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Author Topic: Copal shutter swap  (Read 616 times)

mdshaw55

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Copal shutter swap
« on: August 20, 2019, 12:05:54 pm »

I use my Schneider 28mm a lot and so bought a spare Copal 0 shutter, thinking that if it failed whilst travelling, I could do a straight swap. As a test, I have just swapped the two mounts but did not get the expected results- I thought there might be some off centre loss of definition, but its actually far worse- see test here

https://www.matthewshaw.co.uk/copal_test/content/Test01_large.html

Thinking it was a case of shimming the lenses, I did a half-turn on each, but result was almost identical:

www.matthewshaw.co.uk/copal_test/content/Test02_large.html

Visually, the two shutter units look identical so I can’t understand why the difference?

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Harold Clark

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Re: Copal shutter swap
« Reply #1 on: August 20, 2019, 01:28:49 pm »

Did you measure the distance with a micrometer calliper? My experience with view camera lenses indicated that very small differences in spacing had a notable effect on performance, especially with wide angles.

I would imagine a 28mm lens would be much more sensitive to spacing errors than a 90mm 4x5 lens, so a small discrepancy might be very noticeable.
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alan_y

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Re: Copal shutter swap
« Reply #2 on: August 20, 2019, 03:39:55 pm »

Does the lens work normally when you put it back in the original shutter?
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mdshaw55

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Re: Copal shutter swap
« Reply #3 on: August 20, 2019, 04:24:00 pm »

I don't have a micrometer so can't measure, but I figured a half-turn is more than a shim thickness and it made no difference. Lens is fine when  mounted with the original shutter. I cant see any obvious difference between the two shutters, but there must be something I am missing.
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alan_y

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Re: Copal shutter swap
« Reply #4 on: August 20, 2019, 05:40:51 pm »

I read in Flutot's FAQ that "the most critical spacing is that of the rear lens cell to the iris." I wonder if that is relevant here.
 
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JoeKitchen

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Re: Copal shutter swap
« Reply #5 on: August 22, 2019, 10:58:29 pm »

Did you shot those wide open?  That could have had an effect and stopping it down would certainly be better. 

Also, how many times have you done this test and what kind of stand are you using?  It looks like you moved the camera ever so slightly between shots here. 

Is the spare shutter of the same generation and/or time period?  I have no idea if this would be the case, but Copal may have altered the shutter design in terms of leaf and aperture blade placement over the years. 

I carry two spare shutters with me for this same reason and have had to swap the shutter in my SK 35mm twice on set.  I also swapped out the shutter in my Rodie 55mm and 90mm so I could have the shutters repaired without loosing the lenses. 

I have never noticed any ill effects when swapping out shutters, other then the aperture scale is not there to reference.  So I typically have to gauge where f/11 is relative to the shutter speed scale on the original and use that as my guide on the new shutter. 

I know that the guys at Schneider and Rodenstock will advise against this, claiming that it will effect focus and that each lens is fitted to it shutter.  (But keep in mind, it’s there job to be finicky and they’re German.)  However, if this were the case, then the front groups of each lens would have to be shimmed so the distance would be precise and I have never found a shim at the front in the dozen lenses I have taken apart.  Bob Watkins, at Precision Camera near Chicago (were I get stuff repaired), told me that occasionally you will see a shim in the front.  However I feel like this would be for the rare cases where the shutter was just a little thinner then it should be. 

I also spoke to the techs at Fotocare about this as well, and they often swap out shutters too.  They told me that the critical focus point should not be effected.  If the thickness of the new shutter is far off from the last one, you will see a decrease in the depth of field, but I feel like this would be more applicable to long lenses and table top photography. 

I once read that Copal said two shutters could be off by as much as 24 microns.  Typically ranges like this are three standard deviations from the mean in both directions, so a range of six standard deviations in total, which covers 99.8% of cases, .  This means 4 is the standard deviation, and 68% of all variations are 1 standard deviation from the mean, or a range of 2.  So more then likely any two shutters will be off by less than 8 microns.  This is not that much. 

Neither Schneider nor Rodenstock give any torque measurements for screwing in either side of the lens to the shutter.  They both just say to screw it in until it is snug.  The difference between one person's snugness and another's could be 8 microns.  So I have to assume these lenses need to be made with a certain amount of operational variance in the distance between the front and rear. 

Point being, unless you are unlucky enough to get one of those shutters that is far off from the mean, I dont think it should matter if you swap out a shutter.  Do you have another shutter to test? 
« Last Edit: August 22, 2019, 11:28:24 pm by JoeKitchen »
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Joe Kitchen
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"Photography is one percent inspiration and ninety-nine percent moving furniture."  Arnold Newman
“Don't bother just to be better than your contemporaries or predecessors. Try to be better than yourself.”  William Faulkner

BFD

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Re: Copal shutter swap
« Reply #6 on: August 24, 2019, 07:40:56 pm »

I'm assuming the edge softness is what you are referring to. It would be nice to see what the image looks like in the original shutter. Being that both edges of the frame are so out of focus, I feel like the mounts in the new shutter are not parallel thereby in effect giving you a slight tilt-shift effect. And, yes, most large format lenses are shimmed to make up for slight tolerance differences but that would mostly just affect your focusing ring scale and/or your ability to focus to infinity—I don't think it would do what you are seeing.
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JoeKitchen

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Re: Copal shutter swap
« Reply #7 on: August 26, 2019, 10:39:31 am »

I'm assuming the edge softness is what you are referring to. It would be nice to see what the image looks like in the original shutter. Being that both edges of the frame are so out of focus, I feel like the mounts in the new shutter are not parallel thereby in effect giving you a slight tilt-shift effect. And, yes, most large format lenses are shimmed to make up for slight tolerance differences but that would mostly just affect your focusing ring scale and/or your ability to focus to infinity—I don't think it would do what you are seeing.

Just to clarify this statement, with modern technical cameras that rely on a helicoid to focus, as opposed to bellows, the placement of the lens on the lens mount needs to be precise and shimmed to make up for slight difference is manufacturing. 

The shims I wrote about are different.  Occasionally when you remove the front lens grouping from the shutter, you will find a shim in the shutter.  This is because the shutter is slightly thinner then normal and enough to effect depth of field, so the shim here keeps the front and rear grouping within the spacing range to keep the DoF correct.   
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Joe Kitchen
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"Photography is one percent inspiration and ninety-nine percent moving furniture."  Arnold Newman
“Don't bother just to be better than your contemporaries or predecessors. Try to be better than yourself.”  William Faulkner

mdshaw55

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Re: Copal shutter swap
« Reply #8 on: August 30, 2019, 11:37:35 am »

Thank you everyone for all for your input- Joe, I think it was your original post  which gave me the idea to travel with a spare shutter to swap out when the spring fails (seems to happens around every 18 months).

I do think it is much more straightforward than either Schneider or Rodenstock imply and as Joe points out, the front & rear elements are just hand tightened to no specific tolerance.

There were definitely no shims with the original shutter & it was giving me edge to edge sharpness. With the "new" shutter (serial number is actually lower than the broken one, so assume it is an earlier version), the centre is sharp but then rapidly degrades outwards. A full turn on either front or rear element made no difference so I am assuming it is the distance from the film plane of the combined lens unit that is incorrect. On the other hand, both the shutter units look identical when placed side by side.

I have another working shutter on my SK 35mm so I'm going to try swapping that to the 28mm & see if it has the same issues.

I'm away on a shoot at the moment but will post some more shots taken with the swapped shutter unit & also of the two shutters.

In the meantime, thanks again!
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