Before computer-controlled automatic flashes were introduced, the issue of controlling exposure in flash photography had been a very encumbering limitation. Flash lighting generally falls off obeying an inverse square function of distance. That is, every time the distance is doubled, illumination of the subject by the flash falls by the equivalent of two f-stops. This posed a very severe, non intuitive problem for flash photographers. Photographers had to do a calculation dividing Guide Number by subject to flash distance to derive the appropriate f-stop every time the subject distance from the flash changed, a time consuming and tedious operation at a time when there could be time pressure to finish the calculation.
By designing an extremely flat field lens on a reverse direction non linear pitch focusing helicoid couple able with an appropriate scale f-stop ring, Nikon engineers achieved the objective of adjusting exposure as a appropriate function of distance from the camera provided the flash is mounted on or very near the camera. The lens is also extremely sharp.
On the opposite side of the lens from the regular f-stop and distance scale is the decouple able GN (Guide Number) setting capability.
The lens also has a nine blade aperture to achieve very round aperture shape imaging, which can be an issue in flash photography.
Version 1: Serial numbers 710101 to 746389. NKJ version. Manufactured March 1969 through December 1973.
Version 2: Serial numbers 760001 up. NIC version NKJ changes to Nikon and "Made in Japan". Manufactured January 1974 to March 1977.
