Yellow Meyer Orestor 100mm f/2.8

Discussion in 'Classic Manual Cameras' started by kl122007, Nov 26, 2009.

  1. Just wondering whether it has thorium glass. My Meyer Orestor 100mm f/2.8 (exaka mount) has a rather yellow element inside and I don't have a meter to check.
    Is there any ower in this place and having yellowed Orestor? Please let me know if it is normal.
  2. Sorry I don't have direct experience with this lens. I have read that it might be possible by exposing it to ultaviolet
    (ie sunlight) it can recover. .. Might be worth a try!
  3. I don't beliieve that the Orestor uses thoriated glass - unilkely for a medium speed lens. I feel that the colour is more likely to be ageing balsam, since the design is quite probably a 5 element 4 group formula with the second & third elements (from front) being cemented. I could be right, I could be wrong - JDM will know . . . .
  4. I believe only experts would know.
  5. JDM will know​
    JDM has no idea whatsoever, although I can't remember ever seeing anything with Thorium in it from the DDR. None of my assemblage of DDR lenses shows yellow like the old Takumars, anyhow.
    But JDM does "know" that the Orestor is a 5-element lens, but not the internal arrangement tho' it could be 5/5. Nobody has ever filled in info on this lens at the M42 site (link ) either . I've never had one of these and this post, unfortunately, reminds me that I have overlooked getting 85mm to 100mm or so lenses for my Prakticas.
    Winfried Buechsenschuetz or Bueh B will know.... ;)
  6. I wasn't goin to reply to this as I don't actaully know the answer but as no one so far can be definite I will pitch in with another non-answer.I agree with Alan that it is likely to be discoloured balsam as Meyer lenses tended to be medium quality wheeas thorium and rare earth glass is found in the high end lenses.. Having said that the use of Thorum and other weakly radioactive materials was not uncommon in the 50's and 60's. It was used to increase the refractive index of some glasses in the same way as lead is used in lead glass. The high refractive index allowed the curvatures to be decreased so lesseing spherical aberration.
    I think JDM really does know but is holding out on us.
  7. Winfried Buechsenschuetz or Bueh B will know.... ;)
    Ha! It's pass the parcel time ! ;-)
    There's a tiny bit of information on such lenses on Captain Jack's Exakta pages - mostly photos of lenses though.
  8. There is no radioactivity above the backgroung one in my copy of Exakta mount Meyer-Görlitz Orestegor 2,8/100 mm (Geiger counter test). I guess it's safe to assume thorium glass isn't present in the lens.
    Best regards,

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