Tokina AT-X 357 35-70mm PK to M42

landstrykere · Joined: 10 Feb 2018 Posts: 51 Location: Norge

I have this great Tokina AT-X 35-70mm f2.8. They came in Contax/yashica, Canon, Nikon, Pentax PK/A mounts. Since I no longer use autofocus lenses on my Pentax, where a PK-M42 adapter ring sits all the time, and I also use more and more old soviet M42 bodies for film, I tried to convert this fine Tokina, in non destructive, reversible way.

I still have the box:

The lens mount is easy, without a rear element wider than the internal circle of a M42 mount, three screws retains the mount and I also unscrew the small lever; this I keep with the box in case of reverting:

I didn't need to remove the aperture ring, but I wasn't careful enough and it came off with the mount, and of course the small bearing ball for the clicking stops jumped out somewhere. I have such balls salvaged from different lenses, but this one is small, it goes inside a hole, where a small spring pushes it against the aperture ring for the clicking. The smallest bearing ball I have, as seen on this picture, is too big. so I will proceed without, lack of clicking aperture stops isn't a problem for me, but I will try to find such small balls anyway, for the sake of full completeness. (also be careful to not lose the small stopping pin of the A position on the aperture ring).

the M42 mount: i have used a PK mount from a cheap bad zoom lens disassembled for parts, and the cheapest reversing ring found on Ebay. With a dremel I grind the PK mount and the second thread on the M42-M52 reversing ring:

then 2-components epoxy glue and let it cure a whole day:

then I use the original mount (temporarily unscrew the internal aperture lever) put on this new M42 mount, as a guide for boring holes for the retaining screws.

finally, some kind of spacing rings are needed in order to get this M42 lens mount at the correct position, the one of the original mount. PK and M42 have same register distance, so the new mount has to align where the PK one was.
I considered diverse alternatives, and opted for automotive gasket material, sold cheap in sheets of different thickness. Here I used four layers in 1mm thick and one layer in 0,4 mm thick. Gasket paper is convenient: density, stability, yet soft so easy to cut and bore.

I make rings that I glue together, sand the inside circle until it is uniform and nice enough, remove carefully any dust, then glue the whole under the lens mount, and bore holes for the retaining screws:

now, position the aperture ring correctly, insert the mount, screw it. Done:

the lens on one of my (slightly tweaked) Zenit-V:

this lens was easy to do. Wish all could be like this one...

visualopsins · Expire: 2025-04-11

jamaeolus · Expire: 2015-08-20

Nicely done! When I first glanced at your work table I was "wow what a bunch of work but then realized those were spare parts not lens parts!

landstrykere · Joined: 10 Feb 2018 Posts: 51 Location: Norge