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01-24-2018, 08:38 PM   #1
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Element group issue

Attached is an image of the front element groups of the common M 50mm f2 lens. There is fungus behind the front element. One assumes that the what you see should unscrew into two separate groups but they are as tight as a drum. Before I get a bigger hammer, does anyone have some advice in separating these two groups? Thanks.

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01-24-2018, 10:34 PM   #2
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Aren't element groups normally cemented together?

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01-25-2018, 12:22 AM   #3
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QuoteOriginally posted by Adam Quote
Aren't element groups normally cemented together?
Not always, the definition of optical groups is pretty loose these days. Doublets however, are always cemented [the reason for this is to maintain precise optical alignment]. Behind the front element of double Gaussian 50mm f/2 lenses is invariably, a cemented doublet. If there is fungus between these elements the Canada balsam* between them will need to be heated to soften the adhesive and soaked in solvent to clean it off**.

*If this lens has been repaired before there may be a UV curing epoxy adhesive used which can be really stubborn to remove.
**There is considerable debate over exactly which solvent is appropriate. By far the most common solvent mentioned when de-cementing a doublet when Canada balsam is the adhesive, is Xylene.

Personally, I just chuck it in a beaker of warm DMSO and call it a fluffy day.

Last edited by Digitalis; 01-25-2018 at 03:09 AM.
01-25-2018, 03:08 AM   #4
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QuoteOriginally posted by Adam Quote
Aren't element groups normally cemented together?
Some are and there could be some cemented here. But it is obvious that there are two groups screwed together in this assembly. Fungus on elements that are cemented normally forms on the outside, not where the elements are cemented. I haven't seen this assembly in parts yet so I can't say but I would bet the front element is not cemented to anything and the fungus is on the back side of it. I'll get the heat gun out and see if that helps.

01-25-2018, 06:56 AM   #5
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Rear flange cap... A form of thread locking compound. If you have a heat gun and set it to 200 degrees. A lens wrench is the easiest method to grip or otherwise rubber pads.
If heat is not working.
Chill the group in the fridge to about 38-40 degrees... then with heat gun and narrow nozzle heat the ring as rapidly as possible and then twist.
If with lock the inner is not expanded as much as outer.
The difference might be enough to have less friction on the threads.
01-25-2018, 06:58 AM   #6
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Probably common enough to not even be worth the time and effort. Find one that is clean.
01-25-2018, 09:20 AM   #7
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I always look for an optical diagram to figure out what parts should unscrew. It should be just thread locker keeping you out.

https://www.pentaxforums.com/lensreviews/showfull.php?product=83&bigimage=sm...66k_optics.jpg
01-25-2018, 09:26 AM   #8
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QuoteOriginally posted by Not a Number Quote
Probably common enough to not even be worth the time and effort. Find one that is clean.
For sure but it isn't my lens. I have a f1.7 personally.

01-25-2018, 12:58 PM   #9
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The video linked to here Pentax Lens Disassembly Database briefly mentions the front group at 10:40.

It says access is through the front but that the threads are glued requiring a solvent.

Acetone usually works on the glue used on threads. Dip a cotton bud (Q-tip) in acetone and apply to the thread, wait a little then unscrew.

Hope this helps.
01-26-2018, 09:32 AM   #10
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QuoteOriginally posted by Just1MoreDave Quote
I always look for an optical diagram to figure out what parts should unscrew. It should be just thread locker keeping you out.

SMC Pentax-M 50mm F2 Lens Photo - Pentax Lens Reviews & Lens Database
Yes, it's surprising how a dab of acrylic varnish can hold those threads together. If it's an aluminum assembly (ie: not plastic), I use an eyedropper to run a tiny amount of lacquer thinner onto the threads. I wait a few seconds and try to swirl the thinner around the whole thing (although it helps if you can see where the assembler put the dab of varnish). With a firm grip (grippy gloves, or rubber bands) the retaining ring often comes right off.

Applying heat is risky as it's a good way to encourage partial element separation.
01-26-2018, 11:49 AM   #11
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QuoteOriginally posted by Ontarian50 Quote
Yes, it's surprising how a dab of acrylic varnish can hold those threads together. If it's an aluminum assembly (ie: not plastic), I use an eyedropper to run a tiny amount of lacquer thinner onto the threads. I wait a few seconds and try to swirl the thinner around the whole thing (although it helps if you can see where the assembler put the dab of varnish). With a firm grip (grippy gloves, or rubber bands) the retaining ring often comes right off.

Applying heat is risky as it's a good way to encourage partial element separation.
The heat did the trick. I was able to get the rear group off of the assembly. But there is yet another screw-in group behind the front element. Nothing loosens it. I decided to give the lens back unrepaired. It just isn't worth it. Thanks to all who contributed.
01-26-2018, 02:41 PM   #12
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The 50 f2 is a five element lens with no cemented elements, they are not as sought after as the 1.7 and 1.4 lenses, consequently their value is much less. A replacement on ebay would not cost a lot.
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