There’s no need of loading apps unnecessarily on your phone, as your phone comes with built-in feature of touch focus.
please watch this video of Sony Xperia “S” & perform it the same way on your “Ray”.
http://www.youtube.com/watch?v=0fmHl5XoGTw (I dont own the video link, its just reference for your help)
I’m pretty sure you’ll nail it.
Please post for any further questions & doubts.
(P.S: Save memory on your phone for capturing memorable shots with Touch Focus. I Know its a remarkable feature , so utilize it rather any 3rd party apps.)