Recently had a Conversation with Solscud007 regards mounting Lenses with Very Long, Big Rear Elements to the Chronos Cameras. In his Case specifically the Fujinon-TV 25mm f/0,85; which i also own.
The Lens has a Rear Element, that will Protrude a bit more than 9mm into the Camera, measured from C-Mount Flange Face. While you could for sure just Shorten the Metal Part of that lens, it still has a Rear Element that is not that much smaller and still about that long, while the Metal Part that Protrudes so far is mainly to protect the Highly Convex Shaped Rear Lens Element from Damage. That Rear Part will just Barely Clear the Inside of the 1" Diameter C-Mount Thread, but there Is no way you can ever fit an Standard 5mm C-Mount to CS-Mount Spacer into there to get it to proper Flange Distance to use it.
I Personally just have built a Nikon1-Mount for my Chronos, which is a bit Shorter in Flange Distance than MFT, so i can use an Modified, VERY Slim Adapter for Nikon1-Mount to C-Mount to Mount it, but that is a very custom Solution.
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For the Record, here are Some Relevant Flange Distances for this whole Issue:
Chronos Body Front Face Focale Distance: 11,714mm
CS-Mount Flange Focale Distance: 12,526mm
Nikon1-Mount Flange Focale Distance: 17,000mm
C-Mount Flange Focale Distance: 17,526mm
MFT-Mount Flange Focale Distance: 19,250mm
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[see:
https://en.wikipedia.org/wiki/Flange_focal_distance ]
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Since this Lens, and many others like This just make a small Step of like 0,5mm Forward from the C-Mount Face, and then immediately go up to Diameter 48mm basically almost flush to the C-Mount Flange, there is no way to Mount this Lens to Either CS-Mount with Standard Spacers nor to MFT-Mount without somehow Modifying the Lens itself.
This is an Issue, that is known with a bunch of more Extreme C-Mount Lenses, especially VERY Fast Lenses Like this one, but also Extreme Ultrawide Lenses and such. For Lenses Like this, i originally added that Option on the Poll:
https://forum.krontech.ca/index.php?topic=658.0 .
Now while this Fujinon Lens in Particular has a Removable Mount, which could easily replaced by some other Means to connect it to the Camera, that is a Difficult to make and very custom Part to make, and also would not work for all Lenses Like this.
After Thinking about it, i just figured, it would be Easiest to simply replace The CS-Mount Ring on the Chronos with one that already has the 5mm Spacer Kind of Included, and is just 5mm longer than the Original. Since anything, that could be Mounted to C-Mount has to be able to Clear the Inside Diameter of the C-Mount Thread itself, anything that Has a C-Mount should work on there (Make sure that you still have enough Space "behind" the Lens to not Scratch or damage Sensor or Lens, if you Mount any Extreme Optics like that. For this Lens i can Confirm, it will fit, but for anything else make sure to check that first!!!).
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Solscud007 asked me to Provide Drawings for such an Modified C-Mount Ring, those are Attached to this post.
Measured from the Original one that came with my 2.1, which will probably also be useful for anyone trying to make an Replacement, if they buy one used some Years from Now, and face some Missing Parts, like ExplosionsAndFire experienced here (also why i am posting that here, to keep the Posts about Replacement Parts in one Spot together). Shipping and Taxes /Import Fees to Germany are probably still high enough that it would be cheaper for someone to make one himself here or even get one made, Rather than getting the Original Replacement Part (applies Especially for anyone owning a Lathe or with easy Access to one).
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For Best Fit, its Probably best to either Make Gauges for the Inside and Outside Thread from the Original Part, or at least to check the Fit with The Camera Body and any C-Mount Lens you intend to mount on it. Shouldnt be hard to turn, else You risk it binding up, and not too loose also, else you might get a Slanted Mounting Face and Parts Of the Image out of Focus. Part should also not have any too sharp Edges or Burs, especially the Flat Front Mounting Face Must be Free of any Burs.
The Original Part off my Chronos 2.1 is slanted about 0,03mm Across the Two Flat Faces, but i did not Test any Kind of Run Out on it, so it might even Run very true on the Faces that Matter.
Based on that, i recommend to aim for some Tolerance like maximum 0,02mm Run-Out of the Front Face in Relation to the Threads, then you should have a Part, that will provide comparable Accuracy as the Genuine Part (Probably better, because you cut the Error of the CS-C Spacer out!).
As described in the Dimensional Drawings, the Flat Back Surface does not Really Matter much, but if you can, might as well make it accurate, if it is not, it also wont hurt anything.
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The Two Holes in the Flat Mounting Face are not Strictly Required for it to work, but i still would highly recommend you to put them in there, if you can; as it is just way easier to remove from the Camera, if it for whatever Reason should get stuck somehow, or if it needs just a bit more force to turn than possible by holding onto the Ring itself.
https://forum.krontech.ca/index.php?topic=615.msg4198#msg4198 .
As Far as Backfocus Adjusment goes, that works the Same as with the Original Ring. I still reccomend anyone Removing these Rings, to put some small Sharpie-Dot where The white Line Leading to the Mount goes onto the Threads outside, that way, i was able to remove it fast and put it back into proper Backfocus very Quickly again if i needed it. I found that to be Repeatable enough for my needs (if you need it more Accurate, that should at least give you a very good point to Start Fine-Tuning)
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Any Remaining Parts of the Mechanism, that Locks the Ring in Place can be Found in the Posts Above.
. Probably the Easiest and Cheapest Solution for Lenses Like this, if anyone has Trouble making such a Modified Ring, i could get one made for you fairly easily, let me know.
