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Aspherical rotational considerations during repair
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PostPosted: Sat Mar 02, 2013 2:12 pm    Post subject: Aspherical rotational considerations during repair Reply with quote
Are aspheric (aspherical) lens elements perfectly symmetrical about the lens axis?

Or

When dismantling MF lenses having one or more aspherical elements, must the repairer pay heed to the rotational relationship of the aspherical element(s), as set at the factory, to one another and to the other lens elements?


PostPosted: Thu Mar 07, 2013 11:06 pm    Post subject: Re: Aspherical rotational considerations during repair Reply with quote
guardian wrote:
Are aspheric (aspherical) lens elements perfectly symmetrical about the lens axis?


Yes they are (unless the lens is anamorphic)


PostPosted: Fri Mar 08, 2013 3:36 am    Post subject: Re: Aspherical rotational considerations during repair Reply with quote
SXR_Mark wrote:
guardian wrote:
Are aspheric (aspherical) lens elements perfectly symmetrical about the lens axis?


Yes they are (unless the lens is anamorphic)


I'm most grateful for the reply. Thank you.

OK, I'm obviously pretty stupid about this stuff. I looked up anamorphic. I think that's not generally a consideration when it comes to the older MF camera lenses that concern most participants here. Anamorphic seems to be more a movie camera lens thing.

So sticking strictly with MF still camera lenses, my repair concern involves need to hold constant post-repair relative rotational locations (as set at the factory) of lens elements of any MF aspherical lens I might be repairing. For me, anyway, it's difficult to accomplish when I'm fighting any number of other tough dis-assembly/assembly challenges.

My question for repairers of MF aspherical lenses:

Is this something one needs to worry about??

Presumably, if the (aspherical) lenses are symmetrical about the optical axis, then relative rotational positions don't matter?? Question


ps

By "MF aspherical lens" I mean any MF still camera lens containing at least one aspherical element.

Last edited by guardian on Fri Mar 08, 2013 12:10 pm; edited 1 time in total


PostPosted: Fri Mar 08, 2013 6:42 am    Post subject: Reply with quote
An aspherical element is just what its name suggests: it is not a section of a sphere. It might be parabolic or have more complex shapes, but in any case, it will be symmetrical about its center axis. As Mark mentioned, an anamorphic lens has properties that require correct orientation. Interestingly, anyone who has astigmatism in one or both eyes has as part of their optical prescription a corrective feature in which the lens takes on more of a cylindrical orientation, so this too is an example of correct orientation being necessary.

It seems to me that what you're concerned about is collimation. I've dismantled and reassembled quite a few lenses over the years and when I first started doing it, I wondered if lenses needed to be recollimated after disassembly. But then I got to looking at the way they were engineered. And in every case, when it came time to relocate the glass back into the lens barrel or assembly there was only one way they were gonna fit back into the recesses they came out of. There was just no possibility of misalignment. I've never run into a situation where there was any slop or wiggle room for the lens elements, thus no need for collimation. In fact, I don't even see how collimation can be done with the lenses I've dismantled. There was never anything adjustable that would allow for it. Proof being in the pudding and all, I've never noticed anything amiss optically with any of the lenses I've repaired.

The one thing I found I had to watch out for was, with some elements, I had to make sure that I didn't flip them around when reassembling them. Keeping dust out of the interior and fingerprints off the glass is a bigger concern for me than accidental misalignment of elements.


PostPosted: Fri Mar 08, 2013 12:08 pm    Post subject: Reply with quote
cooltouch wrote:
An aspherical element is just what its name suggests: it is not a section of a sphere. It might be parabolic or have more complex shapes, but in any case, it will be symmetrical about its center axis. As Mark mentioned, an anamorphic lens has properties that require correct orientation. Interestingly, anyone who has astigmatism in one or both eyes has as part of their optical prescription a corrective feature in which the lens takes on more of a cylindrical orientation, so this too is an example of correct orientation being necessary.

It seems to me that what you're concerned about is collimation. I've dismantled and reassembled quite a few lenses over the years and when I first started doing it, I wondered if lenses needed to be recollimated after disassembly. But then I got to looking at the way they were engineered. And in every case, when it came time to relocate the glass back into the lens barrel or assembly there was only one way they were gonna fit back into the recesses they came out of. There was just no possibility of misalignment. I've never run into a situation where there was any slop or wiggle room for the lens elements, thus no need for collimation. In fact, I don't even see how collimation can be done with the lenses I've dismantled. There was never anything adjustable that would allow for it. Proof being in the pudding and all, I've never noticed anything amiss optically with any of the lenses I've repaired.

The one thing I found I had to watch out for was, with some elements, I had to make sure that I didn't flip them around when reassembling them. Keeping dust out of the interior and fingerprints off the glass is a bigger concern for me than accidental misalignment of elements.


Thank you for your excellent and helpful post.

Yes, I quite agree with you regarding the need to be watchful regarding flipping and as well the recesses, etc.. Your use of the term "collimation" in the latter context was helpful to me. By "collimation" you appear to be referring to need to avoid tilting any lens element such that its optical axis no longer aligns with the optical axis of the entire assembly. On reflection, I think that's right. Prior, I was (I now believe wrongly) associating collimation with element rotational considerations.

Aside from any tilting, there is also the consideration, for any individual lens element, that its optical axis after re-assembly would end up parallel to the optical axis of the entire assembly, but not on that axis. This would seem to me also not to be a concern for the repairer.

I'm very much a student of these things. Sometimes the learning comes frustratingly slowly. Sad

Still, your post has given me hope my earlier concerns (regarding rotation of aspherical elements) are without foundation.


PostPosted: Fri Mar 08, 2013 10:43 pm    Post subject: Reply with quote
cooltouch wrote:
.......... But then I got to looking at the way they were engineered. And in every case, when it came time to relocate the glass back into the lens barrel or assembly there was only one way they were gonna fit back into the recesses they came out of. There was just no possibility of misalignment. I've never run into a situation where there was any slop or wiggle room for the lens elements, thus no need for collimation. ......


Broadly speaking I agree with this. Mass produced lenses are engineered to be assembled without the need for individual alignment. They would be too expensive if each one had to have the optical elements individually aligned. The pieces of glass and the metal rings they fit into should be manufactured to a tolerance that renders such adjustment unnecessary. Of course, there is always some variation in manufacturing tolerance and this is why some lenses are better than others. An important part of the optical design is to make the lens work well with a realistic range of manufacturing tolerance. This may require compromising ultimate performance. Zeiss and Leica will work to tighter tolerances allowing the use of designs that are less forgiving but give ultimately better performance. They probably also check every lens rather than just a few at random in every batch.

With mass production when only random samples are checked, a rogue lens with very bad performance can slip through quality control. This could be manifest as generally soft results if the distance between the lens elements is incorrect or images that are soft on one side or in one corner if a lens element is tilted or not correctly centred.

Most lenses I have disassembled do not allow for any significant misalignment when reassembling. There are however exceptions. An example is the Meyer Lydith 30mm f3.5 (and its Pentacon equivalent). There are two issue with this lens. The first is the rear element which is in a small carrier ring that has radial adjustment to centre it on the optical axis. The other issue is that all the lens elements located before the iris are in a carrier that is not very well located in the lens body. Thus, it is quite easy to clamp this carrier and twist its axis relative to the rear element and also change the relative separation. I have had two Lydiths and one Pentacon and they all show decentring to an extent, giving noticeably softer results on one edge. I experimented for quite some time loosening and retightening the front section of one of the lydiths and could see the edge performance change very significantly. I guess this might be why the rear element has the radial adjustment.

Mark


PostPosted: Sat Mar 09, 2013 11:46 am    Post subject: Reply with quote
Thank you SXR_Mark for that example with the Meyer Lydith!
On all the old manual lenses I have repaired yet I found no alignment screws or whatever for centering or such.
I think of the slaughtered AF lenses I saw probable adjustment devices.

On the Samyang 14mm f/2.8 I am not sure about adjustment. It seems that this lens is a damn thing for repair. For my understanding the register distance and parallelism together is located with three screws in the barrel. It seems this can be proper adjusted with a measurement machine and device that hold the front and back correct adjusted until screws are fixed.
Need to open it again to learn and understand a bit more.
I have repaired and converted a lot of lenses, but this lens I can not repair at the moment.

Here I have found an example of a lens that can be centered, and additional with distance adjustment in the same lens - Canon EF 24-70mm f/2.8L: http://www.lensrentals.com/blog/2012/07/optically-adjusting-a-lens
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