Cutting Shims.

[Robert Hart]

Genuine lens technicians will cringe and may well they have good reason, so please, the following comment is me only not any ways near to industry practice.

I operate the SI2K, a digital cinema camera. The Super 16mm film camera lenses that I use on it are slightly out of collimation for a digital sensor which has a fairly thick IR/OLPF panel. It is a little more critical wide wide and ultrawide lenses than for the longer lenses.

After putting it off for far too long, I have procrastinated on this task. I have a great respect for film-camera techs and lens-smiths. Digital cinema cameras make it a little easier in that you see immeidately what you are up to. To correct the lenses for the SI2K, I had to cut some shim material to replace the existing shims or shim stacks which were slightly too thick

Modern plastic shim material can be fiddly difficult stuff to cut tidily if you do not have the proper tools for it.


So if you are rebirthing old motion picture lenses and having to make your shims on the kitchen table, here is a method which might make it a little easier. If I do not have an existing thick and therefore robust shim available as a pattern to cut around, I make a pattern carefully out cereal packet cardboard. Using blue masking tape, I cut thin 4mm strips and tape the pattern down on top of the plastic sheet of shim material. Using a snap-off hobby knife I cut out the centre hole, leaving the strips of masking tape uncut.I then cut the outer edge likewise. I then cut the tapes and shim material beneath through one at a time, moving from cutting one, its partner piece on the opposite side of the circle. This is important to prevent the shim material from creeping or gathering and the cut going bad. I then use good thin haircutter's scizzors to cut the remaining uncut sections of masking tape and shim material around the outside.

Some shims have a small cutout to clear a locating pin. These are you friend as it is possible to link your shim cutouts like the Olympic rings and conserve up to about a third of your shim material. The downside of this is that you do lose the future chance of cutting entire smaller diameter shims from waste trims out of the centres of larger diamete shims.

I found it to be a vexing and timeconsuming task to do it by the suck it and see method of cutting shims, fitting them re-assembling, checking the focus, then taking it all apart, adding or subtracting shim thickness, then doing it all over again.

It is worth doing for the sake of having reliable witness marks on your lens barrel telling truths when they need to be relied upon. This is of course a professional expectation but how many people are having lens techs align the lenses to their cameras or are doing it themselves. There may be more than just a few winging it and relying on the eyepiece and LCD screen.

 

[David W. Jones]

Mmmm, maybe it's just me, but I would have the lenses checked, projected, and shimmed if needed, but are you sure the lens mount on the camera is culminated properly? That sounds like the main issue to me.

 

[Jacek Zakowicz]

Mmmm, maybe it's just me, but I would have the lenses checked, projected, and shimmed if needed, but are you sure the lens mount on the camera is culminated properly? That sounds like the main issue to me.

David, I agree with you.
Robert, in short- don't do it!!! you are in for a world of hurt. Just think about it- there are 100 ways to screw this up and only one to get it right- your chances are about 100:1 for the "wrong".
I've been cutting shims and setting up this stuff for 25 years. Hell I even designed and sell the test equipment- Optimator.
Talk to a lens/camera tech- it's time well spent. Maybe they'll even give you access to their collimator and stuff.
Just from the way you cutting shims I can tell how new you are to this-not a good idea at this point to do it yourself.
Best of Luck regardless!!!

 

[Robert Hart]

David and "Optitek".


I concur with your comments and support your recommendations.

Professional lens servicing is unquestionably the pre-requesite for offering in turn a professional service with the camera and lenses one owns. Otherwise claiming to offer a professional service is just that, making a claim not an assured promise. In Australia by law, that could be regarded as misleading and deceptive conduct for which our Trade Practices Act can give someone a thorough scragging.

I stated a caveat of sorts on the footnote of my original post. "This is of course a professional expectation but how many people are having lens techs align the lenses to their cameras or are doing it themselves."

I would not dream of doing it on high value items. Where the ratio of item value versus the cost is unfavourable and dealing also with the tyranny of distance, I have had to find my own solutions going back to 1978. Things such as harmonising viewfinder screen and focal plane on a CP16R which had been tampered with before I got it, shimming the 10mm-150mm Angenieux zoom to the correct camera focal plane, not the incorrect viewfinder screen someone had set it to.

More recently, I have built my own 35mm groundglass adaptors, which included figuring, polishing and finishing my own groundglass disks from optical blanks. I adapted a Letus Extreme 35mm adaptor to x-y axis adustment and wider groundglass view for higher apparent resolution chasing the 2K Holy Grail.

In this current task, half the lens set ( actually one of two near-identical sets ) had been set up professionally to film camera standard by the vendor who sold them to me. I kept careful notes of the original shimming as a default fallback and did not get into strife. The other half was a collection of cheaper specimens for a second camera and all the possibilities that buying sight unseen off eBay entails.

I remounted from ARRI standard mounts to custom PL-Mounts provided by the lens tech/vendor who supplied the good set. With that as a reference, there was not much to go wrong except for the 25mm lens which had been and remained impossible to obtain infinity focus on. It turned out that somebody else had been romping around inside and had got it badly wrong. After I mined deeper than I am normally inclined to, it was set right internally. All went well but I was confounded for a fair while. A professional lens tech would have sorted it immediately. I guess that is why the lens had sold for $500 versus the over $1,200 they were fetching elsewhere.

It is not for everyone to do their lenses at home and I do not recommend it. However, the chances are that somebody with some fine developed motorskills and appropriate tools, has like me already been backyarding the task. My post was mainly addressed to such as myself who can do their own repairs within sensible limits.

 

[Robert Hart]

Went out and tested the second CP Ultra set after re-shimming and the two vintage Cooke Speed Panchro Ser II s after re-mounting to PL adaptors and shimming the new mounts. It did not end badly. Surpisingly for me, the Cookes, as deteriorated as they are, with the very late afternoon light from behind camera and no over-bright returns from the subject, a shopping centre, the images were as sweet as the CP Ultras at T5.6. Because of the fungus damage and the need to remove pitted coatings, their performance against the light is very ordinary.

 

[Christopher Mills]

If you know how to dismantle and reassemble your lenses, then you are not damaging anything... worst case.. put it back together and send it off to be collimated... people in distant locations has been customizing their tools for as long as there have been tools...
worst case... yes, you have broken the lens... bummer... but not health or life threatening

 

[Mike 'Fireman' Ross]

If ALL your lenses shims are out, what do you think are the chances it's your back focus that's off?!

Mike

 

[Robert Hart]

Backfocus can be an issue with some motion picture film lenses on digital cinema cameras which have additional optical elements between the lens and the sensor. For normal to longer lenses it is not a practical problem for the hobbyist. Except for focus numbers being a bit off in realworld terms it is not a biggie with prime lenses until you get to some of the ultrawides where the depth-of-field at the focal plane end becomes very thin.

Because of the added pieces of planar glass, the angles of incidence from some wider lenses is being affected by the added refraction. Resetting the camera mount itself is not viable as for one lens it will become perfect and be maladjusted on others. With smaller sensors it is a more critical issue. Larger sensors like 35mm or near to seem more forgiving.

The CP primes have a simple mechanism. The entire optical assembly is moved within the lens barrel by operator focus adjustment. It is a relatively simple matter to shim each lens to match the digital cinema camera. The internal optical path is not altered.

Zooms on the other hand are a nightmare. They are designed to lay an image onto a piece of bare film with nothing optical in between. Add that new optical element and their character changes as their image is affected more at the wide end than at the narrow end. You will not get that right entirely. To be sure you are at best sharpness at the wide end, you probably need to defy convention and set your backfocus for wide views, not nearer to the centre of the zoom range of movement. The witness marks at the long end will be out and the lens will not hold focus through a creative zoom movement. In that event you are better to set backfocus for near centre for a best average.

A zoom lens could be engineered especially for cameras with OPLF/IR panels to hold focus through the entire zoom movement. It would be an expensive job. I suspect that the RED developers may have run into that wall with the original affordable 2/3" Scarlett concept with inbuilt zoom lens. I doubt there would have been economies of scale to be relied upon to make the development both affordable for the buyer and profitable for the manufacturer. I'm sure the developers would have been disappointed at having to bail on the original but I think it was a good call. People are fickle enough and there would have been a lot of cranky people if they had been sold a workaround solution. Moving the Scarlett up to the larger sensor size collaterally benefitted everyone.

I also found an issue with groundglass adaptors and extreme ultrawide lenses like the Kinoptik 9.8mm which were not commonly used because of brightness falloff issues peculiar to 35mm adaprtors. Unless the lens was brought physically closer to the focal plane, infinity focus fell outside of the available range of focal movement in the lens. Parfocal or near-parfocal zooms lost that ability.

Please do not ascribe any expertise to my comments. I know just enough to be a danger unto myself and others who heed me. I get there eventually but it takes longer with a few errors along the way.

 

[Jacek Zakowicz]

Robert don't take it personally but you are wrong on pretty much every account with your statements.
Your "need to know" is commendable- respect there :thumbup:
It's not a good idea to broadcast (essentially and inadvertently-I know:wink5 false info on a public forum since a lot of inexperienced peeps will read it and pass it on.
You would really benefit from some interaction with an experienced tech- try to find one in your area and talk to him about these things.
Don't get discouraged- you're on the right track- just hitting some bumps along the way...

 

[Robert Hart]

Jacek.

There is no offence taken by me and your inputs are appreciated. ( Now I know my use of a video camera and rear illumination for checking the correctness of a CP16R viewfinder screen relative to the true film plane would really send a shiver down your spine so I won't go there.)

It is most appropriate that you should challenge my comments, correct them and direct people from visiting misfortunate deadends through bad practices. I would not in a fit do this to more valuable items. The second CP lens set, except for the 9mm had experienced some "mismanagement" by previous owners I say with charitable kindness. The vintage Cookes had been assessed as uneconomic to repair and not presented for sale with the rest of a BL kit. Little did I know that soon after I patched them as best I could myself, that some 35mm glass would re-aquire value possibly beyond its true worth.

My attitude may have been different had they come to me after, not before values rose. They were then and remain pretty far gone with internal corrosion and fungus pits through what was left of coatings into glass. As a lens tech though, would you really want fungussed glass contaminating your shop?

For the future, I shall try to govern my comments accordingly.