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Message |
    
Zcetrt
Tinkerer
Username: Zcetrt
Post Number: 1
Registered: 08-2012
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, August 10, 2012 - 10:29 am: | 
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Hi to all.
I have a Pentacon 29mm F/2.8 MC Auto lens that is clearly decentered. The lens is very soft on left side while the performance in the center and on the right side is great.
I came across brief mentions that Pentacon lenses have screws to calibrate the rear element. However, I haven't seen any additional info for the 29mm Pentacon lens.
Does anybody know this lens? Is it possible to calibrate it? Is it possible to get service manual or instructions somewhere?
The lens is otherwise in superb condition.
Thank you for answering my very first post in this great community in advance,
Ziga |
Br1078lum
Tinkerer
Username: Br1078lum
Post Number: 353
Registered: 11-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, August 10, 2012 - 09:11 pm: |
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Is this the only lens on your camera that is 'decentered'? Have you tried others to see if maybe the problem is in the camera mount, such as a shim that was not replaced when it was last serviced?
PF |
Zcetrt
Tinkerer
Username: Zcetrt
Post Number: 2
Registered: 08-2012
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, August 11, 2012 - 07:54 am: |
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Thank you for your answer.
I tried the Pentacon 135mm F/2.8, Mir-1V 37mm F/2.8, Pentacon 200mm F/4 and CZ Jena 50mm F/2.8 with the same M42-Nikon converter and all other lenses perform great. With the Pentacon 29mm it is very clear that the left side is unsharp. Furthermore I tested if anything changes with focusing to other distances but it doesn't have any influence (I think the elements in the lens does not rotate during focusing).
I own Praktica camera as well but I didn't shoot film with particular lens yet since I am pretty sure that the performance will not improve when mounted to film camera. |
Glenn
Tinkerer
Username: Glenn
Post Number: 1040
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, August 12, 2012 - 06:32 am: |
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Depending when this lens was constructed, I think you will find that this is more a 'quality control' issue rather than a calibration mistake. You could try stripping out the optics, carefully cleaning all element location surfaces and then reassembling the optics. Unless you have access to proper test equipment, centering/calibrating this lens will not be easy. You could try a professional repair shop, but personally I would replace the lens and not waste time or money on it. |
Zcetrt
Tinkerer
Username: Zcetrt
Post Number: 3
Registered: 08-2012
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, August 13, 2012 - 12:45 pm: |
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Thank you, Glenn. I kind of hope that it was "miscalibrated" later. The infinity focus is a bit off as well. If somebody opened the lens and tightened all the screws (including the "potential" calibration ones), he could spoiled the performance.
So nobody is aware of calibration capabilities of this lens? Or perhaps a service manual? |
Michael_linn
Tinkerer
Username: Michael_linn
Post Number: 69
Registered: 04-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, August 13, 2012 - 11:49 pm: |
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It's likely a Meyer design, and if so the rear lens group is probably one unit held in place and centered by three tiny set screws. Once you remove the back you will know if this is the case. If so, check to see if the rear lens unit is loose.
Starting from scratch, you can back the three screws out. Try screwing them all in as equally as possible, counting the turns from when the screw "bites" the threads and try to get them all at about the same point with the rear unit being locked in place. Then check the centering by test. Without a centering microscope, it's just going to be trial and error, but at least you will not be waiting on or wasting film.
Good Luck! |
Zcetrt
Tinkerer
Username: Zcetrt
Post Number: 4
Registered: 08-2012
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, August 14, 2012 - 06:32 am: |
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I looked it up and yes, it seems that it is Meyer design. Thank you for your advice, Michael. I will try to calibrate it with greater confidence now. |
