[GUB]

I have 3 yellowed Super Takumar 50mm 1.4 lenses as well as a bunch of other 50s.
I got all enthused by the thread about the cheap UV light to bleach thorium glass lenses here
Yellowed Thorium Glass Lens FIX- Cheap (under $20US), Fast EASY - PentaxForums.com
And got myself one.
But I thought I would do some comparison shots first to see the difference. And I also had a suspicion the yellowed lenses were a little softer.
But the test showed this was not the case.
Given this and considering the distinctly different rendering of the yellow ones I don't think I will go ahead with the bleaching.
What do you think?
The shot of all 9 lens in the sample was taken with a ringlight and is intended to show the degree of yellowing. The lenses are in the same order as the test.

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[swanlefitte]

How does the t stop compare? Do they meter the same? I would think you might get 1/3 stop light. What about lens flare? If it is uniform a wb correction is easy so less a technical factor.

[GUB]

Those histograms at the bottom are of the cropped area, and as you can see the A is about half a stop faster at the dark end and over a stop faster at the light end. So I am thinking like a half to two/thirds a t stop difference. That is if the vertical lines on the curves tool represents a stop and I am not sure about that.

If it is uniform a wb correction is easy so less a technical factor.

More a contrast tweak surely.
I was a little surprised the A series had that lowered contrast but maybe the highlight was slightly clipped. That means waste light floating around.

[stevebrot]

FWIW, the yellowing is due to a change in the chemical nature of the glass and effects the ability to refract light. Bleaching should improve the optical performance or at least return it closer to the designer's intent. My experience has been an improvement in both contrast and sharpness with each of the five lenses I have treated using the Ikea lamp. I would expect similar improvements using the UV lamd and would definitely treat (bleach is not a proper description of what the light does) all but the top row. That last assumes that the colors in the first photo are correct. Given that the Pentax-A 50/1.4 also shows yellowing, I have my doubts. What were the white balance settings? (...or if from RAW, how was the color temp applied in post determined?)


Steve

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Last edited by stevebrot; 01-04-2020 at 10:02 PM.

[GUB]

FWIW, the yellowing is due to a change in the chemical nature of the glass and effects the ability to refract light. Bleaching should improve the optical performance. My experience has been an improvement in both contrast and sharpness with each of the five lenses I have treated using the Ikea lamp. I would expect similar improvements using the UV lamd and would definitely treat (bleach is not a proper description of what the light does) all but the top row.


Steve

That was my exact thinking until I did the test and could perceive no difference in sharpness.

[GUB]

That last assumes that the colors in the first photo are correct. Given that the Pentax-A 50/1.4 also shows yellowing, I have my doubts. What were the white balance settings?

The shot was courtesy of a pentax 080 ringflash onto white photo copy paper. Settings were 5000K and Natural custom image.

[stevebrot]

That was my exact thinking until I did the test and could perceive no difference in sharpness.

Test the lens against itself, pre vs. post.


Steve

[stevebrot]

Settings were 5000K and Natural custom image.

What are your Flash WB settings? (Default is Auto WB.)


Steve

[GUB]

What are your Flash WB settings? (Default is Auto WB.)


Steve

Yes set to Auto WB (once I found it!)
I opened the jpg in Darktable and it indicated the colour temp was 6500K.
Surely the result in the shot is relative to each other. (That is what I intended and why I didn't obsess on colour temp.)
And note the background paper outside of the lenses - I would say the result is on the cold side.
EDIT

In reality the entire image circle of the A series is clipped so is neutral white.
I put the dropper on the unclipped crescents of the three 8 element lenses and they showed a slight green bias -- sort of around 80 82 80.
And the three 7 elements and the two s-m-c were biased to the red side of yellow -- sort of 90 80 70.

[GUB]

Test the lens against itself, pre vs. post.

Yep intend to do that -- probably the one without a filter ring which is a s-m-c (I think) and one of the sup tak 7 elements.
But I am not expecting them to get sharper than the 8 elements. Are you?

[GUB]

(...or if from RAW, how was the color temp applied in post determined?)

They are ex camera jpgs - probably close to default. Apart from the need of consistency in tests I am invariably in Raw.

Hey Stevebrot - ask new questions rather than edit the old ones - it gets confusing!

[hypermodern]

If you like gold-tinted spectacles then you could treat all but 1 or 2. Just need to remember not to leave your camera on AWB.

[Dartmoor Dave]

I really regret de-yellowing my 50mm/1.4. Yes I ended up with a sharper and more contrasty lens, but I also lost the lovely romantic glow that it gave to photos. The rendering style of the de-yellowed lens had lost any appeal to me personally, and I ended up getting rid of it because I was never using it anymore.

If you're determined to de-yellow, I'd recommend doing it in stages, pausing to take test shots as it gradually gets clearer and only going all the way if you're sure that's what you want. You can de-yellow a lens in a couple of days, but it takes decades to turn it back into what it was before you messed with it.

[GUB]

I really regret de-yellowing my 50mm/1.4. Yes I ended up with a sharper and more contrasty lens, but I also lost the lovely romantic glow that it gave to photos. The rendering style of the de-yellowed lens had lost any appeal to me personally, and I ended up getting rid of it because I was never using it anymore.

If you're determined to de-yellow, I'd recommend doing it in stages, pausing to take test shots as it gradually gets clearer and only going all the way if you're sure that's what you want. You can de-yellow a lens in a couple of days, but it takes decades to turn it back into what it was before you messed with it.

And so even more so considering I have the 8 elements on hand.
I was quite surprised at the results of the comparison and now I will probably reach for the 7 element for portraits.

[Kozlok]

I’d also look at f2 and f4. These lenses have a lot of spherical aberration that causes the soft glowy images wide open, which may be stronger than the effect of the yellowing.