The camera button pressed to take a picture
The button, usually on top of a camera, that one presses to take a picture.
is the button that one presses to take the picture.
The mechanism, usually a button on the top of the camera, that activates the shutter to expose the film.
The mechanism that releases the shutter in a camera, causing film or digital imaging elements to be exposed to the incoming light.
The shutter release button is the one you press down to take a picture and the click you hear is the shutter opening and closing. Most good autofocus cameras allow you to pre-focus on subjects that are slightly off-centre. To do this point the lens at your main subject and lightly press the shutter release button halfway down to 'lock' the focus before recomposing the picture and fully depressing the shutter release.