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08-07-2012, 05:32 AM   #1
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SMC Pentax-M Macro 100 mm f/4 strange "net" behind the front element

I recently bought a well used copy of the Pentax-M 100 mm macro lens and I noticed a strange "net" behind the front element:





I tried to find a proper picture in the net to verify if it's normal for these lenses but failed to find one. There isn't such thing in any other of my Pentax-M lenses so I assume the lens was dismantled/repaired and it is some king of glue or tape to keep the front element together with the rest.
Am I right?

The lens performs well or so it seems, I didn't notice any major softness or light fall off that could be a result of this "net" inside, but still it worries me a little bit ;-)

TIA for any hint.

08-07-2012, 05:57 AM - 1 Like   #2
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I don't think it's glue, unless there are two back-to-back elements right there. It's more likely that it's lens coating that's somehow been damaged, in which case it probably won't have much of an effect on your photos. But if it becomes bigger, that would point at the glue theory...
08-07-2012, 05:57 AM - 1 Like   #3
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That's a cemented element pair at the front isn't it? I had something similar to this on my M 40mm. I think it's a deterioration of the cement between the two elements. There's not much you can do about it, save dismantling the whole thing, getting the elements apart and applying your own optical cement, but that's not really something you can do without insane equipment to make sure no dust gets in and crap like that.

Maybe water got into it and degraded the cement around the edge of the element? Or trapped air got heated up or something. Not sure how it happens.
08-07-2012, 07:05 AM   #4
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Adam, Mareket - thanks a lot for your answers.

Since I'm not willing to dismantle the lens (no equipment and I would surely make things only worse), it seems like the only solution is to take pictures and don't worry about it unless it gets worse...

08-07-2012, 07:41 AM - 1 Like   #5
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I have seen this before and am fairly certain it is a cement issue. There is a company than can re-cement some lenses. However, it would cost at least as much as the lens is worth and maybe more. I would only go to that kind of trouble for a Tak 83mm/1.9
08-07-2012, 09:16 AM - 1 Like   #6
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A quick check of the lens diagram shows the front two elements are cemented together:

http://www.bdimitrov.de/kmp/lenses/primes/_optics/100f4-Macro.gif

On APS-C, I mentally draw a smaller rectangle around the front element to represent the smaller digital sensor. Anything outside of that, I assume is not going to affect images. This is probably a naive oversimplification but once I do that, I can use the lens without worries.
08-07-2012, 09:45 AM   #7
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Thanks, guys!
08-12-2012, 04:02 AM - 1 Like   #8
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I know of a shop that services old Zeiss lenses, where they take them apart, de-cement all the elements, clean them and then re-cement the elements, and align them when putting them back together. I have no idea if the shop would entertain a Pentax lens, but here is the information....


08-15-2012, 11:11 AM   #9
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Thanks for the tip but I'm afraid the shipping cost itself (from Poland to the USA and back) may exceed the value of the lens ;-)
08-15-2012, 12:10 PM - 1 Like   #10
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QuoteOriginally posted by Michelek Quote
Thanks for the tip but I'm afraid the shipping cost itself (from Poland to the USA and back) may exceed the value of the lens ;-)
Given how little it costs to pick up the M100 F4, unless you want to try it yourself it's cheaper to just find another. It's not a rare enough lens to spend the money on a repair IMO.
You can get various lens cements (modern ones set much faster and are easier to use) - There is one guy selling Balsam online (no longer made) as well which is what the lens would have used originally but it's hard to cure in a home environment worht it to do only for a rare lens you want restored to original IMO

Edit this link covers how to do it with Canada balsam

http://www.atmsite.org/contrib/Sapp/LensGlue/

and these guys would be the go to for a valuable lens

http://www.focalpointlens.com/fp_intro.html
08-15-2012, 12:40 PM   #11
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Thanks a lot, Eddie.
I think I'm going to leave the lens as it is now. I'm pretty sure that with my shaking hands I would do even more damage to it ;-)
Besides, I have more macro lenses to shoot with, I bought this one only because it was cheap (ca. 30 USD) so I may as well use it until it delivers a decent picture and then just put it on a shelf.
08-15-2012, 12:53 PM   #12
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QuoteOriginally posted by Michelek Quote
Thanks a lot, Eddie.
I think I'm going to leave the lens as it is now. I'm pretty sure that with my shaking hands I would do even more damage to it ;-)
Besides, I have more macro lenses to shoot with, I bought this one only because it was cheap (ca. 30 USD) so I may as well use it until it delivers a decent picture and then just put it on a shelf.
30USD was a steal but then again you got what you paid for
08-15-2012, 01:10 PM   #13
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QuoteOriginally posted by Just1MoreDave Quote
A quick check of the lens diagram shows the front two elements are cemented together:

http://www.bdimitrov.de/kmp/lenses/primes/_optics/100f4-Macro.gif

On APS-C, I mentally draw a smaller rectangle around the front element to represent the smaller digital sensor. Anything outside of that, I assume is not going to affect images. This is probably a naive oversimplification but once I do that, I can use the lens without worries.
Light from all parts of the front element is directed in various paths to get to each point of the sensor. The cropping of the snsor relative to design field has nothing to do with cropping the areas of the elements that contribute to image making.

A generous hood would likely help this lens (long enough to just avoid vignetting), since then you would minimise light landing on the affected part from sources which will not affect the image (after cropping) but you will stop any spurious reflections on the affected surfaces that might affect the image.
08-22-2012, 08:54 AM   #14
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QuoteOriginally posted by tim60 Quote
A generous hood would likely help this lens (long enough to just avoid vignetting), since then you would minimise light landing on the affected part from sources which will not affect the image (after cropping) but you will stop any spurious reflections on the affected surfaces that might affect the image.
Some black paint or sharpie on the front glass (or a black card ring) would have the same effect.
08-22-2012, 12:13 PM   #15
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QuoteOriginally posted by kh1234567890 Quote
Some black paint or sharpie on the front glass (or a black card ring) would have the same effect.
This idea is much better, becasue it will stop the entering light which would get into that region of the lens. A hood will do nothing for the whole diversity of ray paths which result in light incidnet on all of the front element getting mapped to each image point.
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