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 has been a very well documented phenomena and fix for Thorium lenses. Thorium is a gamma ray emitter and the affected elements have the coloration all the way through the element, not just on the outside. It's also interesting that just about every screw mount Pentax 50/1.4 lens has yellowed, to a degree, with the exception of the original 8 element version which does not contain Thorium. Also every report I've seen reported that UV cleans up the problem. Len On Nov 10, 2007, at 2:43 PM, Philippe Orlent wrote: > 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 >> > > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information