Complex sentence: Unlike 35 mm SLRs, the system's lenses did not have a built-in iris to control aperture; instead, an iris was mounted inside the camera body, and functioned as both an aperture control and a shutter.
Split into simpler sentences: Unlike 35 mm SLRs, the system's lenses did not have a built-in iris to control the aperture. Instead, an iris was mounted inside the camera body, and functioned as both an aperture control and a shutter.