C mount lens thread length

[Jon O'Brien]

I've been told to look out for overly-long threads in the older C mount lenses. Is there a way to check just from photographs if the thread is too long (creating a FFD that is too deep)? What approximate dates did the shorter threads come into use? I'm looking for a lens or lenses that are suitable to use with the Bolex Rex-4, 5, SBM etc era cameras, and the Kodak K100. Thank you for your advice.

[Jon O'Brien]

Actually, it probably doesn't affect the FFD. Probably more to do with the rear of the lens bumping up against the front of the aperture or prism.

[Dom Jaeger]

You don’t want the thread extending more than about 4 or 5mm because it can hit the baffle or filter holder or potentially even the prism.

If an ebay ad shows the thread (some don’t) you should be able to tell if it’s too long. This one is too long for instance:

https://www.ebay.com.au/itm/284359049869?hash=item42351ec28d:g:L5oAAOSwzM9g4bTw
 

Early non-RX 25mm Switars (both f/1.4 and f/1.5 versions) are problematic, I don’t know the exact serial numbers but avoid ones below about 250000 I think. 
 

This one for instance is OK:

https://www.ebay.com.au/itm/263594306163?hash=item3d5f71ea73:g:-7IAAOSwbt5ayhlu

Any RX lens will be fine, as will most C mounts usually, just watch out for very old ones or CCTV lenses.

[Jon O'Brien]

Aha, I see what you mean. Yes that's clear to see in the first ebay ad what the longer thread section looks like. I know what to look out for now. Thank you.

[Simon Wyss]

A longer thread allows one to use shim washers or shallow rings to increase the pull-out. By that one can enter the focus range adjacent to the shortest possible distance of a lens.