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Forum: Pentax SLR Lens Discussion
01-28-2016, 01:05 PM
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Thanks for jumping in! I like your observation regarding the spotty occurrence of radioactivity in the 55/1.8. I have two of those lenses (ser. 832885 and 3637518) and only the newer lens had yellowing.
Steve
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Forum: Pentax SLR Lens Discussion
01-27-2016, 07:30 PM
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I used to think it was glue, but there was work by a forum member a few years back showing that the non-glued elements were stained. Apparently activation of so-called "color centers" happens with when the radiation interacts with additives in the glass. The color depends on the additive. The same thing may happen with natural crystal (amethyst is a good example).
As for the radiation, you are correct in that it is the long-lived daughter isotopes that cause the gamma and beta emissions.
Steve
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Forum: Pentax SLR Lens Discussion
01-27-2016, 12:53 PM
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I think you are probably on to something there. I did a little research on radiation-induced color in quartz and glass a few weeks ago and it may be that the heat or IR from the sun/lamp may be doing the actual work.
Google: "radiation glass color centers"
Steve
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Forum: Pentax SLR Lens Discussion
01-26-2016, 01:45 PM
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Your "after" looks a lot like the best I was able to achieve with my ST 55/1.8 and Auto-Rikenon 55/1.4 using the famous IKEA JANSJÖ desk lamp.* You may be able to bleach it down further, though I suspect there are limits.
Steve
* Yes, the JANSJÖ does work. Why is up to speculation...
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Forum: Pentax SLR Lens Discussion
01-14-2016, 09:57 PM
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Both are great lenses and were among the best f/1.4 lenses you could buy at the time and both are still top tier in the full array of available vintage glass. That being said, there is a mystique surrounding the 8-element lenses. It was very expensive to produce and was only made for a short time before it was replaced by the easier to make 7-element version. For collectors, the 8-element is a special prize that appears on market sort of randomly. The reason, of course, is because their is no external badging indicating it is special.
Is one better optically? I really can't say, having never seen comparative photos between good examples of both. :o
I suspect that one assembly line was brought up as the other wound down.
Steve
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Forum: Pentax SLR Lens Discussion
01-14-2016, 05:18 PM
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No, the aperture ring is not atypical for an early 7-element lens. The photo below is from the review for the 7-element lens on this site:
Note the A/M switch markings, the "2" on the aperture ring, and the IR index mark. All are the same as the OP's lens.
Steve
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Forum: Pentax SLR Lens Discussion
01-14-2016, 05:15 PM
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I agree that "hybrid" is an overstatement. There are a couple of variants of the ST 50/1.4* and you have one that fits the description on the auction. As noted by boriscleto, it is a 7-element lens and typical for an earlier example of such. The more desirable 8-element version would have the red IR mark to the right of the "4" on DOF scale and the rear element would protrude slightly from its guard. It is the location of the red IR mark that is diagnostic.** The description on this site for the 8-element lens is not correct in regards to the A/M switch.
Steve
* The style of lens barrel often reflects the status of parts bins on the line during assembly. Many of the parts are interchangeable and various combinations of rings and markings are not uncommon. I have owned three examples of the ST 55/1.8, none of which were completely alike. One was rather unusual in that the aperture ring direction is opposite the later Pentax direction in same manner as the Auto-Takumar 55/1.8 that came before. That lens is not in the variants noted in the review article on this site. Go figure...
** The IR refractive qualities of the 8-element design were different than the 7-element, hence the difference in the IR index.
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