Shims for Cooke Varotal 20-100, T3.1

[Deniz Cooper]

Dear friends,

I am searching for a shims set for my Cooke 20-100, T3.1 Varotal Zoom lens. Any ideas where I can find one? ?

Yours,

D.C

[Dom Jaeger]

It’s 50 years since this lens was released. Outside of a very few rental houses that may still have the odd factory shim for such such vintage equipment, I really doubt you’ll find anything like a shim set. 

I often make my own shims for a variety of lenses out of shim stock for small gaps or aluminium with a lathe and sanding for thicker rings. 

[Steve Switaj]

Hi Guys;

Fear not - You can easily do it yourself.

It used to be that making shim sets was a pain because cutting the thin stock cleanly is pretty difficult without making a mess. But cheap laser cutters have changed all that. If you have enough skill to make a full sized pattern in a vector-graphics program, you can have your local laser engraving house cut out a dozen perfect shims from a sheet of plastic.

I like the color-coded polyester stock that's used for "official" machinist shims. It's a known thickness and is as thermally and mechanically stable as you can reasonably expect from a thin film. It's available from .0005" to .030" (0.01 to .75mm) , so... something for everyone, basically.

In the States, you can get it in 5" x 20" sheets, which is probably enough to make 7 or 8 if you space them well, available from McMaster-Carr for  $4.26 ( https://www.mcmaster.com/shim-stock-rolls/color-coded-shim-stock-6/material~plastic/ ) or here's an assortment of 15 thickness for $44 https://www.mcmaster.com/9513K42/

In a pinch, you can try using some filter gel. Hate to admit it, but that has worked for me more than once.

Some laser cutters can also cut thin sheets of stainless steel.

Stainless foils can be found from about .001" and up ( https://www.mcmaster.com/metal-foil/stainless-steel/shape~foil/ )

If you prefer this option, try to find a commercial specialty graphics house, many of them will be familiar with using this material for masking friskets or paste stencils and have the tools at hand.

 

 

 

[Dom Jaeger]

3 hours ago, Steve Switaj said:

It used to be that making shim sets was a pain because cutting the thin stock cleanly is pretty difficult without making a mess. But cheap laser cutters have changed all that.

Thanks for the links.

Most camera or lens shims are simple ring shapes with holes for screws, which I find pretty easy to make with a circle cutter and a good quality hole punch. I’ve collected all sorts of shim material over the years, but my main source is a book of Artus plastic shim sheets that’s lasted me years:

https://www.artusworld.com/product/15-sheet-shim-stock-package/

I’ve used laser cutters, but I found with plastic they can create melted edges with a raised lip and the final dimensions can vary from the drawing. Probably needed to use a better quality laser cutter.

From memory, the Cooke 20-100 (like a lot of older cine zooms) uses aluminium ring spacers to set back-focus. Without the proper tools (a collimator and a test projector) and experience it can be hard to gauge exactly how much you need to add or remove. You can keep adding small plastic shims or removing material with careful sanding (making sure to keep the thickness uniform), but it becomes pretty tedious having to refit the mount to check each time.