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Fabricating a PL Mount


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#1 Alex Nelson

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Posted 08 September 2011 - 07:14 PM

Hey everybody,

I know this is an odd and probably naive question, but I'm looking for the dimensions to fabricate a PL mount for a lens. I know the diameter is 54mm, but the more critical specs are maddeningly difficult to find. I'm trying to produce a one-off lens for an upcoming project, so I doubt I qualify to license the mount design from ARRI. I suppose if it comes down to it, I'll just have to take my calipers to an Ultra Prime or something. If anyone can offer some direction, though, I would greatly appreciate it.

Best,
Alex Nelson
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#2 dan kessler

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Posted 08 September 2011 - 11:39 PM

When faced with the very same question a long time ago,
I paid a visit to a camera shop, asked to look at one
of their PL mount lenses, then asked if they minded me
taking measurements (they didn't), and, since I just
happened to have my dial calipers with me, that's what I did.

Now, I'm guessing that the tolerances on an Ultra Prime
are EXTREMELY TIGHT.

So, if you called those Ultra Prime measurements your
upper limit, then held the downside as close as you could
(dead on would be good), I'd say you were close enough.

Fitting it to the lens is probably the more critical part.
Never done it myself, but surely you'd need a collimator
with an accurate standard for the flange focal length.

Then again, maybe an honest-to-goodness optical guy will be
appalled enough at my reply to come in and tell us both
how to do it.
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#3 Dom Jaeger

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Posted 09 September 2011 - 01:04 AM

Well this honest-to-goodness optical guy thinks you're pretty spot on Dan.. B)

Yes, the tolerances are fairly tight. The rear lip of the mount needs to fit snugly in the camera mount cavity, so that the lens is centred and can't move around, but obviously you want to err on the side of too small. Undercutting where the rear of the wings meets the lip will help the lens seat properly.

Other than that the critical things are flatness of the mounting surfaces (mount to lens seating, top and bottom of the wings - ideally within 0.01 mm) and the thickness of the mount wings. Too thick or thin and the PL locking ring either won't slide over the wings or slides too far past. I think 2.00 mm is the upper limit.

The depth will be the tricky thing. Proper PL mounts are machined about 0.1 mm under where the exact back focus should be, then shimmed the rest, so you can set it exactly. It's easy if you have access to a collimator, otherwise you have to play with shims until infinity focus lines up. You can make your own shims out of thin plastic sheets with a compass cutter and a hole punch. I'm guessing you'll be just eye-focusing when you come to use the lens, so as long as you can reach infinity you'll be right.

Not an easy thing to fabricate though.
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#4 Hal Smith

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Posted 09 September 2011 - 08:14 AM

I wouldn't even attempt to pull measurements off of a lens mount without a set of ten-thousands micrometers, a ten-thou dial indicator and stand, Jo blocks, and a good surface plate. A dial caliper, even a good digital one like my Starrett, can't work at the accuracy required (.01mm = .0004"). And, as Dom points out, even then you're going to finish by shimming to get back focus distance right on.

Alas, my frau's Uncle Steve has gone on to his reward with the saints. He was Chief of Tool and Die Operations at Cadillac for twenty years. Steve would have produced a PL mount drawing off my Arriflex manufactured Arri standard/bayonet to PL adapter in about fifteen minutes flat. Anyone know a good medium?

Here's a PL mount illustration from Silicon Imaging's SI-2K Mini manual, it appears to be a dimensionally accurate CAD drawing and it may be possible to scale dimensions off it (I have a hunch all the critical dimensions are in even millimeters). It wouldn't hurt to give SI a ring and see if they're willing to supply a copy of the original with all the dimensions on it.

SI PL Mount.jpg
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#5 Alex Nelson

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Posted 09 September 2011 - 10:45 PM

Thank you, everyone, for your responses. I am, thankfully, only designing a pinhole lens, so back-focus isn't very critical. My biggest concern, at this point, is that the mount fit securely in any camera body. Also, the machining will all be CNC. I have no illusions about my ability to mill something like this by hand.
OEM reps haven't been very forthcoming with information, so my best bet appears to be what Hal has suggested. I also considered photographing the rear of a lens (or a sample of lenses), straight on, using a copy stand and a long lens. Since the external diameter of 54mm is a certainty, it stands to reason that the rest of the dimensions can be extrapolated from there.
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#6 Stuart Brereton

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Posted 09 September 2011 - 11:32 PM

You may have already thought of this, but Les Bosher does this stuff for a living, and he's a helpful guy.

www.lesbosher.co.uk
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#7 Alex Nelson

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Posted 10 September 2011 - 07:41 AM

That's quite helpful, actually. I wasn't aware of Les.
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#8 dan kessler

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Posted 10 September 2011 - 12:41 PM

Okay, Alex, a pin hole lens. That's more like
a lens port cover than a lens mount. Why all
the stress?

No joke, you could make this out of cardboard
and Elmer's glue and the light rays wouldn't
know the difference.

I programmed and ran CNC's for several years,
and, at the other end, I've breadboarded
optical assemblies on my kitchen table
with a straight edge, a razor blade,
toilet rolls and scotch-tape.

Sure, I know all the warnings about dial calipers
not being accurate, but everybody and their
brother uses them with no problems. Most jobs
just don't call for jo block accuracy, and this
is one of them.

Your CNC guys will grab a piece of scrap aluminum
bar stock and knock this out on their lunch break.
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#9 Charlie Peich

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Posted 10 September 2011 - 01:51 PM

Thank you, everyone, for your responses. I am, thankfully, only designing a pinhole lens, so back-focus isn't very critical.


Alex,

Check out Clairmont Camera's Pinhole Lenses . Scroll down the page.

You can put together something similar with, as Dan suggested, a port cap or with with available lens to-PL-adaptors.

Charlie
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#10 Alex Nelson

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Posted 10 September 2011 - 02:12 PM

Hahaha, I know, this project is more involved than it probably needs to be. What complicates everything is that the pinhole is created by a mechanical iris rather than a drilled hole. The other complication is that I need the focal length to be shorter than the flange focal depth of a PL camera. I'm aiming for around 25mm. I've done tests with a Nikon D7000, so I know the concept is sound.
My goal is also to make this a more finished and robust piece of gear than most DIY pinhole lenses. I'm 3D printing the barrel and most of the moving parts, but the mount needs to be able to handle the pressure of a PL locking ring. It's true, I don't need to work within the same tolerances that Zeiss requires, but I do need a snug fit. My hope is that, once I have dimensions to work with, it really will be as straightforward to CNC as you've said, Dan.
I've actually held Clairmont's pinhole lens and it works using pre-drilled swappable plates. It's a nice system, and very well designed (of course), but more cumbersome than what I've designed.
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#11 dan kessler

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Posted 10 September 2011 - 03:00 PM

Okay, so not your garden variety pin hole lens.
Got it. Doesn't change a thing regarding your
original question. Grab that PL Ultra Prime and
your calipers and you've got your numbers.
Want a little extra room? Subtract a couple of
thousandths from the OD and wing thickness. Done.

Oh yes, and I'm sure you'll watch out for shutter
clearance on those short focal lengths, too.
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#12 Alex Nelson

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Posted 10 September 2011 - 05:26 PM

Sounds like a plan. This is primarily intended for digital motion picture cameras, so shutter clearance isn't an enormous concern (although, given that the Alexa Studio and F65 will have mechanical shutters, maybe I should re-evaluate that idea).
Thanks again, everyone, for your help and responses.
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#13 Chris Millar

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Posted 10 September 2011 - 08:09 PM

Not that you might need them I had a look around on the net for the specs anyway out of interest

30mins later - nothing

tantalising link at wikipedia at the bottom of the page to Arri's website for the technical specifications but its a deadend ...

google patents have nothing either for "Arnold Richter mount" and other similar search strings ...

Best bet would be a direct email to Arri yeh ?
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#14 Alex Nelson

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Posted 10 September 2011 - 09:01 PM

I actually found the ARRI patent earlier (it's got some nebulous name like "Device for Securing Motion Picture Lenses to Cinema Cameras"). Unfortunately, it's got frustratingly few details. It mainly describes the general operating concept of the PL mount.
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#15 Brian Drysdale

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Posted 11 September 2011 - 02:24 AM

The easiest method might be to adapt a used PL mount that's been removed from say a damaged lens or buy a new mount from say Les Bosher, then fit your pin hole inside it, or to the rear of the mount.
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#16 Stuart Brereton

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Posted 11 September 2011 - 01:20 PM

There is an Arri Standard mount to PL adapter on ebay right now for about $30. Could you conceivably use that for your project?

Adapter
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#17 Alex Nelson

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Posted 11 September 2011 - 02:41 PM

I did consider retrofitting an adapter (specifically those on eBay), but they would interfere with the design. In the interest of control, I'm going to have a new one machined to the measurements I pull from a sample of lenses.
It does seem silly, though, that so simple and ubiquitous a mount design would be so difficult to reproduce independently.
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#18 Chris Millar

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Posted 11 September 2011 - 03:46 PM

Email Arri or buy that adapter and mill or turn out what you dont need if its in the way ...
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#19 Brian Drysdale

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Posted 11 September 2011 - 06:08 PM

In the interest of control, I'm going to have a new one machined to the measurements I pull from a sample of lenses.


Getting one off items machined can be expensive, you could get Les Bosher modify one to your design and he knows the dimensions.
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#20 dan kessler

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Posted 11 September 2011 - 09:46 PM

I also like the idea of modifying the adapter,
but, Alex, it's your baby.
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