Some companies have specialities that they continually pursue, sometimes over very long periods of time and persistently from company owner to company owner. Arguably, Pentax have been the champions of the Fish-eye lens. At one time a fish-eye lens, that is, one with a field of view of 180 degrees or more and no consideration to trying to correct bending lines, needed the mirror on an SLR to be locked up. This was because of protuding rear elements that would foul the flipping mirror. This therefore meant an accessory viewfinder would also be needed. It was in 1963 that this all changed, when Asahi Optical Co (the maker of Pentax camera and lenses) introduced the Fish-eye Takumar. For the purist, this was actually the second lens made. The first was the Fish-eye Takumar 17mm f/11, 4 elements in 3 groups, fixed focus, waterhouse stops at f/11. f/16, f/22 and f/32. This arrived in 1962, or it might have done, as nobody seems to have ever seen one. This was rapidly replaced by the similarly constructed Fish-eye Takumar 18mm f/11, made between 1963 and 1967.
This lens gave a circular image 180 degrees across and was the darling on Carnaby Street, a weird lens for weird fashion statements! From then onwards though the lenses became full frame images with a diagonal of 180 degrees as opposed to a circular image. 1967 saw a great advance with the Fish-eye Takumar 17mm f/4, 11 elements in 7 groups, now focusing down to 0.2m or 0.66 feet. Built in filters and made from 1967 to 1971 when it morphed into the bayonet fit version, the SMC Pentax Fish-eye 17mm f/4.
The follow up lens for the A series cameras was the SMC Pentax-A 16mm f/2.8, 9 elements in 7 groups, made from 1985-2004. Then we say goodbye to the prime fish-eyes and from 1995 we have the incredibly versatile Fish-eye zooms.
The SMC Pentax-F 17-28mm f/3.5-4.5 (1995-2002, 9 elements in 7 groups) is the full frame zoom, and I use one very happily on my K-1 DSLR. From 180 degree fish-eye at 17mm to a reasonably straight 28mm, it's compact, sharp and always in my Billingham Hadley Pro camera bag. The APS-C replacement is the SMC Pentax-DA 10-17mm f/3.5-4.5 (2006-2019, 10 elements in 8 groups) which, ironically, is larger and heavier than the full frame lens.
This last version has now been updated and has become the HD Pentax-DA 10-17mm f/3.5-4.5 (2019-currently in production, 10 elements in 8 groups)which can now be used as a full frame (that is, full image) on APS-C or a circular fish-eye on full frame bodies.
So I think it's fair to conclude that Pentax and fish-eye lenses have been wedded togather for a very long time. Tomorrow I'll find some fish-eye images to show what they can do.