-contrast object (like an evenly lit backdrop). Just tap on a higher-contrast object of similar brightness to give your camera something easier to lock onto.
Using a tap to set focus and/or exposure adds just one brief step to your picture-taking routine yet allows you to come away with more professional results. Give it a try and impose more creative control on your photography.