How to activate AE/AF Lock indicator in ios 7?

I tried to activate AE/AF Lock on my iPhone 5 running ios 7 earlier this afternoon. To my surprise I couldn't get the indicator to show so I wasn't sure if it was working or not (I think it did). I pressed and holded and I did see the square flashed under my finger. I launched this application from the screen 1.

I just activated my camera from the locked screen and did the same thing and indicator showed!!

Is this by design? I wonder if there is a setting somewhere that I should activate to get the indicator to show.

Thx.