Robertson Mk-20-48A 42.8mm F/1.7 Art Lens

thingiverse

This was designed iteratively using a number of small prototypes, which is visible in the name. This is the Mk-20-48A, indicating it is the 20th "version" and the 48th revision of that version. The "A" indicates it uses a 42.8mm lens element - in opposition to a number of other lenses that were used (with B representing a now unused 39.5mm version). The design began with a fixed-focus variant, which used a bolt-on aperture-plate system (a "waterhouse aperture" system to be precise), and went through several printed revisions before I decided to add a focusing mechanism. After a number of unprinted versions, I settled on the involute internal-external gear combination to achieve focusing by turning bolts which in turn move the inner barrel forward and backwards - the Mk-20. You can read more about the design on this thread on the PentaxForum: https://www.pentaxforums.com/forums/173-general-photography/382204-diy-gear.html#post4931948, the earlier designs (notably the Mk-19) weren't as "finished" looking as the current design. You can also see some sample shots on my Flickr account: https://flic.kr/s/aHsmMhx9gy