Rodenstock 150 Apo-Sironar-S lens shims

[Dave Jeffery]

I needed to replace the shims that were lost in a Rodenstock 150 Apo-Sironar-S lens. Another dirty grey market lens unfortunately. In the Rodenstocks lens specification publication the overall length of the lens is listed as 51.5 mm and it looked like the lens elements would be precise at that length based on my interpretation of the lens design diagram, so just for fun I fabricated shims that made the overall length of the lens to that length. I'm just guessing that this might work.

A few questions
1. Should this lens be at it's optimal sharpness based on meeting the overall length specifications, or is more testing needed?
2 What specifications do people that mount lenses in shutters refer to in order to get the correct critical lens spacing?
3. My plan was to download and shoot a resolution graph to test the lens if there is still a need based on the responses to the above questions. Are there any tricks or procedures that simplify this task?
4. Any other recommendations for shimming the lens to the optimal focus other than my random guesses listed above.

TIA

[Peter K]

Dave,

the aligment of the distance front element to diaphragm with shims isn't for the resolution in the center but in the outer area of the image circle.

As you can see here (http://www.largeformatphotography.info/forum/showthread.php?t=38401&page=2) one needs no resolution graph but only some twigs or leafs in the distance.

Peter

[GPS]

If your only reason to put some shims in the lens is that it doesn't have the stated 51.5mm lens length I would not change anything. After all the lens length is just informative value. Were it always 51.5mm then there would not be a need for shims in these lenses as it is easy to make all lenses with this length precision.
If you don't find any problem with sharpness take the lens as it is.

[Dave Jeffery]

From

http://www.largeformatphotography.info/forum/showthread.php?t=14422

"We also discussed swapping shutters in general. He confirmed the impression which was my own bottomline from discussions on this forum, namely that lens spacing is absolutely critical for modern/wide-angle lenses, and less so for older/narrower lenses"

Thanks for the links! I'll try and adjust the lens as suggested as I have a variety of thicknesses of shims to work with and if I can't get it dialed in I'll send it to the factory for adjustment.

Thanks again!

[GPS]

Dave,
lenses that are provided with shims have them not to get the overall specified lens length correctly (to the nominal value) but to get the lens cells spaced correctly. Those two things are not the same.
The lens cells can have slightly different focal lengths than what is their nominal value. When that is found (measuring with optical devices) the lens cells get their appropriate shims to make up for this difference. In such a case the overall lens length is different too and does not correspond exactly to the cited value.
It's the same with lens focal lengths - they have slight variations and therefore you cannot put them all at the same focal length when focused at infinity. You could measure differences going even up to 1mm if not more.

[IanG]

Razzledog (the big not little man) from Australia found that a problem with 90mm f8 Angulons and sharpness was to do with lens cell spacing, The poorer ones were spaced further apart, which makes sense as the space should increase for lenses designed for macro work, and would be detrimental for normal use.

The problem with the Angulon is the shutter and the way it spaces the cells, Mr Razzledog :D machined the shutter/lens mounts to compensate - before that sounds alarming outside the US a CLA etc means a total complete disassembly/strip down & rebuild so the casting would be devoid of other parts while being machined.

Lens manufacturers can control their tolerances to extremes, but shutter manufacturers don'tb work to the same tolerances.

Ian



so lens manufacturers now use shims where necessary to compensate.

[Bob Salomon]

Razzledog (the big not little man) from Australia found that a problem with 90mm f8 Angulons and sharpness.

But not with a 90mm f8 Angulon!

[Dave Jeffery]

Thanks everyone and special thanks to Peter K for the advice to use shift to check the focus at the outer edges of the lens as mentioned in this link from Peter's earlier post

http://www.largeformatphotography.info/forum/showthread.php?t=38401&page=2

Mr Wenzel at Rodenstock was kind enough to tell me that the standard shim for the 150 Sironar-S is .3 mm. I had the option to use .31mm so I tried that shim thickness first thinking that this may set the focus closer to infinity. Using .31 the lens was very sharp in the center but not so when the standards were shifted, and rotating the lens increasing the gap did not improve the focus. Next I used the .3 mm shims and the focus was sharp at the center and after making the shifts it was sharp at the edges as well. Shooting a test target to double check the lens is next.

The Rodenstock factory will shim a lens for 95 Euro or $140 US. The set of arbor shims cost $16 (Precision Brand Products, St Downers Grove, IL.), and some grinding and finishing is needed so if your time is worth anything then it may be better to send the lens to the factory.

The reason I lost the shim was the grey market lens had some dust inside and I opened it up outdoors to clean it and the shim slipped through my fingers. It was still my fault that I lost the shim but in retrospect I wish I had bought the lens from a factory authorized dealer like Bob Salomon.

Now that's taken care of it's time to go and chase the leaves changing color.

Thanks again!

[GPS]

Sorry Dave, but something is not correct in your fight for sharpness. If a shim is 0.3mm or 0.31mm (0.01mm difference) it cannot have such a dramatical effect on sharpness as you say. The 0.01mm difference you can easily get just by screwing the lens cells harder to the shutter ring...

[Dave Jeffery]

Your idea makes sense GPS.

The center of the lens was very sharp the first time I checked it. I may have also tightened the lens down more after swapping the shims but whatever happened the edges were sharp with the shift applied second time around.

I'm just starting to look into shooting a test target to check the lens to see if it really is set properly. Any thoughts on a good testing method?

Time to get out the tourqe wrench : ) I will pay more attention to the amount of force used to tighten the lens elements though.

Thanks for you feedback.

[GPS]

Dave, to be sure we understand each other - I meant that between a 0.30mm shim and a 0.31mm shim there is no difference as it is impossible to keep the distance between the lens cells with such precision. The 0.01mm is easily lost in the process of screwing the cells. There must be at least 0.05mm difference to be able to speak about some constant effect on the assembled lens.
Now, if I understand it well, you know only one correct shim thickness, the 0.30mm.
There is not any way how you could find the other correct value (either for the infinity sharpness or for the close-up setting) by testing it yourself - either on your gg or with pictures. The reasons are practical ones - on the gg, you cannot see the pictures side by side and by changing the cells spacing you could never "remember" how sharp it was exactly before and after. With pictures you could see it but you would need to take too many to get to the correct spacing value and the whole exercise would be futile because the final picture would be influenced by many other factors. You would get just an approximate value.
The only intelligent way to know is to send the lens to the factory - there they do it with an optical device in a precise way.

[Dave Jeffery]

I took what you said to heart and decided to make more shims and do more testing. As a result I would bet that I can easily visually detect a .05mm difference in the thickness of a shim for the front element of this lens, after focusing it in the center and applying shift as in acordance with the test procedure that Peter K provided the link to. I also believe that without knowing the correct focus point I could change the lens spacing closer to the correct focus of this lens using the combination of magnifiers that were used today. I was focusing on pine needles about 90 yards away so perhaps this is not a critical enough test?

Can I get set the lens as sharp as the factory? Perhaps not but I don't mind trying. The people at Rodenstock provided me with the thickness of the shim and based on my visual testing of shims from .28mm .29mm .30mm .31mm and .32mm, .30 is the sharpest shim for the front lens spacing, which is exactley what the manufacturer of this lens said it would be.

This is only a four element lens and according to Kerry Thalman's review it is one of the sharpest made so maybe it does focus easier than a typical lens?

I just got finished messing around with the lens shim thickness and next I would like to find out how to test this lens in the best possible means so I can eat crow if I'm wrong. I have some great crow recipies having been wrong so many times before so I'll still not rule out the fact that I need to send it to the factory.

I do appreciate your feedback and I did a lot of testing today based on what you posted earlier. Hopefully it is set correctly as the 24" pipe wrench that I'm using on the lens barrel is really chewing it up badly : )

[Bob Salomon]

If it is a four element then it is not an Apo Sironar S!!!

[GPS]

Dave, enjoy...;-)

[Dave Jeffery]

I stand corrected it is a six element lens. If there is anything else you can correct me on please do so.

[Dave Jeffery]

GPS - I appreciate your input and the lens will be tested.

[ic-racer]

My experience with shimming is that it alters flatness of field. It does nothing for the central rays. So, shim based on your intended use. If you want it flat at infinity optimize for that. If you are copying artwork, then you would shim differently. Mounting the lens on a camera where the whole image circle (or most of it) is projected can make testing easier.

[Dave Jeffery]

Thanks,
I was replacing a shim that was lost and I shoot landscapes so focusing at a distance seemed to make sense.
Even knowing what the shim thickness was supposed to be it took about four and a half hours of fousing to find the sweet spot to make sure it was correct. This entailed focusing at the center and shifting about 70 times, and swapping shims and rotating the lens in small increments probably close to 1000 times to find the sharpest focus on the edge. My eyesight is great and I had multiple magnifiers and it was not simple and straghtforward at all.

I think if a person's time is worth anything they should send a lens in for shimming.