Can a modern filter help in front of damaged legacy coatings?

Discussion in 'Adapted Lenses' started by arch stanton, Oct 10, 2012.

  1. arch stanton

    arch stanton Mu-43 Veteran

    420
    Feb 25, 2012
    London
    Malc
    I recently bought an old Pentacon 200mm from ebay and the front element of the lens has the plague! Is there any benefit to fitting a modern multi-coated filter (UV maybe?) in front of it to help with contrast/flare?

    It had no front cap for years, the original coating's mostly worn off the centre 50% of the lens and towards the edges still has some, with random gaps. No scratches that I can see so at a guess someone 'cleaned' it. Also no fungus that I can see, just the damaged coating.

    You can't see any odd pattern in shot, but it looks like it's affecting overall image quality with bright areas in frame.
     
  2. dwig

    dwig Mu-43 Top Veteran

    682
    Jun 26, 2010
    Key West FL
    Absolutely not.

    Adding a filter will make things worse as you will still have the same reflections off of the lens' front element and now a small portion of them will be reflected back into the lens system be the rear surface of the filter.
     
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  3. oldracer

    oldracer Mu-43 Hall of Famer Subscribing Member

    Oct 1, 2010
    USA
    As dwig said.

    Background: The purpose of lens coatings, though to read about them they seem quite complicated, is simply to increase light transmission through a lens. Said another way, they reduce the amount of light that is scattered at the glass interface. It is not so much that we need the light for the exposure (we are talking just a few percent) but it is that some of the scattered light ultimately becomes part of the photograph, reducing contrast. So ... the coating on a modern filter serves to improve that filter's performance but it does nothing for the scattering (and, probably, distortion) from the deteriorated coating on your objective lens.

    More: Anti-reflective coating - Wikipedia, the free encyclopedia and Camera lens - Wikipedia, the free encyclopedia
     
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  4. arch stanton

    arch stanton Mu-43 Veteran

    420
    Feb 25, 2012
    London
    Malc
    Thank you folks, I figured this might be the case. So typically would all glass surfaces in a lens be coated to reduce scattering?

    Is there anything else that might improve the situation? I'm already using a hood, but it's more if there are light-sources near/on-axis. I'd doubt there's anything practical I can do to fix the coating, but anything would help.

    Ultimately it was a cheap gamble, but I fancied playing around with the 15-blade aperture!
     
  5. Ned

    Ned Mu-43 Legend

    Jul 18, 2010
    Alberta, Canada
    Try leaving your lens under a UV or black light, or in the bright sun for a few days.

    This is actually to reduce yellowing in old lenses, but it may be worth a shot.
     
  6. oldracer

    oldracer Mu-43 Hall of Famer Subscribing Member

    Oct 1, 2010
    USA
    It seems to be kind of a black art with lots of proprietary techniques. Check out some of the discussions among the binocular mavens, usually birders. (Birding Binoculars 3: Coatings) My guess is that coatings are used on the glass/air surfaces but not in cases where two lenses are cemented together.
    Probably correct. I believe it is possible to strip and re-coat a lens but I doubt it would be cost-effective given the value of most camera lenses in general and yours in particular. Maybe at the Zeiss, Leitz, Schneider, et al high end it would be different.
     
  7. Just Jim

    Just Jim Mu-43 Top Veteran

    941
    Oct 20, 2011
    If you really love the lens... I mean looooove the lens. You can have it recoated at focal point lens service.
     
  8. arch stanton

    arch stanton Mu-43 Veteran

    420
    Feb 25, 2012
    London
    Malc
    Bright sun? Ahhh, that's a rare resource in London! If the sun puts in a rare appearance this winter I'll try this, thanks :)


    I'm just flirting with this one, couldn't call it true love enough to buy her sparkly new coatings!
    Plus my AF lenses'll get jealous and I'll have to sleep on the sofa again :biggrin:
     
  9. steve16823

    steve16823 Mu-43 Regular

    186
    Sep 26, 2011
    Brookfield, IL
    The other thing to consider that as an old single-coated lens it might not perform much differently even if the coating were fully intact.
     
  10. lenshoarder

    lenshoarder Mu-43 All-Pro

    Nov 7, 2010
    I don't see how this will help a worn off coating.

    In my experience, it doesn't help with yellowing either. I've tried it in the blazing hot California summer sun. Didn't do a thing.
     
  11. dwig

    dwig Mu-43 Top Veteran

    682
    Jun 26, 2010
    Key West FL
    True.

    Also, the coating of the front surface of the element has nothing to do with the image quality if no filter or other reflective element (e.g. closeup lens, ...) is placed in front of it. Reflections from the front surface are merely light lost to the outside world. These reflections don't get back into the lens so they don't affect the image quality.

    It's on the internal surfaces where coatings are critical. The outside rear surface is also important, but less so than the internal surfaces.
     
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