kl122007 Posted November 26, 2009 Share Posted November 26, 2009 <p>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. <br />Is there any ower in this place and having yellowed Orestor? Please let me know if it is normal.</p> Link to comment Share on other sites More sharing options...
chuck_foreman1 Posted November 26, 2009 Share Posted November 26, 2009 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! Link to comment Share on other sites More sharing options...
astral Posted November 26, 2009 Share Posted November 26, 2009 <p>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 . . . .</p> Link to comment Share on other sites More sharing options...
kl122007 Posted November 26, 2009 Author Share Posted November 26, 2009 <p>I believe only experts would know.</p> Link to comment Share on other sites More sharing options...
JDMvW Posted November 26, 2009 Share Posted November 26, 2009 <blockquote> <p>JDM will know</p> </blockquote> <p>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.</p> <p>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 (<a href="http://m42.artlimited.net/site_lenses.php">link</a> ) 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.</p> <p>Winfried Buechsenschuetz or Bueh B will know.... ;)</p> Link to comment Share on other sites More sharing options...
colin carron Posted November 27, 2009 Share Posted November 27, 2009 <p>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.</p> <p>I think JDM really does know but is holding out on us.</p> Link to comment Share on other sites More sharing options...
astral Posted November 27, 2009 Share Posted November 27, 2009 <blockquote> <p>Winfried Buechsenschuetz or Bueh B will know.... ;)</p> </blockquote> <p>Ha! It's <em>pass the parcel time !</em> ;-)</p> <p>There's a <em>tiny bit </em> of information on such lenses on <a href="http://captjack.exaktaphile.com/">Captain Jack's Exakta pages</a> - mostly photos of lenses though.</p> Link to comment Share on other sites More sharing options...
miha_steinb_cher Posted November 28, 2009 Share Posted November 28, 2009 <p>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.</p> <p>Best regards,</p> <p>Miha</p> Link to comment Share on other sites More sharing options...
ph. Posted August 19, 2022 Share Posted August 19, 2022 according to the Meyer catalogue (apparently this forum program doesnot like to upload PDFs, so no attatchment) the 100mm has the second and third element bonded together. And according to another reliable source, the special and expensive glass types went to Zeiss and not to Meyer. p. Link to comment Share on other sites More sharing options...
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