M42-Mount Lens Aperture Mod

This is the M42 lens mount. It is a screw mount used by many vintage lenses and cameras from the ’40s through to the ’80s including many rather desirable lenses such as the Zeiss Tessar, Pentacon lenses, and Helios-44 series.

Later lenses for this mount, including the most desirable ones, had a feature called the auto-aperture pin. This allowed the lenses, like those on a modern camera, to remain at their fastest end for focusing and framing even when the aperture ring is used to stop it down, and the camera can then push the pin in as the photo is taken to temporarily stop the lens down.

If you are using the lens natively on (for example) a Praktica camera, or on a lens adapter that has an extra flange to push that pin down permanently for you, then this is fine, the lenses will work just fine. However, some lens adapters, including all the ones I could find for Sony/Minolta A-Mount, do not have that flange, so the lens simply will not stop down. In additon, that pin gets uncomfortably close to the camera internals on A-Mount such as the mirror.

This is, to say the least, non-ideal. I had a great time the other day shooting with my Helios-44M-4, but it was only usable at f/2. The aperture ring was doing nothing for me, so I hope you like shallow depth of field!!!

What to do

The easiest way to solve this is just to glue the pin down but… that sucks. Here’s an easy, 5 minute mod that is fully reversible. All you need is a cheap ballpoint pen - one of the kind with a transparent ink tube inside it, an appropriate screwdriver for the backplate of your lens, and something to cut the ink tube with.

The way that these tend to work mechanically is that there is some kind of surface underneath the backplate that presses up against it when the pin is all the way out, so as to retain the pin within the lens - this may be a separate piece such as in the Helios-44M-4, which has a T-shaped pin that presses against a separate pad inside the lens, or it may be integral to the mechanism like in the Zeiss Tessar.

If you can get something firmly fastened between that surface and the inside of the backplate, the pin will be held in! …And conveniently, the ink tube from a ballpoint pen has exactly the right inner bore to fit snugly around the auto aperture pin.

Essentially these are the steps:

1. Measure how much the pin moves down when you press it to be fully stopped down - check this at all apertures! In my experience this was 3mm for both of my lenses. Many lenses have a doubly sprung pin such that you have leeway.
2. Cut a piece of tube off of the clean side of that exact size
3. If it does not nicely test fit over the aperture pin, poke something through the hole to stretch it back out to the correct bore - it may have been squashed when you cut it!!
4. Remove the backplate from your lens
5. Fit the piece of plastic tube over the auto-aperture pin
6. Reassemble your lens. You will have to push against spring pressure to close the backplate, whereas it should have happily sit flat before the mod.

Demo

Here’s videos showing the process on a Carl Zeiss Jena Tessar 50mm f/2.8 and on a Helios-44M-4 58mm f/2. Apologies, my phone kept losing autofocus on the Zeiss video :/

And how exactly that little plastic tube fits over the aperture pins on both lenses, if you didn’t get a clear look:

Now, you can use your M42 vintage lenses on a modern camera with full manual control. It feels excellent to use lenses like this, I can highly recommend picking up one to try! Plus, it opens you up to a world of fantastic looking optics for remarkably cheap.

Hope to see you back here soon

— Hazel