Is this fungus?


Frogherder

Link Posted 20/04/2012 - 18:19
I've recently aquired an M42 500mm/f8 mirror lens which when viewed from the front shows a spotless mirror. However the back lens appears to have an appearance of "partial mirroring", see below. The effect manifests itself about 5-6mm below the surface of the lens, so either at the interface of two pieces of glass or on the surface inside the lens.



It's not a patch on my Tamron 500mm mirror but at the cost it looked worth playing with. It's slightly blurred as the following macro (ish) shows






regards
Bernard
Last Edited by Frogherder on 20/04/2012 - 18:19

Offertonhatter

Link Posted 20/04/2012 - 18:35
Not sure.
If it is a mirror lens, it might be decay of the mirror coating that you can see.
Whatever it is, it does not look good.
Some Cameras

Frogherder

Link Posted 20/04/2012 - 19:48
Thanks Iain for the reply, further information just obtained.

If I look through the front I can justmake ouot the "stain" at the far end of a tunnel.
If I rotate the focus whilst looking through the front the "stain" doesn't rotate but a dust spec on the mirror does.

If I look through the back the "stain" appears just below the glass surface and doesn't rotate when the focus is moved.

From the above I've deduced that the "stain" is not on/in the main parabolic mirror or the small secondary hyperbolic mirror, both of which rotate with the focus.

It therefore is on the inside surface of the back lens or the interface of two glasses, assuming the back lens is multi-layered.

I haven't figured out how to get at it yet, but having heard so much about fungii on this forum, and as far as I know never experienced it, I thought I'd ask the question.

As I said it didn't cost a great deal so ......

regards
Bernard

Offertonhatter

Link Posted 20/04/2012 - 20:00
Bernard,
In that case, it could be fungus, or the "glue" in the optic sandwich failing.
Normally Fungus on lenses are round growths in areas of the glass, this could be one large growth.
However, it is unusual and possibly someone might have the correct answer.
Some Cameras

Frogherder

Link Posted 20/04/2012 - 20:06
One of the things that puzzle me is the fact that little or no light appears to be lost.

With the camera on a tripod I get the same exposure setting with my Tamron and this lens, so it isn't as dark as it might appear.

It does have a slight milky tone which doesn't come out in the 'macro' shot of the monitor and some of the fuzziness could be down to a tripod sat on carpet at the other end of the room.

regards
Bernard

beachboy2

Link Posted 24/04/2012 - 10:20
I think this is separation between a couple of the correcting lenses. Have it on my 350mm F5.6 mirror. No effect on my photos.
cheers
bb2

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gartmore

Link Posted 24/04/2012 - 13:34
Fungus is tiny little spidery fingers spreading across the glass, if it is so bad that it it cuts out light then you can probably eat it.
Ken
“We must avoid however, snapping away, shooting quickly and without thought, overloading ourselves with unnecessary images that clutter our memory and diminish the clarity of the whole.” - Henri Cartier-Bresson -

sorted78

Link Posted 24/04/2012 - 21:07
Quote:
I think this is separation between a couple of the correcting lenses. Have it on my 350mm F5.6 mirror.

+1 (and I've also got it on my 350mm/5.6 mirror if that's any consolation!). I'm pretty sure it's not fungus, as others have said. Even in a really heavily fungus-infested Sigma zoom I saw, there were thread-like filaments at the edges.
Last Edited by sorted78 on 24/04/2012 - 21:10

Frogherder

Link Posted 24/04/2012 - 22:58
OK, thanks to every one who took the time out to respond.

I'm sold on the idea of it being between two lenses and as I've discovered there is no appreciable light loss.

Coupled with the fact that despite spending time making a 55mm pin spanner to remove what I think is a locking ring holding that section together to find that I cannot budge the ring. It's either glued in, or Al-Al bonded itself into place.

So I'll just put up with it.

Next job is to test it for resolution/clarity, but as it doesn't focus on infinity I'm waiting for the postman to deliver a flush fit M42-PK adapter, then I'll be able to play outdoors rather than with a 13mm extention (hence the monitor shot).


regards
Bernard
Last Edited by Frogherder on 24/04/2012 - 22:59

Cuchulainn

Link Posted 25/04/2012 - 00:06
One thing to remember is that damage to coatings or cemented surfaces is often far worse in appearance to the naked eye than is seen in images. This is because you are comparing the reflection of the damaged area (which is maybe 10% in a particularly bad case) versus the reflection from the nearby unaffected glass (which could be 0.1% with a good coating) - so there is a 1000 fold increase in reflected light! However, it's transmitted light that counts, and so the decrease there is only 10%! Naturally, not all defects are equal, and some will add distortions to the image that are out of proportion to their apparent size, conversely, sometimes "terrible" damage won't appear in the end image. It's why a damaged lens at a good price is often worth putting on the camera to see what actually has happened to it!

Frogherder

Link Posted 25/04/2012 - 11:12
Quote:
Naturally, not all defects are equal, and some will add distortions to the image that are out of proportion to their apparent size, conversely, sometimes "terrible" damage won't appear in the end image. It's why a damaged lens at a good price is often worth putting on the camera to see what actually has happened to it!

I agree, many years ago I bought an enlarger to do B&W printing. The condensor lens had quite a large chip out of one side (probably been dropped) but this made absolutely no difference to the prints. I accept that a condensor is not on the image side of the negative.

Bernard
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