Some cameras can't focus on QR codes

Some testers found that their cameras couldn't focus on QR codes. This has also been an issue on some test devices:

• Sony Xperia Tipo
• Huawei Ascend Y520
• Huawei Ascend Y330

All of these devices have fixed-focus cameras. The size and resolution of the screen being scanned, and the resolution of the camera's preview image, may also be relevant. Scanning a small, low-resolution screen using a low-resolution, fixed-focus camera seems to be the hardest case (e.g. scanning the HTC Wildfire S using the Sony Xperia Tipo).

@ernir reported that the testers' devices were able to scan QR codes when using the zxing app - we should investigate what it's doing differently.