[pathdoc]

If any one of you has (or plans to obtain) a thoroughly yellowed thorium-bearing Takumar lens and has not yet treated it, I'm wondering if you could take note of whether focus lock detection is better/more certain after de-yellowing than it is before. This need only be a subjective thing (unless @Digitalis has already run this experiment on his test bench and knows the answer ).

[jatrax]

Interesting thought. I would have assumed that it would make no difference, mostly a change in WB. But it will be interesting to see if anyone does the test. None of mine are yellowed, so I'm no help.

[pathdoc]

I would have assumed that it would make no difference, mostly a change in WB.

Which reminds me, I must pop my hot Tak on my K-5 and take a RAW picture and see what sort of corrections are needed.

[vonBaloney]

Don't know whether it would make any noticeable difference in focus-confirmation or not, but I've definitely seen improvements in overall sharpness from a de-yellowed lens. (i.e. the effect is more than just a color cast for a heavily yellowed lens)

[grahame]

Never noticed it because all cameras I have before K5II have split or EE-S focusing screen installed.
But I will guess K5 and newer camera should not be affected because the AF sensor was designed to work with different wavelength light.

[stevebrot]

If any one of you has (or plans to obtain) a thoroughly yellowed thorium-bearing Takumar lens and has not yet treated it, I'm wondering if you could take note of whether focus lock detection is better/more certain after de-yellowing than it is before. This need only be a subjective thing (unless @Digitalis has already run this experiment on his test bench and knows the answer ).

Are you having problems with focus? There is no harm in doing the de-yellowing and there is potential for improved optical performance too. I would just do the fix and see if it helps.


Steve

(...had to de-yellow one of my ST 55/1.8 and also Auto Rikenon 55/1.4...Ikea JANSJÖ lamp...)

[stevebrot]

(i.e. the effect is more than just a color cast for a heavily yellowed lens)

This is true. The radiation changes the physical character of the glass resulting in the color shift and, reportedly, changes to the refractive properties of the affected elements.


Steve

[vonBaloney]

This is true. The radiation changes the physical character of the glass resulting in the color shift and, reportedly, changes to the refractive properties of the affected elements.

Yep, the difference in sharpness was obvious when I did a before and after on a rather yellowed one.

[Digitalis]

The radiation changes the physical character of the glass resulting in the color shift and, reportedly, changes to the refractive properties of the affected elements.

According to my tests* the yellowing causes a very slight loss of image contrast, though this is heavily dependent upon the degree of yellowing. Thorium is dominantly an alpha emitter**, so the depth of particle penetration is rather limited. I lack the equipment required to determine if the refractive index of the thoriated lens element has changed*** As my testing has been specifically geared towards the optical properties of the lens as a whole: Testing for contrast,resolution,coma, astigmatism field curvature where relevant.





* Tested: 40 samples of the M42 Takumar 50mm f/1.4 largest contrast drop observed was 22% at MTF=40 @ f/8 - In my test notes this particular lens had a colour shift of 48% Yellow and 17% magenta.
** There are only two known stable isotopes of thorium that are Beta particle emitters, though in this case they are short lived - With increased energy the particles emitted from the isotope results in shorter half-life.
***though if it did: focal length would change over time but the changes would be very slight. Though the problem stands in the way of proper testing is that it is impossible to acquire a brand new Takumar 50mm f/1.4 as a control lens to test the others against.

[pathdoc]

My problem at the moment rests mainly in being unable to lay my hands on a clearly suitable globe for de-yellowing, and to have attempted the process during the rather poor, cloud-cursed light of winter. Now that we are getting more in the way of actual sun, this may change. But I thought seeing as the K-1 seems (by many of its users' lights) to set new standards in Pentax AF, it might make a good test platform for the question.

[Digitalis]

Nice thought, but an LED is unlikely to output anywhere near enough UV on its own for this purpose. You would be better off getting a dedicated UV globe.

[dclauria]

I used a small desk lamp with a CFL (compact fluorescent) blacklight bulb.

[vonBaloney]

Nice thought, but an LED is unlikely to output anywhere near enough UV on its own for this purpose. You would be better off getting a dedicated UV globe.

I have de-yellowed a bunch of them now with a simple LED lamp. I was skeptical also, but it works. And not only does it work, but it works fast. (24 hours is usually plenty.) I jam the lamp practically right into the lens (with foil on the other side to reflect back). This ability to really get the light just where you want it probably accounts for its ability to outperform a dedicated UV bulb that is less directional. Anyway, whatever the reason, it does the job...

[Digitalis]

The LEDS in these lights are (in all probability) cheap GaN/InGaN LEDs - which have a peak emission in the UV spectrum - A dedicated monochromatic UV light source will be more efficient. UV leds would be great for this approach.

you still clinging to your completely wrong assertion that it is the balsam that yellows and not the glass?

I have been awaiting for you to give hard evidence of this. Until you do find compelling evidence that elevates your concept beyond a mere theory: my opinion stands. Molecular chains in polymers require less energy to break, do you not agree? The data I have been able to find on this phenomena is limited to crystal structures, not amorphous glass structures from what I have been able to find. The amounts of energy to deform the molecular bond of a material in its glass state is tremendous. You DO know they vitrify high level nuclear waste in glass do you? If there was even a suggestion that emitted radiation itself could cause a breakdown of the material that is used to contain it is troubling indeed.

[monochrome]

What has me shocked is that no one has done this demonstration before.