Focus during the development of the follow-up to the S2 was on the viewfinder.
Nippon Kogaku's objective was to achieve "a field of vision that was compatible with interchangeable lenses with different focal lengths", and as they saw it, there were several ways to go about this.
One of the ways was to employ a mask-switching real-image viewfinder, which enabled the user to see the view area clearly.
This style was used for an external universal viewfinder attached to an accessory shoe.
However, the developers determined that the device was simply too big to be incorporated in the planned camera, and the idea was scrapped.
They then produced two samples, one of which was a virtual image zoom-type finder.
The other finder was a bright frame switchable type.
Since the zoom-type finder was too large in those days, the developers opted for the bright type.
The operational ease of the Nikon S2's life-size (1x) viewfinder was reconfirmed.
This full-size finder was also employed with the SP, and the bright frame for each lens was replaced within the field of vision.
The term "full-size" represents the effect of looking through a piece of glass with no angles to refract light — in theory, a very simple mechanism.
This was not the case with the Nikon SP, however.
It is a complicated structure which incorporates a coincidence-type rangefinder and adds — one after another — frames which are compatible with a variety of lenses.
Furthermore, the parallax (the difference in the position of view from the viewfinder and that from the shooting lens) is automatically fixed by a linkage with the rangefinder.