Shim found between elements of Sironar 150 and Copal 0

Hi,

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<b>Summary:</b>

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I have found a shim between my shutter and lens when removing the

lenses for a shutter repair. Should this be there? It does *not*

affect setting of aperture or shutter speed if left out of the

assembly. I cannot say if it detracts or enhances focus as I do not

have a darkroom or LF enlarger. Its a bit of a pickle really.

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<b>In Detail: </b>

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I bought a second hand Sironar-N 150 in a copal O shutter on a lens

board. The shutter jammed. I took the front and rear lens elements

off to take the shutter to the local shop. When I later moved the

lenses from a tabletop I noticed a 'washer' (aka shim or spacer) left

behind on the table ie I don't know which lens group it was

associated with.

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Elsewhere on the internet I read that standardisation of shutters and

lenses is supposed to mean there is no need for spacing shims like

this. So I'm wondering if this shim is supposed to be there at all.

In each case, front or rear of shutter it fits directly on top of the

shutter threads. In the pic it is slightly offset so you can see it.

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I don't know if it should be where it is, ie inbetween the lens

elements. Undoubtedly it must affect focus but for better or worse?

What do you reckon?

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Its going to be a total *uuuurghhh!* if its not supposed to be there.

But I'd like to know for the future so I can get a better perormance

from the lens from here on.

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BTW It is only thin..I'd say approx 0.1mm by eye.

 

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<img src="http://www.bwt.pwp.blueyonder.co.uk/shim.JPG">

There are manufacturing variations in making optical elements. Of course the manufacturers aim at making each element identical from lens to lens, but there is a limit to what can be affordably done in manufacturing.

 

For some lens designs the spacing between the optical elements is critical. For these lenses sometimes the spacing needs fine tuning to adjust for manufacturing varitions. Optimizing the spacing will optimize the correction of abberations and thus the performance of the lens. If the shim is lost the change will be for the worse but may be subtle -- the lens will still focus. Some manufacturers machine the shoulder of the cell to alter the spacing while others use shim washers. I have seen this in several different models of LF lenses from at least two manufacturers. It is most common in wide-coverage types. Rather than being a sign of sloppiness, the shim technique shows the manufacturer is going to the effort of measuring each lens and adjusting for optimum performance.

 

When shim washers are used, the manufacturer normally installs them in front of the shutter because the user is less likely to encounter and perhaps damage or lose the spacer in that location. To install a lens in a lensboard you have to remove the rear cell, but normally there is no reason to remove the front cell. So it is clear that the washer belongs on the front of the shutter.

 

So when you put your lens back together, install the shim in front of the shutter. Take care that the shim doesn't get dislogded or damaged as you screw the front cell in. It is easier to do this if you hold the front cell with the threads that screw into the shutter facing up, then place the shim on the shoulder at the bottom of these threads, then screw the shutter onto the lens cell from above the lens cell.

Of course, in terms of spacing between the cells, the spacer can be on either sie.

My Rodenstock catalogue "AUFNAHMEOBJEKTIVE" issued 7/1985 says, overall length of Sironar-N 5.6/150 is 51.0 mm, and length from shutter rear plate to the brim of the rear lens cell is 20.3mm if the Sironar is mount in Copal # 0; for Compur #1 that length should be 19.3mm.

Uli

When I had the incorrect spacing (too large) for my 68mm f8 the central 3x3 area of the image achieved proper focus but the rest of the frame did not. The effect was subtle and I did not notice it until I got the processed film back. As mentioned the spacer can go on either side, just be sure it is in there somewhere. Good luck!

Ah thank you for the replies..I thought I was on auto-update but had no emails so I thought I'd come and check.

 

Mr Briggs ...Wow! Thank you, that is all news to me.

 

It also gives me some hope that (when I finally get a darkoom set up etc) my shots to date may not be the unholy mess I thought they would be (althought I must confess I also looked for a lens performance test chart for the lens, could not find one so assumed optimal f-stop would be around [gulp] f32 [eeek!] on account of the self named large format site F32.net in the UK. Bri = wally).

 

Well I'm quite surprised that lenses are honed/'dialled in' in this way. Has the process got a name eg if I have a shutter that sticks I ask for a clean and lube.

 

LOL I'm still quite happy from reading this news - in fact its made my week. Me being me of course will worry that this is the right sizes shim etc etc etc. but thats me - always expecting the worst. But yeah...you have made me happy! :)

in fact

 

 

 

WHHOOOOO HOOOOO!!!