Whoa! Revisiting the Fisheye lens Wikipedia article, I see a Pentax K-1 camera pictured with Fish-eye Takumar 11/18mm. Now we’re really talking if we are seeing general-purpose Wikipedia articles depicting the Pentax K-1 camera.
20181210/https://en.wikipedia.org/wiki/Fisheye_lens
20181210/https://en.wikipedia.org/wiki/File:Pentax_K-1with_flat_Fisheye(34927515900).jpg
So let’s go searching around for other information on that lens, if it is indeed a jewel worth owning.
20181210/DuckDuckGo Fish-eye Takumar 11/18mm
Ah, it’s considered “low value” according to Pentax Forums.
20181210/https://www.pentaxforums.com/lensreviews/Fish-Eye-Takumar-18mm-F11.html
Check out S-M-C or SMC 17/4 which is almost as small but has superior optical performance.