Capture image event in objective c

I am trying to set a method when capture image is touched.

1) user press the capture button in the UIImagePickerController.
2) method is triggered by pressing this button.

I feel silly asking this it but i couldnt find an asnwer without creating custom overlay.

im talking about the big circle button in this image: http://i.stack.imgur.com/FGpMW.png