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06-07-2018, 09:18 AM   #1
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Pentax 28mm f2 rare lens -- cloudiness INSIDE a lens element?

I have a smc Pentax 28mm f2 "Hollywood" (yeah, that one) that I am getting ready to sell, except one element has a bit of haze, sort of. It is kind where it doesn't seem to affect the lens in any discernible way optically, and you can't even see it looking through the lens -- looks completely clear. However, with the LED flashlight and looking at a oblique angle, you can see a bit of cloud around parts of the edge. Using it for myself I don't care about it, it isn't fungus and the lens performs fine. But since the lens is quite valuable, and more valuable still if it has no issues, it thought I'd take it apart and see if that mark could be cleaned off so I can get full value.


But once getting it apart, the shadow of a cloud doesn't seem to be on the surface of the element -- I figured it would either be something I could wipe off or some sort of coating damage that there was nothing I could do about. But from what I can tell the outer surfaces look fine (and cleaning doesn't help in any case). It sort of looks like coating damage in a lower level of coating but still quite near the surface. Is that possible? Or can a slight haze develop somehow inside of a solid element? (And again, even after isolating the element, you can't see it unless you shine a flashlight on the element from the side edge and then you can see a bit of cloud on the edge). I'm pretty sure it is a single solid element rather than two cemented together. This is supposedly a 9-element lens and the count is correct if this is a single element, unless two cemented elements counts as one? It is a biconvex element (curved inwards on both sides) -- 3rd from rear. The flaw does look "almost" on the surface rather then in the middle, but these things can play tricks on you when looking at them. Maybe it really is on the surface and just looks like it isn't?

Any thoughts on what exactly I'm looking at? Is it inside the element as it appears (and how is that possible)? Any miracle cure?



06-07-2018, 09:28 AM   #2
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Get it checked out by a specialist. I take lenses apart all the time but I wouldn't dare mess about with such a rare lens. Worst case scenario is that there will be one fewer copy of the lens in the world.
06-07-2018, 09:38 AM   #3
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Original Poster
QuoteOriginally posted by HarisF1 Quote
Get it checked out by a specialist. I take lenses apart all the time but I wouldn't dare mess about with such a rare lens. Worst case scenario is that there will be one fewer copy of the lens in the world.
If I was keeping it and this actually affected performance, maybe. But getting it checked out will cost as much as the amount I'll lose just by discounting it for having a flaw. I would just like to understand the flaw, or the possibilities. I've seen similar cloudiness lots of times ("only can see it with a flashlight" type flaws -- extremely common with old lenses). But usually they are clearly on the surface, but here I can see the purple tint of the outer coating and it all looks perfect.
06-07-2018, 09:53 AM   #4
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Can you get some images of the issue?

Oftentimes with these older lenses you get a milky haze over time. From experience there's minimal impact on sharpness and maybe only a bit of loss of contrast. Not really discernible!

06-07-2018, 10:11 AM   #5
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QuoteOriginally posted by HarisF1 Quote
Can you get some images of the issue?

Oftentimes with these older lenses you get a milky haze over time.
Yeah, but inside a solid piece of glass and not on the surface?

QuoteQuote:
From experience there's minimal impact on sharpness and maybe only a bit of loss of contrast. Not really discernible!
I don't think it is affecting the image quality basically at all, but hard to prove that statement. In any case, the lens performs as expected. I did manage to get a picture where you can sort of see it with the flashlight coming in the from the edge and shining on the flawed side (it seems) and looking through the element. It is completely invisible looking at it any other way. I thought maybe it could actually be a projection through the lens, but that usually occurs when there is paint loss on the edge which I don't see here. Maybe this is actually a cemented element? I always thought that would still count as two elements?

06-07-2018, 10:37 AM   #6
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Is that the doublet at the center of the lens? Could the haze be partial separation?
06-07-2018, 10:49 AM   #7
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QuoteOriginally posted by tvdtvdtvd Quote
Is that the doublet at the center of the lens? Could the haze be partial separation?
Nope, 3rd element from the rear. It does have a recessed notch running around the outside (see the diagram) though and you can see the inner wall edge as a lighter color much better on the side of the "seam" with the flaw. Is is possible that it could be an internal reflection off the inner side wall -- just sort of an illusion/hologram effect?
06-07-2018, 11:08 AM   #8
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QuoteOriginally posted by vonBaloney Quote
Is is possible that it could be an internal reflection off the inner side wall -- just sort of an illusion/hologram effect?
That sounds very probable, especially as the outer edge isn't polished. The haze might be nothing more than the reflection of the edge of the lens
bouncing off the inner face of the polished surface.

06-07-2018, 11:51 AM   #9
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QuoteOriginally posted by tvdtvdtvd Quote
That sounds very probable, especially as the outer edge isn't polished. The haze might be nothing more than the reflection of the edge of the lens
bouncing off the inner face of the polished surface.
The outer outer is painted black, but the inner edge on one side of that notch is visibly unpolished, but the other side of the notch on the inside the edge is darker/smooth. So it is kind of strange. I would think an internal reflection might appear differently depending on the angle of the light (or appear to move around) -- this looks like an unchanging thing when you can see it.
06-07-2018, 07:43 PM   #10
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I once had seven Pentax-M 135/3.5 lenses (small eBay problem) and finally got around to selling them. I wanted to sell six of them for lots of money so I did everything, including the flashlight test. I kept the worst one which looked terrible when shining a flashlight through it. It looks like lots of dust but doesn't ever get cleaner. Out of curiosity, I tested them for sharpness. This same lens is also the sharpest. But of course, it would have been the hardest to sell.

It seems like there is nothing you can do - you can't clean or recoat the element and it's not separation. It may be just some weird thing that they all do to some extent. I like the theory about the notch. I'd trust your statement that the lens performs as expected - you just have to find a buyer to do that too.
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