Pl mount for C lenses on SI-2K

[Alex Raskin]

I have made a DIY adapter that allows C-mount lenses to be used on SI-2K camera.

What's the big deal - SI-2K already has C-mount adapter available as an option, right?

Yes, but that adapter requires you to screw/unscrew FOUR tiny screws in the immediate vicinity of the sensor, using inherently sharp screwdriver. Every time you need to change from C to any other mount, you'll have to do that, because it is not possible to just leave C adapter installed in SI-2K - it blocks other lenses.

Try doing all this on location during stressful production (which refers to ANY production of course :).

To that approach I say, Nein danke.

What I did was, made a C-mount adapter for SI-2K that has PL flange. It puts the C-lenses exactly where they are supposed to be in terms of the back element distance from the cam's sensor, thus preserving focal distance and the infinity focus. However, mounting my adapter is as easy as a PL adapter - no screws.

Me happy now. Just wanted to inspire other SI-2K users who may be quietly suffering like I did - it does not have to be that way!:)

[Bob Hart]

Alex.


I made an alternative IMS - C mount adaptor a while back for the Kinoptik 5.7mm lens. I cut the flange face a little furthur back so that the Kinoptik would focus on infinity, rather than its normal 3ft. This lens was provided with no adjustable focus as it has such a deep depth-of-field

If I want it to focus at its normal position of sharpest at 3ft, I then set the lens about half a turn forward ( approx 0.2mm ) in the mount and if it is going to be in a dynamic environment I shim the flange face for a firm tighten down. I bring it furthur forward for fine focus trims. With the SI2K, you want it as sharp as it can be.

I deliberately made the threads very snug so that there is no wobble, a mongrel way to do it but hey, I am no engineer.

[Alex Raskin]

Awesome!

Bob, how did you go about the CAD drawings and CNC machining? These are costly here in the States...

[Bob Hart]

Alex.


No such things as CAD drawings and CNC machining exist in my world but would be nice to have. It is all measurements with a vernier, knowing the respective flange to focal plane dimensions, samples of the mounts or lenses to match up to like when cutting the threads, hand drawings, close-up glasses in front of my eyeballs and a small Taiwanese CH250 lathe.

One small gotcha is the refraction of the thick combination filter panel in front of the SI2K sensor which causes the effective flange to focal plane distance to be shorter, especially with wider lenses, such that a lens which is set to end-stop right on infinity focus may not focus to infinity unless you skim or shim a IMS-PL Mount adaptor especially for it or take out some shims from the back of the lens and its mount piece if that means of collimation adjustment is available.

This subject got quite a bit of airplay on the SI Forums some time back. P+S Technik was working on a precision backfocus-adjustable IMS-PL Mount but I have read nothing since on the subject.

Engineering something that is robust, very finely adjustable, fits within the available workspace inside an IMS adaptor and is operator friendly is a very, very, big ask.

It may not be achieveable within a reasonable price point compared to the solution of buying several IMS-PL mount adaptors, individually skimming or shimming permanent fixtures for individual lenses which do not have facility for adjustment and swapping out from the IMS base mount itself rather than the PL Mount.

Most stingy rats like me would probably go cheap and buy in the extra IMS-PL mounts. But extra travel in a backfocusable mount to enable a makro-focus adjustment like ENG lenses would be a very cool option.

There is a relatively small population of SI2K owners and those RED owners who have been brave enough to defy their guru and fit the P+S Technik IMS mount mod to their cameras. The cost of R&D required to put a backfocusable IMS-PL mount into such a small production run might never be recoverable.

[Alex Raskin]

But you see, I don't think IMS mod is required.

All you have to do is make a PL-to-Cmount adapter, which is what I did.

Then you can put it on either IMS base, or on IMS base + PL mount by P+S, which makes any lens Macro in the case of latter.

This is possible because both IMS base and PL mount use the same size/shape PL receptacle.

The only trick is to measure your DIY adapter's diameter for the cam's IMS base, not PL adapter, because IMS base is narrower. That's it.

[Bob Hart]

Alex.


I examined the C-mount in PL mount route but found this limited my options with some larger diameter C-mount lens bodies which were not going to fit deep within the PL mount whereas with a direct IMS - C mount adaptor they will. Particularly I wanted the Fujinon f1.8 16mm to 160mm Fujinon. However it is a soft lens, beautiful contrast but soft, so I could have sufficed with a PL-Mount adaptor.

The other factor was that the IMS to C mount was easier to make, - same operations to make the PL disk as the IMS is PL mount with a narrower shoulder, but less metal to turn internally and reach inside with the limited tooling I have.

Measuring up is a breeze as you can install the "pipe washer" C mount to the camera and measure forward with the vernier from the flange of the C mount to the flange of the IMS. Of course you can also do this with an assembled PL Mount over the "pipe washer".

The downside of the IMS - C mount route is that the disk has to be quite thin at one point and a stress riser is created at the internal corner of the lens body clearance relief. With the aluminium plate stock I used instead of stronger material, I would not want to put a heavy lens on it in an agile environment.

[Pedro Rey]

Hi Alex and Bob!
Been having somewhat different issues, but same suffering.
We normally use cmounts, rarely do we swap cmounts for nikons or PL.
But I have found that screwing and unscrewing several times the cmounts during production might lead to lens damage. For example my 16mm Fujinon had its iris ring broken by an uncarefull AC this way....twice...
So I am looking precisely for either an IMS to cmount permanent fixture for each lens (which I think P+S has available for their Linos cMount set) or as you said a PL to cmount with apropiate back flange adjustment.
Would you be interested in selling me some adapters Alex ?

TIA

Pedro
FFRACTALL.COM / CINE-DIGITAL.NET / COSTA RICA
Costa Rica

[Alex Raskin]

Originally Posted by Pedro Rey

PL to cmount with apropiate back flange adjustment.

Hi Pedro, I don't think a bonafide PL to C is even possible for most C lenses, as they would have to sit way too deep inside the PL mount.

Thus I made a IMS to C mount, which is fine. I call it PL because the flange on IMS mount is exactly the same as regular PL flange.

IMS to cmount permanent fixture for each lens (which I think P+S has available for their Linos cMount set)

Actually Linos lenses have a IMS to proprietary mount, not IMS to C, unfortunately. The way they work is their regular C mount is replaced by IMS-to-proprietary Linos mount adapter. So unfortunately it is not possible to just get Linos mount and make a IMS-to-C out of it, AFAIK.


Regarding selling the adapters. I made mine as DIY and it is not something I would offer for sale (although I just used it on a tough 2-day commercial shoot and it performed wonderfully!)

I have drawings of the measurements. Maybe if someone has access to a CNC machine shop, we could co-op on the order and make what they call a "group buy". This way everyone shares the expense, and the price per mount is driven down. (Then we can even offer such mount for sale to others, with profits going to the original co-op members. I can facilitate: storage of the adapters; eCommerce system for online sales; and shipping of orders.)

Let me know what you guys think, I'd be interested to provide the drawing, test the samples, serve as a central point of communications, and share the costs as a co-op member of this project.

What we need is a great CNC machine shop.

[Bob Hart]

Alex and Pedro.


These are the measurements for the one IMS-C mount adaptor I made for the Kinoptik 5.7mm.


Front contact face for IMS clamping ring to C mount flange face = 3.7mm.

Thickness of PL clamped ring = 2mm

Therefore IMS flange face to C mount flange face = 1.7mm

As I mentioned previously, I overcut the flange face of the C mount portion so that I could get true infinity focus.

To get back to the normal 3ft focus of the Kinoptik when collimated correctly, I turn the lens about 1/2 to 3/4 turn which for a 0.7mm pitch means my flange face is 0.3mm - 0.4mm approx behind the normal C mount flange for the SI2K for that lens.

If I was making several IMS adaptors, each as a permanent attachment for individual C mount lenses, my inclination would be to not yet cut the slot for the notch pin in the disk or the bayonet cutouts, overcut the C mount flange face on the adaptors and then shim them between the lens and flange until the lenses' witness marks are correct for measured distance at widest aperture.

Once that is established, then cut the slot for the notch pin and the bayonet cutouts.

Otherwise, when the mount is offered up to the camera, the focus marks on the lens barrel may be anywhere but the place you want them to be. With a slot for the pin in each of the bayonet lugs, you get four choices of orientation but to have it right where you want it is better.

My mount measurements are :-

IMS Disk outer diameter = 67.9mm. ( Generous clearance to allow for my lack of machining skills. ).

IMS Disk thickness = 2mm

Internal diameter of clearance hole for C mount lens body = 40mm. Clearance hole is in front face of disk.

Depth of clearance hole for C mount lens body from front of disk = 3.7mm ( Is also the C mount flange face overcut rearwards by about 0.7mm.).

Outer diameter of rear shoulder for IMS mount = 51.9mm. ( this is a generous clearance to allow for my lack of machining skills.)

Length rearwards of rear shoulder from flange face of IMS mount = 5.1mm ( machining error - should have been 5.0mm.

Diameter of additional shoulder rearwards of rear shoulder = 30.4mm ( machining error - should have been 30.0mm.)

Length of additional shoulder rearwards of rear shoulder which contains a little more C mount thread = 1.0mm

Internal diameter of C mount thread peaks = 24.7mm = tidy fit, which will vary depending on shape of thread cutting tool and condition of thread of individual lenses.

The additional small rear shoulder was a hedge on my part to keep extra metal in case I had to furthur overcut the C mount flange face to achieve infinity focus with the Kinoptik lens. It does not touch anything inside the SI2K Mini body.

The wall thickness of the thinnest cylindrical loadbearing section of 1.7mm is 6mm and a ring shaped flat section is 3mm. This would allow for a two-piece construction, an outer ring with disk an inner ring with C-Mount hole and thread. This could be either itself threaded for adjustment with three grub screws on radial centres in the shoulder for locking or a simple snug fitting disk with a vee-channel for taper-ended grub screws to lock into, adjustment provided by shims.

It is this small 3mm section which concerns me regarding mounting heavy lenses as the loading is both bending in use and shear in frontal impacts..

The second option provides little advantage over shimming the C Mount itself and represents an un-necessary extra part, unless the IMS-C mount is to be used for multiple lenses, still a messy business changing them over.

The Kinoptik is about as wide a lens as one might sensibly use on the SI2K, so my mount flange is probably cut as far back as one would need to go to shim forward from.


FOOTNOTE:

What I have called "the disk" is the same diameter and thickness in both the IMS and PL-mount. The rear face of "the disk" is the PL-Mount and IMS flange face.

The locator shoulder on the PL-Mount however is 54mm diameter. The locator shoulder on the IMS mount is smaller at 52mm.

[Alex Raskin]

Correcto mundo.

Apparently with that smaller inner diameter, P+S Technic says: "Don't try putting your PL lens here in our IMS base. See? It does not fit!"

Albeit by the time the trying party figures the diameter difference, the rear element of the lens will have been crashed into the SI-2K's sensor already... Which makes me wonder why P+S even bothered with the difference in the inner diameters between IMS and PL mounts.

[Bob Hart]

Alex.


I did the very same thing on a Mini35 with a 9.8mm PL converted and rebarreled Century-Kinoptik.

I experienced a glass-on-glass clash which did not feel at all good and punched a mark in the coating of the front glass of the Mini35, fortunately outside of the 16.9 image frame edge. For some obscure reason, Century removed a protective metal rim from around the rear element.

To be fair, this only came about because of the convenience of a rapid change exchangeable mount system and I was less than attentive, working on automatic, the same scenario as when I double-mount a IMS-Nikon adaptor in the front of an already installed IMS-PL Mount and wonder why the wretched thing WILL NOT focus.

A lens clash does not so easily happen with a Letus but the whole swap thing with the three grub screws takes longer, so on the balance of all benefits, they are about equal in this department.

I had another look at the Makro-Kilar and there are distortions to the figure of the front element where some deep scratches had gone through the coating and the fungus had subsequently etched the edges of. There was also that big etched area in centre of the element which is also a distortion.

I may have a go at refiguring the front element but that is a whole time-consuming, labour-intensive and high science I think I am capable of but need to build tools for.

[Alex Raskin]

Hey Bob, seems like your Kilar is a goner. If you buy a good one, the going price of $500-$700 may be uncomfortably high.

May I suggest a cheap but effective alternative for the Macro work - using your own IMS-to-C adapter.

Just get an old Nikkor 250mm or 200mm (that's a Nikon F-mount telephoto lens.) Going price: $50 on eBay. Also get a F-to-C mount adapter, they are aplenty on eBay for $50 or less.

Nikkor goes into the F-to-C adapter, then this adapter screws into our custom BobLex C-to-IMS adapter. That's it, you get a sharp macro lens for $100 total, with the same magnification as Makro-Kilar 90mm.

[Bob Hart]

Alex.


The Makro-Kilar is not usable as it is right now. The smaller 40mm will be in the same state as the front element is identically damaged.

I still have the 4+ and 7+ Century Optics achromatic dioptres from the AGUS35 35mm adaptors I built.

The designed purpose of those dioptres is as high quality close-up attachments. With a 52mm-58mm ring they fit three of the four CP Ultra T* lenses, 12.5mm, 16mm and 25mm. They will also work on a Nikon 50mm f1.8 and Nikon 85mm f1.8.

All may not be entirely lost. The single front element is a simple plano-convex lens. There might just be a generic plano-convex lens of the same power and diameter out there.

[Pedro Rey]

Im in for a group buy!
Unfortunately I do not know a CNC Shop, and I am in Costa Rica, so shipping out of here would not be smart either.
Kaspar Kallas made himself a IMS to C mount as well and has access to the drawings and a shop, but he canoot secure a good price unless we get 25 units or so. He has a price of $200 per mount, which too rich for me, specially given that the fujinons are usually $300 per lens!

This mount is "calibrated" to the Fujinons specifically.

Pedro

[Alex Raskin]

IMO, retail price for one unit of IMS to C adapter should be roughly $150 or less, depending on the costs and the market size for this. My guesstimate is that if we get 20 people to group-buy adapters at $120-$150 each, that should cover the professional CNC production costs.

Of course if someone is making one single custom adapter, I can see how the price can be widely negotiable.

Anyway, I made mine from the existing parts + a modded $35 flange, so I'm pretty happy right now ;)