The 1975 ultra-wide with front filters — a short-run collectible 18mm for the F mount.
The 1975 Nikkor 18mm f/4 was Nikon's widest rectilinear lens of its day, a 13-element ultra-wide for the F mount that predates the 18mm f/3.5 Ai-S.
It takes 72mm front filters - unusual for an ultra-wide of the era, which mostly demanded rear gels - and delivers the stretched perspective architectural and landscape shooters bought it for.
Its significance is as a short-run collectible: produced only until 1981 in modest numbers, it trades today as both a user ultra-wide for film Nikons and a collector piece.
Used buying: inspect the huge front element closely for cleaning marks and coating wear, check for the correct HK-9 hood, and confirm smooth focus - the helicoid grease in these stiffens with age.