Nikonos

Nikonos utilizes a blade type focal plane shutter with each thin blade covering the entire aperture. During shutter  release, both blades are released at the same time but the 2nd blade is retarded by a flywheel mechanism to produce 1/30~1/500 Sec. speeds plus B. Instead of utilizing a sprocket, film advance is accomplished by a registration pin that provides equidistant film winding. The design is simplistic, and reliable. This is Nikon's first underwater camera, followed by later models that offered built-in light meter with auto exposure. The interlock mechanism for Nikonos is its interchangeable lenses that are taken off by a 90 degree rotation. There are two O-rings that are replaceable: One behind the lens, and one on the body below its top plate.

Above, Nikonos camera with external viewfinder. Left, the lens acts as the body lock for the camera, and after taking it off, the body can be pulled out from its outer shell by using the strap lugs that work like two can opener levers.

Left, the two-blade focal plane shutter of Nikonos II is cocked by a single stroke of camera's advance lever, and then released by the same lever by pulling on it towards the back of the camera. This is a robust water sealed design that's easy to operate, and and has minimal vibration when operated under water with gloves.