Apo-Ronar shims--one, two, or none?

[Bernice Loui]

0.1mm converts to about 0.004 inch which is HUGE for a lens barrel. Tolerances for lens cell barrels and shutters are typically 0.001 inch range. These numbers are not appreciated unless one does machine work, engineering-technical calculations and similar. For the hobbyist and those with no real world experience doing stuff like this these numbers and their physical dimensions are difficult to relate to or appreciate.

"Oh, I've ripped apart a Philippe Patek watch, put it all back together but for some reasons it is no longer time accurate, it does work, so good enough for me.

As for easy for any "good" machine shop to make shims. Shims less than 0.01 inch thick or 0.254mm are not going to be made on a lathe or similar machine tool process due to the limits of shearing material from bar stock. Parts like this will be made by etched metal process where a mask is applied to a sheet of metal specified fo the part, then etched until the part is cleared. They can also be made by laser cutting or similar. These are not common machine shop machines.

LF has become more of a Foto hobbyist or speciality artist endeavor today due what has happened to the image production universe. There are not similarities to the custom car world or similar and the LF camera world. Not a bad thing if this keeps LF image making alive and well. What can and does happen, the difference between technical excellence and individual projections of what they believe is excellent can become accepted as a standard of excellence disconnected from true technical excellence based on Science, Engineering, and the way Nature is.


Bernice

[Pere Casals]

I'd like to summarize my findings in lens shimming these years, for the case it can be useful for some, as old&used lenses may have lost their shims.

Why shims ?

Some manufacturers were not usually shimming their lenses, Fuji for example...

Sample to sample variation in LF lenses performance always has been there, over time manufacturers found several ways in the lens assembly to minimize aberrations (sometimes manufacturing engineers were moved to the design department to help designs be easier to manufacture with high quality consistence cand reducing defective products). In some barrels possition for each group can be individually adjusted. Probably a manufacturer would pair the right front and rear cells that do minimize a certain aberration, perhaps another remaning aberration would have a better compensation by adjusting the inter-cell spacing.

Old convertible Symmars (for example) have their cells individually corrected, but in many designs the cells are individually uncorrected, the rear cell compensates a miss of the front in the counter sense, when the amount of the miss reaching the rear cell does depend on the intercell distance then the shim adjusts the amount of the miss, thus the shim allows an adjustment for an optimal compensation.

It is not a trivial compensation, because a shimming may be better for the center and another one for the corner or the mid. Or a different aperture has different optimal shimming. Or a different shimming have better corrections for close or distant subjects...

IMHO the work the shimming makes depends on the design, this is beyond what I may understand... in practice we adjust the shimming for best performance for the particular lens usage.



Problems...

1) We never know if a used lens has the original shims.

2) We may want you optimize a process lens for distant subjects, a common case for ULF people.



Manufacturer's shimming service

This is the best way, an expensive lens (or a lens we love) that has to be used a lot it deserves the manufacturer's care at any cost, if available. As Bob Salomon adviced, on any doubt about the shimming, Rodenstock will make a perfect job, we can guess that they know how to do a perfect job, they have all the shims, they know the design and the assembly procedures, this is a perfect choice, and it is a privilege to still have that service available.



When DIY shimming ?

This is suitable if we like/enjoy DIY, we want to make the effort and/or manufacturer's service is not available, not feasible or not wanting to spend much money. Or simply we may want to check if our shimming is good.



"Internationale Phototechnik" 8/1993.

In this issue it was described a DIY method to optimize G-Claron and APO Ronar repro glass for distant subjects, this was posted by Peter K in this forum: (https://www.largeformatphotography.i...l=1#post369572)

"Focus the center of your ground-glass on fine structures in the distance like leafless trees or high grasses with a strong loupe at wide open aperture.

Shift the rear frame of your camera to the left and the front frame to the right up to the point you can see the structures get blurry.

Unscrew the front group of the lens circa 2 mm (1 line or 0.08 inch) from the shutter or barrel and look for the fine structures, twigs or leafs. Adjust the distance between the front group and the shutter up to the point the structures are as sharp as possible without any coma and blur. Refocus if necessary during the adjustment.

Place a steel or brass shim of the determined thickness between shutter and front group. If the thickness of the shim is the correct one there is no difference in image quality between the center and the edge of distant subjects. For close-up work remove the shim or place a much thinner one."




Hr. Dieter Wenzel --Dieter.Wenzel-at-linos.de

https://www.largeformatphotography.i...r-ad-infinitum!

Hr. Dieter Wenzel says that shimming in APO Ronars makes a difference visible in the periphery, wheras the central parts will be equally sharp at infinity and close-up with either adjustment.



P.Casals contribution to the shimming technology (sorry for the self-irony, laughing at myself)

> To check performance at different spots in the circle, don't use GG+Loupe , but an Ocular/Eyepiece of 20x to 50x without the GG, the GG won't allow to see many lp/mm, using a simple Ocular substitutes shooting film

> Check different off-axis distances at different apertures

> A cheap ocular is only good in the center, or has curvature, it will work but one has to be aware of that, not difficult to understand that...

> Off-axis we have to tilt-swing the ocular to align it in the rays direction.

> Use a resolution target to have a quantitative reference http://www.takinami.com/yoshihiko/ph..._test/USAF.pdf

I'm pretty confident that the x30-x50 ocular is a very good way to see what happens with the shimming, a x30 or x50 enlargement in LF is a lot, I'm pretty sure that what we don't see with the ocular it's impossible to see it in a print, and it's beyond lens and film performance... so an easy/fast way to do it.



Field curvature

The inspection with the Ocular won't account for field curvature. My guess is that shimming won't modify much the field curvature, but the aberrations correction, anyway after adjusting a shimming with an ocular it would be great to spend some sheets on a resolution target to see the practical difference.

[Tin Can]

Good useful summary, Pere.

I recall a recent LFPF post by I think Bernice that allowed that machining was always done on a shutter and never on the lens halves to make up for too much distance.

It may have been Mark talking about Kodak lenses.

The desire was to make switching to a new shutter easier.

[Leigh]

The thickness of Copal shutters is as follows:
#0 - 20mm
#1 - 20mm
#3 - 28.6mm

The tolerance on all of those is ±0.025mm (0.001 inch), which is pretty tight.

This info from the respective Copal instruction manuals.

- Leigh

[Tin Can]

Good data Leigh!

[Pere Casals]

The tolerance on all of those is ±0.025mm (0.001 inch), which is pretty tight.

Leigh, yes, "but anyhow measured shutters show that practical manufacturing tolerances are much smaller than specs" https://www.largeformatphotography.info/shutters.html

Probably 0.05mm won't make much a difference, the APO Ronar adjustment for long distance may be around 2mm (it looks)... We can guess that newer shutters may have a more accurate manufacturing than the older ones.

[Bernice Loui]

So tell us all Pere...

*How many lenses have you design that have gone into high volume production?

*What kind of optical test instrumentation have you used and experience with?

*What is your real world machine tool experience with making machined parts?

*How many machined parts have you made, what kind of machine tools are you qualified and skilled at using?


Bernice

[Bernice Loui]

O.001" is typical of optical world tolerances. Any more will NOT yield production consistency. As to older shutters being lesser tolerance, they are NOT. Having measured un-molested Kodak Ilex, Compur and other older shutters they are remarkably accurate and close toleranced. Fact is, optical folks understood well what the mechanical demands were.


Bernice

The thickness of Copal shutters is as follows:
#0 - 20mm
#1 - 20mm
#3 - 28.6mm

The tolerance on all of those is ±0.025mm (0.001 inch), which is pretty tight.

This info from the respective Copal instruction manuals.

- Leigh

[Leigh]

O.001" is typical of optical world tolerances. Any more will NOT yield production consistency. As to older shutters being lesser tolerance, they are NOT. Having measured un-molested Kodak Ilex, Compur and other older shutters they are remarkably accurate and close toleranced. Fact is, optical folks understood well what the mechanical demands were.

Hi Bernice,

We've been making precision components and assemblies for well over a century.
It's surprisingly easy, even using good manual (not computerized) equipment, depending mostly on mind-set.

I personally am a Tool and Die Maker with more than 50 years' experience in that trade, doing precision work.
I commonly work with tolerances less than .001 inch.

- Leigh

[Bernice Loui]

Have a 13" lathe and RF45 in the garage with needed tooling, cutting tools and measuring tools from Etalon or Mitutoyo or Kafer (German) or Compac (swiss). At some point these machine tools will be upgrades to a Mori Seiki MS850 or HL460 and Deckel FKP_?

For those who are skilled using good tools, 0.001" is not difficult. Problem is, most folks have no idea the physical aspects of what 0.001" is.
Remember what it was like to learn how to run a lathe or mill trying to make any part?

This is not my day job which has more to do with designing stuff by numbers and similar. The machine tool stuff is a "hobby". As for designing optics, cranking out the math and numbers related to a design is only one aspect, actually producing a real world lens is an entirely different ordeal that I'm not in any way qualified to attempt any lens or optics design even if the math-calculations are not difficult to do.

My machine tool and machine parts history is making microwave bits where 0.0001" is common. Taught how to do machine work by the proto type machinist decades ago initially on a Hardinge HLV and Bridgeport mill. He got tired of me designing parts that are not easy to make and not understanding paper designs -vs- how parts are actually made.


Bernice

Hi Bernice,

We've been making precision components and assemblies for well over a century.
It's surprisingly easy, even using good manual (not computerized) equipment, depending mostly on mind-set.

I personally am a Tool and Die Maker with more than 50 years' experience in that trade, doing precision work.
I commonly work with tolerances less than .001 inch.

- Leigh