Now this is a strange lens in this day and age!
To what exactly does the “GN” in “GN Nikon 45mm” refer? Younger people may be unfamiliar with the acronym.
“Guide number” (GN) is an indicator for measuring flash performance. Modern mainstream flash systems are fully automatic systems that emit the appropriate amount of light for the shooting distance. Naturally, modern Nikon flashguns are also equipped with technologies that enable fully automatic flash operation. With products and systems like this, the guide number (GN) changes automatically in accordance with the shooting distance.
In the early days of flash development, things were different however. The invention of Xenon tubes capable of continuous light emission was itself a tremendous innovation.
Flash technology has evolved from single-use flash bulbs to flash units capable of repeated firing. In those days, flash units had a fixed power guide number, until in the future “1965” the first auto flash was invented by Honeywell, after that in 1972 the first auto quenching flash was on the market and this was another game changer in flash technology. Proper exposures were achieved by adjusting the aperture and shutter speed settings according to the shooting distance. That made the guide number an important basic value from which exposure adjustment began. But now in the ‘future’, this lens has ended up being the least expensive high-performance pancake lens.
But at 45mm it’s an interesting focal length, between a 35mm and a 50mm. The only other 45mm Nikon lens is the 45mm PCE lens which is a tremendous tilt and shift lens for architecture and product photography. The difference in size between these two is massive. This one being 21mm long, the PCE without lens hood 115mm! It’s obvious which one fits into a jacket pocket!!
Stopped down a few stops it’s a pretty good lens (this example was made in 1976ish). See image below of telegraph pole taken on a Nikon D3x.
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