[Darkover]

Today I was out for some pictures with me good old Pentax-M 1:4 100mm Macro.

Before I took a picture I found that the lens had very low contrast. I looks like fog.
All picture I took with this lens are garbagge!

At home I examine the lens carefully, oben it, clean the front lens, because I thought there was fungus
on the lens. But this was not the reason! The front lens depends on two lens glued together.
And the canada balsam between the lesen become bad!

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It is not so easy to take a picture from this to show you.

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On the lens it is only visible with light in very special direction! It is easy for me to took a picture where you can not see it.

The last time I used this lens 6month ago it was okay. So it happen quickly when it happen. I don't think the lens becomes to hot. It did not stay in my car for example.

I check all my other lens and found another one. The old 200mm/4. With this lens it happens at the back side and it is
smaller, only at the border of the lens.

I search at ebay for the 100mm and found one with the same problem. So check the lens carefully!

I think I will now seperate the two lens, clean it and glue it again. Not an easy task of course!

Olaf

[mkgd1]

I'll check mine now.

[Pentaxrookie]

Thanks for this info. Good to know.

[marcusBMG]

That's unfortunate. I haven't found any good account of how this happens or what might prompt it. there are lenses which are known to be prone to this happening, the sigma apo 400mm f5.6 is a case in point.

Do let us know how you get on with any diy. There are some pointers here:

Asahi Pentax-M 50mm f1.4 Lens Haze with Balsam Separation Suspected, Repairing... - PentaxForums.com

Old Lenses can be restored by Re-cementing « SKGrimes

Re-cementing doublet elements with Canadian balsam

https://www.optical-cement.com/cements/manual/manual.html

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Last edited by marcusBMG; 08-02-2020 at 09:26 AM.

[pentaxus]

The 100mm macro seems especially prone to that as in I've seen it three times.

[bilybianca]

And mine has a spot that I first thougt was from a droplet drying on the front lens leaving a residual, but then understood was a beginning lens separation. This doesn't affect the IQ a lot, and can be fixed by an experienced repairman (Mighty Mike?). But it wouldn't be worth the cost, I've got a 100mm DFA macro that leaves the M lens in the dust anyway.

Kjell

[Darkover]

I try now to find glue. Not so easy.
I think canada balsam is history for 50year. It is hard to believe that it was used by Pentax. Even the glue that was used after canada balsam is obsolete since 1980 because there was PCB inside.
Now it is common to use UV activated epoxy glue. I think I will try it. But the two lens has to be glued very exactly. I think better than 1/10mm because there is very less space in the mounting hole.

Olaf

---------- Post added 03-08-20 at 09:43 ----------

Oh, and my first thougt was to buy
the same lens at ebay.
But at the picture from ebay you can see that the lens had the same problem. So I agree, it looks that this problem happen no often to the old 100mm. What a pity! It is a nice lens.

[Praktica*ist]

Hello.
This is unfortunate, but alas a quite common issue on the Pentax 100 mm f/4 macro lens.
I have seen countless of them with the exact same issue sold "as is" on ebay.
This haze issue can also be present on other optical formulas of the same period.

It is a degradation of the cement inside a lens doublet.
By that time, I suspect the doublets in this lens generation were built using optical epoxy glue or UV-curing glue.
Canada balsam is a very viscous fluid, that remains pliable and does not fails like this.
If you look at the Takumar lenses, which are older : similar issues are quite rare.

[Darkover]

It was possible to oben the front group without destroying it!

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It heat the lens in my oven 10min for 100degree (Celsius!), 10min for 150, 10min for 200, 10min for 220, than it stayed there for 1h to cool down.
After this I put the lens in acetone and wait 1h. Now it was easy to open the lens.



Now it will stay there for some time to remove the old glue.

Of course the most interesting part is gluing it again. I will wrote about it in the next days...

Olaf

[SSGGeezer]

Did you mark the edges in some way to assist realignment?

[Praktica*ist]

Very interesting tentative !

Are the two elements the same size ? This would make the realignment possible without optical bench.

[Darkover]

I didn't mark. I think it is not necessary. And yes the element are the same diameter. Perhaps the biggest problem, when gluing they need very exact alignent because the lens fit very close in the holder.

Olaf

[Praktica*ist]

May I suggest to use the tips of a caliper to realize the alignment of the doublet ?

[Darkover]

I glued it together! I use Cytoseal XYL for this. Unfortunatly this is expensive. You need to know a doctor or biologist to lend his bottle for one drop.

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I made a small paper strip put it around the lens to align it correct. A small amount of the glue touch the front and back part of the lens.
But I think it is no problem to clean it.



After a few minutes I cut my paper strip and it open themself little bit. So it is easier to remove the paper later.

Now I will to wait until tomorow before I try to clean the lens. The lens looks realy good, no air bubble. I am full of hope!

Olaf

[Darkover]

It is working! I clean the lens very carefull and install it.

The first testpicture, a part of my headphone amplifier:




Of course it will need some more test, but it did looks hopefull.

Olaf