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03-26-2015, 06:00 PM - 1 Like   #1
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Lens shopping over the weekend

A few years ago, I was able to acquire a Carl Zeiss Jena Flektogon 35/2.4 and the Carl Zeiss Jena Sonnar 135/3.5 for a relatively cheap money (actually, dead cheap, but hush!). Both were for M42, but for some reason I don't really use them as much. In fact, every few months I want to sell my M42 gear and buy something useful with it. But then I take the Flektogon into my hands, mount it on my K5 and I'm thinking "I cannot part with this lens". So I keep the M42 stuff, and instead it happily multiplies with each walk to the flea market...

Either way, the only way to get rid of my M42 stuff is to get a Flektogon for K-Mount, so I told myself. Of course, there is no such thing, but there is something that comes eerily close, the Flektogon in PB mount. I found one in my favourite second hand shop when I was over in Scotland last weekend (For cheap Fleks, go to Scotland!). Together with a Sonnar 135/3.5, also for PB mount, I paid even less than for my M42-mount lenses. The Flektogon had a sticky aperture which I accidentally repaired within 20 mins after my arrival back home (Let's take it apart, and see what the problem is - oh, here we go, fixed!).

So I now have two functioning PB-Mount lenses. They are not very easy to convert to Pentax K-mount, although I hear that conversions to Canon have been successful. Furthermore, adapters are available for mirrorless cameras. Indeed, for a moment I thought about getting a cheap Fujifilm X-M1 + adapter, since I'm quite curious about the new Fujis anyway. But then I remembered my original goal to reduce my arsenal of lenses and cameras and free some money, and I put that thought aside again.

Now, I hear you say "Skip to the end!"

First thoughts and estimates

I disassembled the lenses and found that they both have very similar mounts. Some notes:
  1. A conversion on one lens should work with the other one as well.
  2. As far as I'm concerned the 28/2.8, 50/2.4 and 50/1.8 have very different mounts.
  3. One of the key things is to retain infinity focus. Some lenses like the CSJ Tessar 50/2.8 have a focus adjustment screw - do the PB have something similar?
  4. what is an appropriate donor mount? I have a M42 adapter without infinity focus, would this suffice? Superficially, with a bit of grinding down, it should fit the lens very well.

Overall, the task splits into two sub tasks: (1) getting infinity focus (2) mount conversion.

Infinity focus

On first glance, this seemed to be the most difficult part. Other reports suggest grinding down the lens and all sorts of things, which I would like to avoid. Further disassembly reveals that there is a tiny screw that constrains the focus movement of the lens barrel. removing that screw allows to focus beyond the lens's infinity point and achieves infinity on camera (lens simply held agains the camera).


The offending screw is pictured on the right hand side of the lens. It constrains the movement of the helicoid, within the area of the helicoid that is cut out. Increasing this area should increase the focus movement.

But it's not as easy as that: when you remove the screw, there is nothing to constrain the helicoids anymore. Whilst this isn't a problem on the infinity side, on close focus the lens helicoid makes a few turns too many and slides off the rails and your lens comes apart. Also, it does not suffice to just screw it back together, instead you have to disassemble the entire lens again, take off the rails, screw it the helicoid back in, put in the rails again. I did this about 9 times until I figured out why this is happening. Duh!

Simply removing the screw does not suffice, I could find out how the mechanism actually works: The screw acts as a barrier within a cut out of the helicoid. Hence, it should be easy to simply grind off a few mm (my guess is that 2-5mm should be enough from the helicoid to get infinity focus on K-mount. This should be relatively easy and my hunch is that if I only grind down the extra bit partially, i.e. that there are two different levels, I could use the screw to either adjust the lens to Pentax K infinity or Practicar PB infinity. This way, the modification would still be partially reversible. I haven't yet gotten all the details yet though, so use this information with caution.


Here is a photo of the Practicar Sonnar 135/3.5, taken by holding the Flektogon 35/2.4 onto the Pentax K5:

Mount Conversion

This should be the easy part, simply use the M42 adapter, grind it down a bit and drill a few holes. Replace the original mount, take out anything that gets in the way, such as the electronics and the aperture coupler. So I thought.

However, the aperture control lever links the aperture ring with the aperture mechanism - if you remove it, the aperture will stop working.

Well, fine then, I will leave it in and grind off a little more of the M42 adapter. Well, so I thought. However, the aperture control lever is tightly integrated into the mount, and currently I see no way to transplant it.


Everything in there is important.

Next I wondered whether I could simply grind down the PB mount, so that it fits K mount specifications. But the lugs of the PB mount are actually much closer to the lens, so I would have to grind them down almost entirely. That's not an option either.


Not much to grind down.

This was the point where I thought of giving up, selling the PB mount lenses so that someone with a mirrorless camera system can enjoy them.

But then I looked into defunct lenses and teleconverters on ebay. It would break my heart to trash a good functioning lens for such a butch job, but I found a mouldy Vivitar converter, which should be with me in a few days. From the photographs it looks like it might fit very well, but knowing my luck it won't. I will report back once I know more, but let me know if you have any suggestions.

Apologies for the haphazard photographs, I wasn't really planning on doing a systematic exploration and only took a few photos along the way.

03-27-2015, 02:27 AM   #2
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haha, you'll finish with a M42, a PB and a Vivitar converter quite a butch ^^ but hey, that's fun and I'd do it myself so ...
bravo for your explanation and photos of the process.
Wish you luck !
03-27-2015, 07:54 AM   #3
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QuoteOriginally posted by redcat Quote
haha, you'll finish with a M42, a PB and a Vivitar converter quite a butch ^^
That's precisely my worry as well. In which case some of them will have to go, really...

I forgot to mention that I also picked up a Pentax FA80-200 together with the Flek and the Sonnar.


QuoteOriginally posted by redcat Quote
but hey, that's fun and I'd do it myself so ...
bravo for your explanation and photos of the process.
Wish you luck !
Thanks, I and the lenses will need it!
03-27-2015, 09:06 AM   #4
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I'm curious - since you have the m42 version, why not just put a little bit of threadlock on an M42 adapter (with the spring removed, if it has one) and dremel a small notch so the lens will lock onto the camera? This seems like an awful lot of work.

03-27-2015, 01:49 PM   #5
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Narual, this already works without drilling. I really don't know why I want to do this, maybe because those PB-mounts are so incredibly useless. This has changed a lot since the arrival of mirrorless and the appropriate adapters, but now I just want to do it anyways, for the sake of it.
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aperture, carl zeiss jena, flektogon, focus, infinity, k-mount, lens, lens butchery, lenses, m42, mount, mount conversion, pb, pb-mount, pentax lens, practicar, screw, slr lens
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