Complex sentence: Some older cameras were fitted with "convertible" lenses of normal focal length; the front element could be unscrewed, leaving a lens of twice the focal length and angle of view, and half the aperture.
Split into simpler sentences: Some older cameras were fitted with "convertible" lenses of normal focal length. The front element could be unscrewed, leaving a lens of twice the focal length and angle of view, and half the aperture.