Complex sentence: On automatic or semi-automatic lenses, when the lens-mounted button was depressed, halfway the lens iris would contract to shooting aperature and then, if pressed farther, the lens button engaged the camera's shutter release button and tripped the shutter.
Split into simpler sentences: On automatic or semi-automatic lenses, when the lens-mounted button was depressed halfway the lens iris would contract to shooting aperture. If pressed farther, the lens button engaged the camera's shutter release button and tripped the shutter.