I got my new xperia U just 2 days ago, but when i use its camera , i found this problem. Holding the camera key launches the camera app but when i press camera key to click photo it do nothing, i tried holding it but it didnt worked at all, but after clicking some pics by touch button there was a small squre in middle in green colour , then i tried taking pics wit camera key and it worked 
, then i noticed that only when there is that "green small squre" during camera app, i m able to take pic with the hard key, is there anything with the camera settings, please help me 
the green square is focus; if it's green then the image is focused at that point, if it doesn't turn green, the image won't be focused - it will probably be blurry
when you are using the camera, you can click on the menu button and you have multiple options there, including capture mode and focus options - play with it a bit and see what suits you best 
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