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Author Topic: The Importance of Shims in Lenses  (Read 638 times)

JoeKitchen

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The Importance of Shims in Lenses
« on: January 20, 2021, 01:51:47 pm »

For the last couple of hours I have been reassembling my SK 28mm f/5.6 lens because one of the internal elements worked itself lose on a plane flight.  I discovered something that I use to think was way over-exaggerated, the importance of making sure the front half of a large format lens is placed at the exact distance from the rear half.  This is usually done through the use of placing paper thin shims at the base of the front half. 

In process of taking the lens apart to fix the loose element, I separated the lens from the shutter and dissembled the front half.  I noticed three shims in the front with first one falling out.  In the process of screwing the front half back into the shutter, that shim got creased and caused the front to be slightly angled.  This created a slight loss of focus on one side.  I then removed this shim, feeling it would be better without and the focus totally dropped off on the edges.  It remained tack sharp in the center of the IC, but out of focus on the edges. 

I just got finished flattening the shim and placing it back in position so as to lie flat (not an easy task) and the lens is just as sharp as it was a few days ago. 

It turns out that I have been wrong in assuming the shims did not make that much of a difference.  In fact they are extremely important. 
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Ken Schuster

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Re: The Importance of Shims in Lenses
« Reply #1 on: January 20, 2021, 03:41:09 pm »

"It turns out that I have been wrong in assuming the shims did not make that much of a difference.  In fact they are extremely important."

You're a brave sole, Joe! I was happy to read that you persevered and got the lens reassembled with no "extra" parts left over. At least as important as lens shims are focal plane shims when mating a digital back to a body. Even with film, Hasselblad's "secret sauce" for edge-to-edge sharpness was in their film cassette mechanism, as well as matching serial numbers for backs and bodies. 
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JoeKitchen

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Re: The Importance of Shims in Lenses
« Reply #2 on: January 20, 2021, 09:20:17 pm »

"It turns out that I have been wrong in assuming the shims did not make that much of a difference.  In fact they are extremely important."

You're a brave sole, Joe! I was happy to read that you persevered and got the lens reassembled with no "extra" parts left over. At least as important as lens shims are focal plane shims when mating a digital back to a body. Even with film, Hasselblad's "secret sauce" for edge-to-edge sharpness was in their film cassette mechanism, as well as matching serial numbers for backs and bodies.

Not that brave, just cautious.  Like beyond sober cautious.  Given the value of this lens and the fact I needed to use the pencil point side of a spanner wrench, it was methodical process. 
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EricWHiss

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Re: The Importance of Shims in Lenses
« Reply #3 on: February 12, 2021, 03:47:04 pm »

Interesting and thanks for sharing that.  I've never actually seen a paper shim, but often thin metal ones.    Did you by chance measure the thickness of the shim?
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JoeKitchen

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Re: The Importance of Shims in Lenses
« Reply #4 on: February 15, 2021, 04:57:06 pm »

Interesting and thanks for sharing that.  I've never actually seen a paper shim, but often thin metal ones.    Did you by chance measure the thickness of the shim?

It was not a paper shim, but a paper-thin shim made of metal. 

I dont own a micrometer, so I cant measure these things. 
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Harold Clark

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Re: The Importance of Shims in Lenses
« Reply #5 on: February 16, 2021, 10:01:44 am »

It is surprising how much difference shims make. I have 150mm & 210mm Kowa Graphic lenses I got in barrels some years ago and mounted in Copal shutters for my LF cameras. The barrel lenses each came with a shim. I tested the lenses with and without shims, and there was noticeable difference on 5x7 film.

I believe the conventional wisdom with some LF lenses is that shims can be used to fine-tune performance of edge vs center. The 210 mm for instance could be optimized for performance across the field of coverage for 8x10 use, but optimized for centre for 4x5.

Considering the short focal lengths used on digital cameras relative to LF, the tolerances would be much more critical.
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EricWHiss

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Re: The Importance of Shims in Lenses
« Reply #6 on: February 23, 2021, 01:47:46 am »

I think it makes more difference for some optical formulae than others.   I haven't done it in a while but I used to swap out lens groups in the electronic shutters and a lot of them didn't have shims, only a few did.  You can for sure change the spacing as much as with a very thin shim just but putting more torque on the lens so that's something to pay attention to as well I guess.

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