|image by Shoji Kimura (Image rights)|
While the earlier Nikonos series of underwater cameras had been evolutionary refinements on the original French Calypso, the 1980 Nikonos IV-A was a design originating entirely from Nikon. Where the models up through the Nikonos III had been entirely mechanical cameras, the IV-A added TTL light metering and aperture-priority autoexposure via an electronically-timed shutter. The only manual exposure option was a mechanically-timed speed of 1/90th second, or "B".
The camera body was also greatly redesigned, offering a more conventional shutter release and hinged back. The viewfinder included LED display of exposure and flash status.
The Nikonos V followed in 1984, offering manually-selected shutter speeds and TTL control of flash exposure, making the IV-A a comparatively less desirable model today.