Fabricating a PL Mount
Posted 08 September 2011 - 07:14 PM
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.
Posted 08 September 2011 - 11:39 PM
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.
Posted 09 September 2011 - 01:04 AM
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.
Posted 09 September 2011 - 08:14 AM
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.
Posted 09 September 2011 - 10:45 PM
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.
Posted 10 September 2011 - 07:41 AM
Posted 10 September 2011 - 12:41 PM
a lens port cover than a lens mount. Why all
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.
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.
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.
Posted 10 September 2011 - 02:12 PM
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.
Posted 10 September 2011 - 03:00 PM
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.
Posted 10 September 2011 - 05:26 PM
Thanks again, everyone, for your help and responses.
Posted 10 September 2011 - 08:09 PM
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 ?
Posted 10 September 2011 - 09:01 PM
Posted 11 September 2011 - 02:24 AM
Posted 11 September 2011 - 02:41 PM
It does seem silly, though, that so simple and ubiquitous a mount design would be so difficult to reproduce independently.
Posted 11 September 2011 - 03:46 PM
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.
Posted 11 September 2011 - 09:46 PM
but, Alex, it's your baby.