Asahi Pentax S1a Brochure
The Asahi Pentax S1a was a single lens reflex (SLR) camera with a focal plane shutter manufactured by the Asahi Optical Company of Japan. The Asahi Company made much advancement in the design of professional cameras. This 35 mm film camera shipped with the Super Takumar 55mm f/2 lens and had a fully automatic diaphragm. It was also compatible with Takumar lenses.
In the early 60s, this camera had the most advanced viewfinder design consisting of an instant return reflex mirror, and "pentaprism" which transmitted the image to the viewfinder without any loss in brightness. In this system, the diaphragm remains open all the time and closes to the programmed size a fraction of a second before the shutter opens. The design was extremely popular and many manufacturers followed it.