Lost the shim in my lens

[wclavey]

I have a 210mm Sironar-N lens mounted in a Copal shutter. It had a shim behind the front cell; it was mounted that way when I purchased it. A few months ago, I was out shooting in the field; it started to rain; I was trying to remove a tight filter from the front of the lens and mistakenly unscrewed the lens cell from the shutter - - losing the shim in the knee-high weeds.

While I cannot see a difference in the focus without the shim, either at infinity or up close, I am certain that the shim was there for a reason. I also tried to see if I could detect any change by examining the image with a loupe focused on ifinitiy, close to 1:1 and at the midpoint, adjusting the position of the front cell in the shutter by screwing it in and out. My thought was that perhaps I could calculate what size shim it needed by knowing how far out I had unscrewed the cell. Again, I could not see a difference, but I am certain that it is needed.

So my questions are: who can do the necessary testing or how do I determine what size shim it might need and where would I get said shim? Is there someone who does that kind of testing?

[C. D. Keth]

That lens should have a standard shim for that particular shutter. Contact the manufacturer, they should be able to tell you the thickness and where you can get one.

[resummerfield]

I thought shims were installed at the factory to correct for variations in shutter thickness. For what it's worth, I checked my 210 Sironar-N, and the shim is 0.007-inches.

[C. D. Keth]

That could be. I remember teching a group of identical lenses when I was in school and they all had identical shims. I assumed they were uniform but perhaps not.

[Peter K]

The shims is there to adjust the lens-faults like astigmatism and coma in the outer lens-areas, so if one checks the image in the center with a loupe, there is no difference between with shim or without. Also the thickness is different for infinity work and for 1:1.
To adjust the space between front-cell and shutter is described here http://www.largeformatphotography.info/forum/showthread.php?t=38401&page=2&highlight=infinity

Christopher, every lens will be checked in the factory and is eqipped with a shim in the thickness needed. But the difference between shims has to measured with a micrometer not by eye.

Cheers
Peter K

[C. D. Keth]

Christopher, every lens will be checked in the factory and is eqipped with a shim in the thickness needed. But the difference between shims has to measured with a micrometer not by eye.

Cheers
Peter K

Thanks for the clarification. I didn't just eyeball the shims, though. The thickness was written on them in sharpie.;)

[Peter K]

So the lens is always equipped with a shim for more freedom to align the lens. Without shim one can only increase the distance frontlens - diaphragm. But this depends on this specific lens, the next batch from the same focal-lenght can be different.

[wclavey]

Peter, thanks for the link. Now I can see why I could not tell a difference - - I was looking at an image in the center to try to detect a difference. I'll try it the way described in the link.

So where do I get the shims? I have been to the major camera equipment and lens sales sites and none of them seem to have something like this described in their catalog. Can you buy something like this at the hardware store?

[Peter K]

The problem with a shim is it has to have a certain inner and outer diameter and the most critical parameter is the thickness. So a washer from the hardware store can be a solution. But normaly the shim has to be made with the needed dimensions.

[Bob Salomon]

Peter, thanks for the link. Now I can see why I could not tell a difference - - I was looking at an image in the center to try to detect a difference. I'll try it the way described in the link.

So where do I get the shims? I have been to the major camera equipment and lens sales sites and none of them seem to have something like this described in their catalog. Can you buy something like this at the hardware store?

Camera repair technicians will have shims.

[wclavey]

Well, I have contacted camera repair technicians, several LF lens dealers, SK Grimes, the company (Linos) that now owns Rodenstock, some specialty LF equipment makers who say they test and calibrate lenses - - about 12 people altogether. Everyone was very polite and helpful when they each referred me to someone else (except for Linos who never responded at all...). I decided that it was fruitless when people started referring me to other people I had already heard from.

A fellow forum member (Venchka) has someone who works in his company who will fabricate a shim for me, and it seems that this is the path I will take. But it just doesn't seem right for something that is so commonly discussed (the need for shims, etc.) that I cannot find anyone who sells them.

Last call before I go the custom fabrication route... Any other ideas? Perhaps I just didn't contact the right dealers, technicians, etc. Thanks.

[venchka]

"Who you gonna call?"

Shims-R-Us! ;) :D :cool:

Free home delivery too. Just like Domino's! :eek:

[John Schneider]

If you know the thickness, and measure the ID and OD, you can always make one from shim stock (Mcmaster-Carr etc.). If you need a thin shim, you can cut it yourself with scissors if you get brass. I used 0.005 brass shim stock for something a while back, and it cut okay with sharp scissors (sheet metal shears would be better of course).

[Ernest Purdum]

A long time ago I needed a bunch of shims of this shape, but varying thicknesses. I found (probably from McMaster-Carr) some stainless steel shim stock laminated into maybe 3/32" thick sheets. Trying to punch it was no good, deforming it badly. We eventually cut out a piece a little bigger than the O.D. needed, put it in the lathe, bored and then parted it to the dimensions needed. Then we hit a snag because the outer edge had a nice finish which made it almost impossible to peel off the unneeded thicknesses. If I recall correctly, we wound up giving the outer edge a finish almost like very fine threads.

I appreciate the problem. I don't think there is any very easy solution if the dealers, technicians, etc. can't help.

[John Schneider]

Do a search within McMaster for shim washers -- 0.001 to 0.125 thick in a range of English ID and OD's. Maybe you can find an ID/OD close enough for your purposes.

[wclavey]

Thanks, all.

John, I did a search on McMaster and found the right thickness shim stock, but not any pre-made spacers in the correct I.D./O.D. I need. I think, not having any real metal working tools, I'm going to go with the friend of a friend fabrication route - - and have 2 made, just in case this problem arises again.

I have 2 emails/calls still un-returned, but I'm no longer hopeful about that... Wayne, fire up the equipment...

[venchka]

CNC machine humming.

[Nathan Potter]

As mentioned by John Schneider you can very conveniently use shim stock. I use brass from .001 to .005 inch thick. Easy source is Small Parts Inc. and they will mail to your house. Under .005 can be cut with scissors or light duty shears. You can stack even thinner layers to achieve greater thicknesses. The OD can be trimmed to size with scissors. The ID is more problematical. I sandwich the foil between two pieces of hardwood and clamp very securely. Then I use a hole saw on my drill press (something above 1000 RPM say) to cut thru the top wood and thru the shim stock into the lower piece of wood. This usually avoids ripping the shim stock unless it is very thin, say under .003 inch thick.

Nate Potter, Austin TX.

[Nathan Potter]

Hey Wayne, local CNC machine shop is the way to go! Sandwich the shim stock between aluminum plates - clamp very well then go at it. Mill thru the top aluminum and shim stock into the lower aluminum. You can actually find on line design software that allows you to do your own layout and can be directly compatible with CNC mill driving software. May not be all that expensive for such a small job either.

Nate Potter, Austin TX.

[venchka]

Nate,

My friend, in the office next to where I'm typing now and close by after work, has a small CNC machine in his garage. He uses Lexan for the sandwich material. We have software-I think he will work from an AutoCad file that I will produce. He is primarily making bicycle parts. The lens shim project will be a sideline.