Can I mess up the lens calibration by dismantling it?

ovim · Expire: 2015-01-28

Over at Pentax forums I mentioned dismantling a 1.4/50mm S-M-C Takumar and a member told me the following:

"when you dismantle a lens, you'll not be able to put them exactly calibrated as before... you'll need a special laser device to align them perfectly"

Is this really true? I'm perfectly happy with my repaired and de-yellowed Takumar and compared it against my near-mint Pentax SMC-A 1.7/50 and couldn't find any really noticeable differences.

peterqd · Expire: 2014-01-04

This is just my own opinion, but I feel with a "spherical" lens (as opposed to "aspherical") there should be no misalignment issues as long as the glass is properly seated squarely on the shoulder. If there was a need to calibrate and adjust the glass then I feel there would probably be some means of doing this, with shims for instance, but I've never seen anything of that sort. I've never dismantled an aspherical lens but it seems more likely these would need to be accurately aligned.

By the way, it's possible to unscrew either the front or rear lens blocks on the Tak and get to the diaphragm without affecting the alignment of any of the elements. You only need to remove individual elements if you need to clean the internal faces.

Spotmatic · Joined: 18 Aug 2008 Posts: 4045 Location: Netherlands

What peterqd says. This is only applicable to modern aspherical lenses with floating elements etc.

ovim · Expire: 2015-01-28

That's what I thought. Thanks!

rick_oleson · Posted: Sat Mar 14, 2009 4:49 pm Post subject:

The fellow on the Pentax forum might be correct: each element has an optical axis, and they must all be perfectly aligned in order for the lens to be perfectly centered. The accuracy of grinding and the closeness of fit in the cell barrel limits the amount of error that is possible, but it cannot ensure 100% perfect centering. Having said that, neither does the manufacturer provide 100% perfection, everything has a tolerance. I would expect any modern manufacturing system to deal with this issue by making the grinding and the barrel fits extremely accurate, it's a much more efficient manufacturing approach than hand-adjusting each assembly.

I don't think there is much, if any, difference between a spherical and an aspheric, or between a lens with fixed or floating elements, in this respect: centering is basic to any optical system.

SkedAddled · Expire: 2021-08-12

rick_oleson · Posted: Sun Mar 15, 2009 1:47 am Post subject:

A floating element is one that changes its distance from the other elements during focusing. Most lenses focus as a unit, but in some, particularly macro lenses, one element is on a separate helical that moves it differently from the others to provide better correction across the entire focusing range. The Olympus 50/3.5 Macro and the 55/2.8 Micro Nikkor both have floating elements.

SkedAddled · Expire: 2021-08-12

Rick, thank you.
I think I understand, though I'm not certain.
Can you point me to some reference which illustrates your explanation? I am so far unable to locate anything.

rick_oleson · Posted: Sun Mar 15, 2009 2:29 am Post subject:

I don't see anything at the moment ... there is a mention of the floating element of the Micro Nikkor at http://www.mir.com.my/rb/photography/companies/nikon/nikkoresources/micronikkor/55mmmicro.htm , and the OM at http://www.mir.com.my/rb/photography/hardwares/classics/olympusom1n2/shared/zuiko/htmls/macrozuikoC.htm ... the OM page has an optical diagram of the lens but it doesn't point out which element is the floater. Looking at my OM macro lens, I think it's the second element from the rear, this one is on a cam so that it rotates slightly as the lens focuses. The floating element in the 55/2.8 Micro Nikkor is in front of the diaphragm, and does not rotate but has its own double helical separate from the focusing helical.