Lens calibration help.

chris77 · Feb 16, 2021

Hello Apug
After disassembling cleaning and reassembling of a super takumar 50mm (8 elements) I realise that I have overly tightened the front group and therefore involuntarily changed the factory set distance between the groups.

So I am wondering, in order to find the maximum sharpness, without the help of a digital body which I do not own anyway, is there a simple way to find the sweet spot ?

Try and error?

Cheers,
Chris

monopix · Feb 16, 2021

How do you know you've over tightened it? As, however it is you know that seems to be a clue as to how you would find out the correct tightness.

Donald Qualls · Feb 16, 2021

I'd expect to see a change in focal length if the distance between groups has changed -- so if you know the original focal length, to the fraction of a millimeter, getting back to that figure ought to put you back in spec.

chris77 · Feb 16, 2021

I will post pix tomorrow, so someone interested can take a look.
What happened as a result of bringing the two groups slightly closer together is that sharp focus became impossible.
Still kind of sharp but not really.
I took time today to unscrew the rear group slightly (can be done without opening the lens) and I think I found perfect focus. So I will open the lens tomorrow again when I'm in my studio.

shutterfinger · Feb 16, 2021

This link https://takumarguide.weebly.com/1--14--50-358-8-element.html has a diagram for your lens.
You likely have a element reversed or not seated properly in the barrel.

E. von Hoegh · Feb 16, 2021

shutterfinger said:
This link https://takumarguide.weebly.com/1--14--50-358-8-element.html has a diagram for your lens.
You likely have a element reversed or not seated properly in the barrel.

Took the words out of my mouth.

E. von Hoegh · Feb 16, 2021

monopix said:
How do you know you've over tightened it? As, however it is you know that seems to be a clue as to how you would find out the correct tightness.

.

Lens elements seat on hard metal seats. Overtightening? leads to cracks if done hard enough, strain in the glass if moderate.
You have something improperly inserted.

chris77 · Feb 17, 2021

Hello
Here some pix to explain.
No, the lens elements are in correct position.
But when screwing back the front group the aluminium barrel moved, I didn't know this has a thread as well. This way the two groups were brought together a tiny bit closer.
So, I did an extensive test today, unscrewing the front group bit by bit. Did this with film, so the results will tell.

chris77 · Feb 17, 2021

chris77 · Feb 17, 2021

Another explanations would be, that this lens was never sharper and I just realise it after working with pancolar for a while.

It seems though focussing on a bar code from like 1.2 meters away I did see some amelioration when unscrewing the front group a tiny bit. My left eye is almost perfect for sharp vision, but still, it remains difficult to judge. The developed negs under a loupe will tell. Keep u posted

shutterfinger · Feb 17, 2021

I downloaded and printed at the printers highest resolution the lens test targets at http://www.normankoren.com/Tutorials/MTF5.html
I made several copies and attached them to a 24x36 artist canvas running opposing directions so I have the fine and coarse lines next to each other. The target has to parallel to the film/image plane of the camera. I use it to calibrate rangefinders and test focusing.
Search engine Lens Test Targets to get a wide selection of targets for free download or purchase.

chris77 · Feb 18, 2021

shutterfinger said:
I downloaded and printed at the printers highest resolution the lens test targets at http://www.normankoren.com/Tutorials/MTF5.html
I made several copies and attached them to a 24x36 artist canvas running opposing directions so I have the fine and coarse lines next to each other. The target has to parallel to the film/image plane of the camera. I use it to calibrate rangefinders and test focusing.
Search engine Lens Test Targets to get a wide selection of targets for free download or purchase.

Thank you! You are very helpful.
Good day

Lens calibration help.

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