Shimming PL mount for fixed focal length lenses

[Kaspar Kallas]

Hi

I am once again in the position when I need to use a set of lenses that I have no access before start of the shooting period, worse yet it is for 3D so I need to have all the mounts shimmed double.

What would be a good workflow to get this done fast?

So far I have done this:
1 Mount PL mount
2 Mount lens
3. focus to something at fixed distance (2M)
4. See the number on barrel
5. dismount everything
6. unscrew PL mount
7. add/remove shim
8. goto 1 until I am happy
This is VERY time consuming so if anybody has an idea, would be very grateful.

Thank You
-Kaspar

[Bob Hart]

Kaspar.


I can't think of any other way to do it. I am surprised P+S have not yet come up with an adjustable IMS-PL Mount adaptor by now. This could also function as a macro adjustment as a bonus.

Being the engineering perfectionists they are, my guess is they are trying for something better than a simple cylinder and sleeve with 0.7mm pitch thread and lock screw on a smooth shoulder.

The PL mount has four positions so it does not matter if the front of the mount actually turns the lens. Once the backfocus is set and locked, any PL-Mount lens can be dismounted and re-oriented in steps of 90 degrees.

This would be a quick temporary solution. I just have not got round to making one myself as I hate the job of cutting those fine threads.

[Rohan Dadswell]

Hey Bob,

If you do get around to it let us know - it would sure be handy (and I'm sure it would be cheaper than anything that P&S would produce ;-)

[Kaspar Kallas]

Hi

There is problem with the design you propose - the locking mechanism.
A friend DOP found nice Angenieux zoom lens 12-120 off the e-bay with broken mount. So I rebuilt the mount straight to ims withe same backfocus adjustment system. The problem is temperature change, when you close the screw at room temp and then work 30min in -10C then the screw become loose.
All parts of the mount were built from stainless steel.

Anyhow I just moved to another country and have to find a good machine shop again to start experimenting....

Thank You
-Kaspar

[Bob Hart]

Kaspar and Rohan.


The simplest and dirtiest method would be to re-draw the existing IMS - PL mount barrel into two pieces.

The front piece would have an extended threaded shoulder of about 10mm furthur back than the existing adaptor and continuous from the area covered by the PL lock ring , no taper, and be identical on the front to accept the stainless mount ring and the exiting standard PL lock ring.

The rear piece would have the four bayonet lugs and narrower shoulder of the IMS end of the mount and enough narrow shank to permit the clearance of the IMS lock ring which is same as the PL lock ring.

The outer diameter of this rear piece would be enough to allow an internal thread the same as the PL lock ring and enough thickness to carry three 4mm thumbscrews on axial centres. One would be enough but the PL lock ring thread is too coarse and it would likely begin to rock as the threads dressed down to a smooth clearance with use.


More finessed methods would be :-

Method 1. - Cut an approx 5mm groove in the middle of the inner circumference of the threaded section of the rear piece where the axial screws centres are and insert a two-piece nylon ring for the screws to bear down on, so that the threads are protected from crush damage.

The extra diameter required to accommodate this and provide enough guts for the axial screw threads would make the thing very bulky and heavy.

Method 2. - Cut an approx 5mm groove in the middle of the outer diameter of the threaded section of the front inner piece for the tips of the screws to bed down on.

The groove would have the collateral benefit of the doubling the screws as security against the front piece coming off. The screws could be cut down a little on the ends to allow a bit more travel of the front section before the sides of the groove fouled them.

The downside of this method is that a stress riser is introduced. With dynamic stresses introduced with successive tightenings of the screws. The mount might tear away at the rear corner of the groove with time and abuse and drop a long lens when somebody takes a chance and not support the lens on rods.


These would be reasons why P+S have not done it yet. Crude methods and risk of expensive failure - not their ethos at all.


My imagining of how they eventually go about it :-

A Nikon style actuating cylinder of two fine threads, one left-hand, one right hand so that little movement of an adjusting ring will cause a useful movement of double the pitch of the threads. The moving mount section will be keyed or splined from twisting.

The fine threads will be of such tight clearance and lubed with viscous grease so that rocking does not occur or is minimised and disappears when the adjustment is locked, probably with a backfocus screw in a slot like the MINI35 compact and ENG zoom lenses. (I wonder if "The Doctor" at P+S is reading this discussion and bouncing a design around in his head.)


There. - A few thoughts. I have not tried this because the lathe I use is a small CH-250 and the diamter of the workpiece is right on the limit. Also I dread trying to acccurately centre, drill and tap those six screw on axial centres which hold the stainless PL ring in front of the IMS-PL adaptor barrel.


Another simpler alternative would be to remake the front stainless PL ring with an extended wider threaded shoulder with 0.7mm thread pitch, cut the receiving threaded hole into the existing IMS - PL barrel and tap the screws on radial centres into the existing narrow shank.

Trouble is, there is not much guts there and the screw threads would likely fail. I have not gone there because it means remaking a perfectly good piece of P+S stainless engineering without any hope of repeating the precision and finish.

The problem is the lack of workspace between the two locking ring systems. The Le brothers designed a mechanically simple backfocus system for their Letus Elite and Ultimate 35mm adaptors which got around this problem. I think they were forced to use special nylon tips to the locking thumsbcrews to avoid crush damage to the threaded shoulder.