2 lenses in one group always cemented together?

kuuan · Expire: 2014-12-26

I have a Pentax M f1.4/50mm which I believe is considered to be a Planar design, thus having 6 lenses in 4 groups (1-2 | 2-1).

The lens has a strong fog covering about half of the surface of one of the inner rear elements. After removing the single rear element I saw that the haze is within the second element of two glasses housed together. I succeeded in opening it and the 2 classes came apart. I tried to clean the haze with lens cleaning fluid but it does not come off.

this makes me wonder:

Aren't these two glasses, if a Planar or even Ultron design, supposed to be cemented together?
If originally they had been cemented together the cementing must have become that bad that the lenses separated by themselves, and the haze would be the remnants of the cement. Is that possible?
Or could two lenses in one group simply be fit snug together in one housing, without cementing?

I thought to try to clean the haze off with acetone. If success then simply to reassemble it, without cementing. What would I have to expect?

curious about your thoughts,

cheers,
Andreas

visualopsins · Expire: 2025-04-11

Iirc, the cement has an index of refraction; different cements have different indices of refraction. Thus I would expect some change in optical formula without cement.

kuuan · Expire: 2014-12-26

hm, ok, thank's serious..ah visualopsins

I had found this photo and inspecting it closer makes me think that these two lenses originally may not be cemented together :
http://www.flickr.com/photos/pangestu-bdg/4218948866/

is it possible that two lenses in one group are not cemented together, just tightly placed together within one common housing?
if so I wonder what the haze on my lens was, and how I could get it off, if at all. Aceton best bet?

visualopsins · Expire: 2025-04-11

I don't know; seems reasonable, except where surfaces meet moire can be produced. Use of 'ANR' glass inserts for scanning film, for example.

Here's info/source for lens cements/solvents: http://www.optical-cement.com/

kds315* · Expire: 2021-03-09

Andreas, that 6 lens in 4 groups design makes it clear that two lenses are cemented. If that cement is deteriorated (or fungus has grown into the cement - it was organic canada balsam years ago), teh only was is to cook that lens group in some strong and usually dangerous (healthwise) solvents until the two lenses seperate. Then they need to be cleaned and re-cemented with ideally the same cement which was used once because that refractive index has been part of the lens design. Doing so means having the ability to work with an autocollimator to position the elements correctly and then let the cement harden. All quite some effort and not to be done on the kitchen table. With all due respect - finding a new such lens is much easier and cheaper!!

kuuan · Expire: 2014-12-26

thank you Klaus for your explanations.

understand that this lens is nothing more but an ideal guinea pig to enjoy and to learn repairing lenses. It had come very cheap in a set with some others, and if I cannot rescue it I would not look for another, good copy. It already, surprisingly, made me interested in the various lens designs.

I do not have to separate the two lenses, they are already apart! Looking at the photo I linked above there these same lenses look already separated too. Which now makes me suspect that, contrary to reports, this lens is not a Planar after all, resp. that it is a Planar but that two lenses in one group do not necessarily have to be cemented together, but in this case are simply housed tightly together. Could this be true?

I would love to get the haze off. Maybe the glass is corroded and the haze cannot be removed, anyway I want to give it a try. Lens cleaning fluid does not do anything, I wonder which would be best for cleaning. The same acetone which is recommended to clean off residues of cement of separated, originally cemented together lenses?

If success I would simply reassemble it and see what it will do. I won't try to go through re-cementing, but as mentioned now I suspect they never had been cemented in the first place!

kuuan · Expire: 2014-12-26

oh, now looking at the lens scheme

( taken from http://www.luminous-landscape.com/columns/sm-02-11-24.shtml )

Now I understand what's going on here:

First I had unscrewed to very rear element which comes in his own housing, than had opened the following housing and a second lens came off.
3 already had come off the front, erroneously thinking that the total was 6 I assumed that there was only one more single element left.
As it remained inside the main body of the lens I could not judge it's thickness.

Looking at the scheme I understand now that there are actually 7 elements and that therefore there are 2 more remaining, and these are cemented together.
As the fault of my lens must be in that cementing I won't be able to repair it.

in the luminous landscape article btw., exactly about these two elements, it is said:
note the cemented concave and convex surfaces between the fourth and fifth elements.
Modern lensmakers shave costs by making these cemented surfaces flat.

cheers,
Andreas