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Help! A strange artifact in Oly OM 50mm bokeh!

Discussion in 'Adapted Lenses' started by jambaj0e, Jul 24, 2011.

  1. jambaj0e

    jambaj0e Mu-43 Top Veteran

    816
    Aug 31, 2010
    So I bought an Olympus OM 50mm f1.8 (Serial #3,634,718 "made in Japan") from eBay. I was doing a bokeh comparison to my Konica Hexanon 50mm f1.7. One thing I noticed, the bokeh is not that different between the two, at least in the space I was using.

    The more disturbing thing is the artifact I see in the bokeh highlights of the Olympus, a dark dot on each of the bokeh hexagons.

    Let me show you both lenses at f4

    Konica
    5968940551_cb5401b5d8_o.

    Olympus OM
    5969498398_947defeafc_o.


    Konica Closeup
    5968950057_933c0e1660_o.

    Olympus Closeup
    5969507700_930f396da0_o.
     
  2. f6cvalkyrie

    f6cvalkyrie Mu-43 All-Pro

    Feb 12, 2010
    Brussels, Belgium
    Hi,

    Viewing the Olympus close up shows it's more like a dark dot surrounded by dark/luminous circles.
    That makes me think of interference between two reflected lightbeams. White light interference fringes.
    Is there a very thin layer of 'something' covering that metal part ?
    A coating, grease .... ?

    C U,
    Rafael
     
  3. jambaj0e

    jambaj0e Mu-43 Top Veteran

    816
    Aug 31, 2010
    A thin layer covering the metal part of what? The lens? The inside of the lens? The sensor? The glass?

    The P&S camera in the photo?
     
  4. f6cvalkyrie

    f6cvalkyrie Mu-43 All-Pro

    Feb 12, 2010
    Brussels, Belgium
    I mean the metal part of the camera in the picture that is reflecting your light.
    The effect is most obvious in the Oly close up, in the specular reflection that show right above the center of your cropped image. But also in some other hexagons of the Oly shot.
    Less evident in the Konika shot !

    C U,
    Rafael
     
  5. Pan Korop

    Pan Korop Mu-43 Veteran

    479
    Mar 31, 2011
    Phare Ouest
    Uh... By any chance, are you sure there's no doublet (I presume there's at least one in the optical formula, if not two) in the Zuiko starting to un-cement?

    It would show up like interference fringes.

    Also, uncementing doesn't always start from the sides of the lens, where it appears more spectacular and obvious.
     
  6. Markb

    Markb Mu-43 Top Veteran

    532
    Jun 9, 2011
    Kent, UK
    Mark
    Dust? Pitted coatings? Either are possible on a 20-odd year old lens. As for balsam separation, the Zuiko 50/1.8 has one cemented pair as illustrated here, so it's a possibility.

    How do you like the lens otherwise?
     
  7. jambaj0e

    jambaj0e Mu-43 Top Veteran

    816
    Aug 31, 2010
    What's a doublet and what do you mean by uncementing?
     
  8. jambaj0e

    jambaj0e Mu-43 Top Veteran

    816
    Aug 31, 2010
    It's more evident on this crop at f8

    5970436452_c1eb264f9d_o.
     
  9. Within a camera lens you will usually have at least one combination of two lens elements (one concave and one convex) that sit directly against each other without seperation.
     
  10. jambaj0e

    jambaj0e Mu-43 Top Veteran

    816
    Aug 31, 2010
    Is this something that I can clean easily or did I just bought a bad lens?
     
  11. Similar highlight effect here.

    I don't have images taken using my two copies of this lens that have out of focus highlights so I can't say if mine exhibit a similar effect.
     
  12. Pan Korop

    Pan Korop Mu-43 Veteran

    479
    Mar 31, 2011
    Phare Ouest
    I'm French, I talk funny-peculiar at times. :)

    Doublet: a pair of lenses, cemented (glued) together to form a single solid block.

    Uncementing: probably a gallicism of mine. Old doublets were glued (rather, gooed...) together with "Canada balsam". Something like a natural resin but maybe some caribou bodily fluid ;)
    Anyway, mechanical or thermic shocks can cause these old cementings to separate. Sometimes it's only visible through careful examination under a strong point light source. Appears as oil/fuel stains, actually interference aka "Newton" rings. Common enough with old lenses, esp. fast "standard" or "portrait" lenses, and practically unrepairable, except at great expense.

    PS: modern lenses use synthetic glues which suppressed this sore issue.
     
    • Like Like x 1
  13. I tried out my two OM 50/1.8s last night, one being a Silvernose F.Zuiko and the other a later model "Made In Japan". Neither lens appeared to exhibit the same dark dot in the centre of the out-of-focus highlights when tested between f/1.8 to f/5.6.
     
  14. patrick359

    patrick359 Mu-43 Rookie

    17
    Feb 6, 2010
    I found a couple of shots with those nifty hexagons floating in the background, and they didn't have any dot in the middle, or any interior pattern at all. This is a late model OM 50/1.8, from the mid 80's. I'm pretty sure it was set at wide open, or close to it.