C-Mount lens threading issue

Cory Zapatka · November 9, 2014

Hey All-- So I've acquired a Black Magic Pocket Cam, which has allowed me to dust off all of my old C-Mount lenses and put them to good use. My Kern 75mm f/2.8, however, seems to be acting strange. The C mount can now become unscrewed, which I don't remember it being able to do before-- see photo attached.

I don't think that this part of the lens should be able to detach completely from the camera mount. It basically comes right off of the focusing threads. Any insights about this would be greatly appreciated-- I know I haven't explained the situation in the best way, but hopefully the photo will help.

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Jean-Louis Seguin · November 9, 2014

Either the distance scale ring was so far out of adjustment that at minimum distance the two parts of the focusing barrel came apart

or

The guide pin is broken.

Jean-Louis

Cory Zapatka · November 9, 2014

Thanks for the reply, Jean-Louis. Do you think this is a self-serviceable issue or something that is even worth looking in to getting fixed?

I could buy a new one for $150 or so, but I have a personal connection to this glass as it was my dad's old lens from his travels.

Jean-Louis Seguin · November 9, 2014

With a jeweller's screwdriver, loosen the screws around the distance scale ring and pull the rear part off the lens. Now put the mount back on the lens. The guide pin inside the mount should align with one of the grooves in the lens barrel. The mount should be able to slide smoothly in and out but not rotate. If that looks OK, take the mount off the lens and screw the rear part of the distance scale ring to the mount all the way, then back it off a little bit. Now take lens and rotate the distance ring to the infinitu mark. Reassemble the two parts and tighten the setscrews (snug, not too tight). The lens should now be able to focus from infinity to the minimum distance without coming apart. The distances marked will probably be incorrect and the lens might focus past infinity but at least it should be back in one piece.

Cory Zapatka · November 10, 2014

Ah, well there is the problem. As you first suggested, it seems like the guide pin has snapped off. Most likely from over tightening or trying to force the c-mount off (I think this may have happened when I left the lens on during the summer and the expansion made it extremely hard to take off). Either way, I think I'm going to have to either solder/weld a new guide pin that will fix the dimensions of the grooves on the lens barrel, or drill through the mount and install a pin that way. I think this would hold better.

Thanks for your help, Jean-Louis!

In the image you can see the small, grey area where the pin once existed.

Edited November 10, 2014 by Cory Zapatka