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Coating Question


M Joel W

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Does a coating that looks blue mean a lens is warmer and vice versa? It's counterintuitive because a blue filter makes the image more blue.

Do blue flares correlate with a warm image?

Do warm flares correlate with a cool image?

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Good question.

I'm not sure there is a direct correlation between the colours you see reflected from coatings and the transmitted colour cast of a lens, particularly multi-coated lenses where you might see a variety of colours reflected.  Even though each glass element in a lens would have the same multi-coating applied, you often see different colour reflections at different internal surfaces because there is a complex combination of interference cancelling going on. How it all affects the transmitted colours is probably equally complicated.

I have a Leica Summilux-C on my bench now for example that has reflections that vary from green to purple to cyan, which if you simply assumed were the colours the coatings reflected, would mean the lens should transmit more warmer colours. Yet Summilux-Cs are usually regarded as being cooler than average.

Certainly the coatings are affecting the colour transmission in some way, but we might need someone more qualified than we are to explain it to us!

 

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Thanks, good point about different reflections for different surfaces. I hadn't considered that. Or I'd assumed it was due to different coatings on different elements but that doesn't make much sense...

I did get a 2x extender attached to the 12-240mm Angeniuex btw and it almost covers S35. In the middle of the range there's a bit of vignetting but very minor.

But literally this has the worst optical performance I've ever seen. I still don't have any idea how Kubrick got his to look as good as it did.

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Anecdotally, a lot of older lens coatings in my collection like Pentax Super Takumars look warmer and low contrast compared to their more modern counterparts like the Pentax SMC. And the same relationship seems to hold true for vintage lenses like Cooke Speed Panchros vs more modern Zeiss lenses, which have very high contrast coating with purple and green/cyan reflections. This leads me to believe that the color and contrast which you see looking into the front of the lens will be generally similar to what you get out the back, so to speak. 

That said, I have seen some older lenses (mostly telephoto, I think?) with hazy blue coatings, so it would be interesting to compare the color on those.

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My old Zeiss (pre-T*) seem to reflect gold and purple. Newer Zeiss (T*) blue, teal, cyan, and yellow. (Haven't compared color cast as they are for different formats.) Other multicoated lenses appear somewhere in-between. 

Old lenses flare rainbow so I have no idea how that works.

I just read that Zeiss's new T* blue coating flares blue but slightly warms the image. So maybe that's where I got the original idea.

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As much as I understand optics coatings do not affect colour rendition of a lens to any extent greater than 0.01 permille or something of that order. Think, the thin layers are there to improve transmission, not to cut anything back. What works on the colours heavily is the number of elements, the glasses as well as the colour corrections. Basically, dense glasses cut on ultraviolet, violet, and some blue. Triplets generally are colour corrected from blue-green on through to red, they can’t depict blue in the same plane with the rest of the spectrum.

A Tessar-type lens with its strong rear cemented achromat allows a designer to align the entire spectrum already satisfyingly. From five elements on you have additional liberties, six elements being the most widely used formula up to around f/1.4. If you pull in the classic Baltar six-element lens, a relative of the Opic by TTH and Zeiss’ Biotar or the Xenon by Schneider, the heavy flints deduct so much from the short wavelength end that images tend to look alike. To see what I mean, compare an older non-bloomed system such as Dallmeyer’s f/0.99 or a Cooke anastigmatic lens with a mid-century Angénieux M 1 for example. That has double the number of elements, eight vs. four. All the glass renders the picture warmer due to less blue light. A zoom lens of sixteen to twenty glass feels still warmer.

Of course, with the blue sky in the frame, the subject changes to futile. We are used to have the sky above us, meaning in the upper part of the image, so our perception of the scene goes: everything fine, in spite of a possible few percent less blue.

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1 hour ago, Simon Wyss said:

As much as I understand optics coatings do not affect colour rendition of a lens to any extent greater than 0.01 permille or something of that order. Think, the thin layers are there to improve transmission, not to cut anything back. What works on the colours heavily is the number of elements, the glasses as well as the colour corrections. Basically, dense glasses cut on ultraviolet, violet, and some blue. Triplets generally are colour corrected from blue-green on through to red, they can’t depict blue in the same plane with the rest of the spectrum.

A Tessar-type lens with its strong rear cemented achromat allows a designer to align the entire spectrum already satisfyingly. From five elements on you have additional liberties, six elements being the most widely used formula up to around f/1.4. If you pull in the classic Baltar six-element lens, a relative of the Opic by TTH and Zeiss’ Biotar or the Xenon by Schneider, the heavy flints deduct so much from the short wavelength end that images tend to look alike. To see what I mean, compare an older non-bloomed system such as Dallmeyer’s f/0.99 or a Cooke anastigmatic lens with a mid-century Angénieux M 1 for example. That has double the number of elements, eight vs. four. All the glass renders the picture warmer due to less blue light. A zoom lens of sixteen to twenty glass feels still warmer.

Of course, with the blue sky in the frame, the subject changes to futile. We are used to have the sky above us, meaning in the upper part of the image, so our perception of the scene goes: everything fine, in spite of a possible few percent less blue.

Thanks, Simon. Of course, as regards ratio of what spectra are transmitted, improving transmission selectively is effectively cutting back whatever isn't improved, but point taken. This is way over my head, though what you've written makes sense. I appreciate the detailed response and will continue to trust my own eyes rather than my weird theories.

Zeiss does state that the additional warmth of the blue t* coating is trivial, whereas the blueness of the flares is not. And it makes sense that lens design is a bigger factor...

How different is a Baltar from a Gauss-Tachar from a Schneider Xenon from a Biotar from a Primoplan? (Or those may not be equivalent designs, but of similar vintage designs from different manufacturers.) I notice the Xenons going for $300 on eBay and some of the others going for twenty times that. Curious what justifies that big a difference in price.

Is a 50mm Cine-Xenon that different from a Cooke S2 or Baltar?

Edited by M Joel W
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Feared that question to come. I don’t have the data for comparison. The six-glass class is huge, a list to work oneself through seems pointless. In older days, people observed what’s sharp in a picture, today bokeh has almost more weight.

The main difference between Baltar, Opic, and Xenon from younger lenses lies in the glasses used. After the unpleasant adventure of thorium oxide melts that turn yellowish brown new glass sorts were developed and still are being. Refractive indices of 1.65 were in practice not exceeded until 1940. But as you said, the very design plays the main role. It’s much more interesting to observe what comes out a Kino-Plasmat besides the original Plasmat or the difference between a long focal length Tessar and, say, a mirror telephoto. Else, modern lenses are calculated computer aided. Many more iterations are performed than when they had to do the math with a slide rule. Cooke S 2 belong to the pre-war era.

If you have the guts, shoot a portrait with a monocle lens, perhaps better on black-and-white film. That’s something different.

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