Archived posting to the Leica Users Group, 2007/11/10
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]This sounds like a very plausible theory, Anders. As I stated before, I don't doubt Len's observation. But -and as an answer to Henning- the 2 visual proofs of this observation were not were not really made in identical circumstances. That's why I suggested to documented them in a similar conditions. Even from his low res jpegs, I did the test, balanced them (black point, white point, mid gray) and put them in Lab colour -the widest color space as you may know- to make them comparable*. There still is a visible difference. But at least the comparison has been made on a par level. So let's not start doing critical image comparisons in different conditions, on different screens and with low res sRGBs if we want to use these a studying material. Even if visual comparison does not give the right explanation for the reason of succes for this treatment. Philippe Op 10-nov-07, om 19:00 heeft Anders Nygren het volgende geschreven: > 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 > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information >