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SOM Berthiot

SOM Berthiot 20-100

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#1 Ole Hoerning

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Posted 30 September 2013 - 06:11 AM

I´ve got a SOM Berthiot ZOOM M 20-100mm 2,1.

It used to be mounted on a cine-x-ray camera, so it has the focus fixed at 12 feets.

Does anyone knows how and where to loosen the focus?

It has a C-mount, where there is printed "17-85/1:1.8"

Does those values complies when mounted on 16mm film camera and or old school 1 inch tube videocamera?

(I'm not cinematographer...only still photographer)

And finally, is it possible to mount some sort of tripod collar on the beast?


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#2 Robert Rif

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Posted 01 October 2013 - 06:25 AM

I am not familiar with this particular lens, but if you look on the focus ring, do you find a grub screw? This may have been use to set and lock the focus distance. If you find one, just unscrew it slightly and move the focus ring, not holding the focus scale, as this may move, and you've now changed the placement of the focus distance markings instead.

 

The 17-85/1:1.8 should be for 16mm cameras.

 

Sure, it's possible to mount a tripod collar on the lens, but you'll have to find one or make one yourself. Depending on your camera, a base plate, rods and lens support is another way to rig it.


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#3 Dom Jaeger

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Posted 01 October 2013 - 08:19 AM

The focal lengths don't change by using the lens on a different format, it's still 20-100 no matter what camera you mount it on. Som Berthiot made 17-85 zooms for 16mm, perhaps this lens is a variation on that with a focal length extender added to increase the image circle, much like (for example) Zeiss 10-100 16mm zooms were converted to 11-110 for Super 16. It may have originally been a conversion for use with 1" video tube cameras which require a larger image circle than Super 16. At any rate it's now 20-100.

 

A grub screw in the focus ring is the most likely way the focus was locked off, look for one that is in a different position than the screws holding the focus ring on to the inner barrel, which will be uniformly spaced around. Otherwise you may need to remove the focus ring (by undoing those uniformly spaced grub screws) and look underneath to see if it's been locked off internally. If you do that, mark the position of the outer focus ring relative to the inner threaded barrel it attaches to so you don't lose the distance settings. You might need to tweak the position later by checking infinity on a reflex camera at the 100mm end. 


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#4 Robert Rif

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Posted 01 October 2013 - 02:37 PM

That was a much better explanation, thanks Dom.

 

It's also possible one of the grub screws holding the focus ring have been replaced with a longer one and effectively locking the focus ring. So check them all just in case..


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#5 David Mellet

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Posted 07 October 2013 - 07:09 PM

If this came off an x-ray machine I might steer clear. I have heard of some lenses being radioactive.

 

This possibly could be how they get radioactive.

 

Caution is advised.


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#6 Alan Duckworth

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Posted 07 October 2013 - 07:44 PM

If this came off an x-ray machine I might steer clear. I have heard of some lenses being radioactive.

 

This possibly could be how they get radioactive.

 

Caution is advised.

 

Although X-rays are potentially dangerous, and can cause short and long-term damage to a variety of things - including humans - they do not make anything radioactive.

 

The radioactive lenses that you have heard about are due to Uranium being present in the sand used to make the glass. As the Uranium decays it gives off low levels of radiation. Apparently some of the Takumar [now Pentax] lenses from the fifties and sixties are made from such glass, but there are probably others as well.


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#7 David Mellet

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Posted 07 October 2013 - 08:15 PM

 

Although X-rays are potentially dangerous, and can cause short and long-term damage to a variety of things - including humans - they do not make anything radioactive.

 

The radioactive lenses that you have heard about are due to Uranium being present in the sand used to make the glass. As the Uranium decays it gives off low levels of radiation. Apparently some of the Takumar [now Pentax] lenses from the fifties and sixties are made from such glass, but there are probably others as well.

 

Wow. Did not know that. About the x-rays or the lenses doing it on their own. Thanks!


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#8 Zac Fettig

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Posted 08 October 2013 - 07:30 AM

If it was a designed as  ground up medical imaging lens, you might not be able to adjust focus much, if at all. There probably was a setting for adjusting back focus at the factory, but it might not be easy to get at. Traditionally, Medical-Nikkors were focused by moving the camera back at forth. I'd assume this was used the same way. Moving the target towards the focus, rather than vice versa.


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