A new app called FocusTwist by Arqball lets your iPhone do what a US$400 Lytro light field camera is used for. Like the Lytro, the FocusTwist app lets you adjust the focus a picture after you have taken it. However, the method of after-focusing is completely different in FocusTwist than it is in the Lytro. The Lytro uses specialized hardware and software (you can read about it here). FocusTwist, on the other hand, has a bit of trickery up its sleeve.
The app actually takes several photos at different focus points when you expose an image. Users can change the focus by tapping different parts of the image. Arqball's back-end servers handle the image processing, so the images can even be refocused by any friends you've shared a photo with. Because the FocusTwist app takes a succession of rapid images at different focal lengths, the after-focusing feature only works well if you manage to hold your iPhone very steady while taking a pic. For best results, a tripod is handy.
FocusTwist is US$1.99 and available in the App Store now.