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Lens calibration help.

chris77

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Jan 11, 2015
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705
Location
Paris
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Medium Format
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
 
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.
 
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.
 
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.
Click to expand...
.

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.
 
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.
 

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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
 
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
 
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