Archived posting to the Leica Users Group, 2007/11/10
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]On Nov 10, 2007 11:34 AM, Henning Wulff <henningw@archiphoto.com> wrote: > >For those of you interested in the Pentax 50mm yellowing problem, > >Here are photos taken before and after 116 hours exposure to UV > >light to reverse the yellow cast problem due to the radioactive > >element Thorium in one of the lens elements. > > > >Before photo. Lens has brownish, yellow cast. > > > ><http://gallery.leica-users.org/v/LeonardT/Pentax/Takumar_50_2.jpg.html> > ><http://tinyurl.com/233emd> > > > >Here is a photo taken after only 116 hours of being exposed to UV > >light. The exposure was to 16 hours of sunlight and 100 hours of UV > >from a blacklight. > > > ><http://gallery.leica-users.org/v/LeonardT/Pentax/Takumar_116hrs.jpg.html> > ><http://tinyurl.com/2bx7do> > > > >I did not expect any improvement this quickly if at all. I'll > >probably keep the lens in front of the blacklight for a few days > >more to see if there is any further improvement. > > > >Thanks for your interest. > >Len > > > Very impressive. I'm still not clear on the actual process of > reversing this colour change, though. As Chris Saganich pointed out, > available UV radiation is too long wavelength to have enough energy > to boot electrons around to reverse these colour changes, as far as I > know. On the other hand, clearly there is enough energy in the UV > radiation. Something awfully low energy is happening here. > > In any case, it looks like the lens is well on its way being > 'filterless' (internally) again. > Isn't UV the recommended treatment for fungus? It sounds like a more probable explanation. But of course it sounds much more esoteric to blame the radioactive glass for the colour change. /Anders