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Repair of the SOM Berthiot 10mm f/1.9


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I recently pulled the trigger on one of these potentially trouble prone lenses. It was bought very cheap, with foggy glass and tight focusing, but an aperture which was working, for the most part. This is the H16RX version, but the construction is essentially the same as the non-RX lens, particularly the aperture assembly, which is what I'm writing about repairing.

Last night I set to taking apart the lens to polish the haze off the elements adjacent to the aperture. This is common in old lenses with original lubricants, as over the last 50 or so years, the greases have migrated and outgassed, depositing haze on the elements near the aperture. I've seen this in most older still photo rangefinder lenses, especially Canon and Leica. Anyway, I got the lens down to bits and the aperture block out. I saw the awful "crown" of bent metal which makes up the pivot points in each aperture blade. Getting the lens elements haze free and the focusing helicoid working fine was all good but I still didn't want to have to take this lens apart WHEN, not if, the weak aperture blade pivot points give way. 

I have quite a few parts lenses available to me, but no Berthiot 10mm samples. I was on the lookout for replacement blades with better construction. First thing I had to find was a lens that opened the aperture in the same direction as the Berthiot, and had the same number of blades, which is six. Turns out I have three of them, one needed repair. I selected two Bell & Howell Comats, both 25mm f/1.9. One is the first version which is almost like a small golf ball, the other is the later version with the finish that looks like the Angenieux lenses which shipped with Filmos. The third donor lens I have is a Lumax but the aperture blades would require a bit more modification over what I describe below.

Both of the Comats turned out to have aperture blades which are an exact match pivot-to-pivot. Radius, distance and angle are almost exactly the same. The only issue is about .5mm extra on the inside angle of each aperture blade. These blades are easily cut in order to remove the appropriate amount of metal to create a matching blade to the Berthiot original. If this is not done, the lens' rate of closure does not match the aperture scale due to the subtle S bend in one side of the aperture blade. So far I've cut a few blades and using a 20 microscope, I can see that I got the cuts close enough to discourage tearing. The best option would be a rounded corner cut but this is impossible for me in my home workshop.

Once I get the whole lens back together, I'll have an essentially new Berthiot 10mm RX lens, though I'll have spent five hours of labor on it. The opportunity cost of that time spent on this repair is not offset by the cost of a 10mm Switar RX or Cinegon RX, for me, but might be for most out there. That said, since I work on most of my own gear (aside from my REX4) I figure I'm saving money and enjoying the time I spend improving a lens.

I thought I'd put this out there for all you with Berthiot 10mm f/1.9 lenses which have non-operational apertures due to their cost cutting. It's a fine optic and with some TLC can really be made to work quite nicely for folks who want an inexpensive C mount wide angle.

Phil Forrest

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I've been trawling around looking for some old lenses recently and saw a few advertised as having haze. That's less of a concern, as you show here, but anything fifty-plus years old that's been heavily used is likely to also have damaged (over-polished) coatings and I'm not quite sure I want to get into having them redone.

Possible, though!

P

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About 12 years ago I had a Leica 21mm f/3.4 Super Angulon M that had been coated by Focal Point here in the states. They did an incredible job and that was one of the finest, if not THE finest optics I've ever used. The lens body was well worn but the glass was immaculate and beautifully, deeply coated. I'm pretty open to working on lenses with coating marks, but I'm always concerned by the ones that have decades of journalists' shirts grinding away at the front element or even the rear (!) with whatever rodeo or racetrack dust they may be wallowing around in.

I don't know what it might take to recoat a lens these days, especially since I'm not going to be having it done on many elements from many lenses, so I don't know of any services here that do this kind of one-off service. Might be good for me to invest in this in the coming years, if it's even possible. Until then, I'm using a hood!

Phil Forrest

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Interesting. My way would be to punch out the crowns in order to install solid pivot rivets. Have never done that because I would need a die for the repeated precise alignment of the blades plus fitting rivets. No client pays for that.

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Simon, I agree that no client would pay that much labor for this lens. I'm doing it as a proof of concept and also because I find working on lenses (or almost any mechanical device aside from my diesel Benz in the rain at night) quite meditative and enjoyable. 

I needed to add that if the blades are to be transplanted as I described in my earlier post, the holes and slots in the aperture carriage need to be widened with a jeweler's file. Only a very small amount is needed and the brass is quite easy to work.

Phil Forrest

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Interesting! I have an Eyemax 6in f/4.5 C mount which is fixed focus at infinity. I'm assuming for a gun camera.

Simon, do you have any experience with the SOPELEM 10mm f/1.9? I've seen a few of them around and they all seem to be in more of a "parts lens" state than a usable lens. I've read they are the same optic, licensed by Rank Taylor-Hobson who made them for years after Berthiot ceased production of this lens.

Phil Forrest

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Yeah, it’s the same design. TTH’s Monital line were lenses by SOPELEM, the fusion of OPL and Berthiot in 1964.

Through all the lenses I have become acquainted with some favouritism has gotten hold of me, especially when it’s about diaphragms. A Pan-Cinor 150 I had here lately excels as a lens that has a rugged diaphragm. Its smaller siblings for 16-mm. and 8-mm. film yet have such crown leaves.

Berthiot had begun with that already in 1935.

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