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02-08-2012, 02:00 PM   #16
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Looking through several lenses and filters there were many different hues. The only pattern I could see is that the only lenses that gave a neutral, color correct image were the very best and the very worst of them. I assume with the best, they were reflecting little light (some only had a very faint ghost image) and were doing it evenly between the colors, and with the cheap ones, I can only assume that they were reflecting a lot of light but doing it evenly between colors. I saw good lenses and iflters will all diffrent kinds of hues as well as cheap ones with all different kinds of hues. One of my hoya hmc filters gives a green hue. my best guess, if all the layers are perfect thickness, it gives a neutral image, and if one or more layers is even slightly imperfect, it causes a hue. Seeing the way these things are mass produced, even the good ones, and guessing that even slight variation in the thickness would cause some color cast, I'm guessing it just varies what the color case is by which coating is the most out of spec, even if just slightly so. Perhaps, with certain brands, and their manufacturing technics, there is always one layer that is out more, so that manufactures lenses tend to have that hue. Perhaps in some cases, it is even intentional. Maybe getting perfect coatings with no hue is nearly impossible and only the rare sample will achieve it, so they intentionally sway the color cast to a color that will give the most pleasing image. Maybe that purple hue gives the least negative effect so its what many better manufactures try for

Of course I'm just guessing on some theory's here.

02-09-2012, 11:43 AM   #17
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QuoteOriginally posted by vonBaloney Quote
Maybe not, if you consider the world of all possible coatings. In the real world, most camera lenses I've seen appear purplish or yellowish or basically clear. The multi-coated lenses are always the purple ones.
That's just Pentax's recipe. You can take my word for it or not, your choice. What I'm telling you is that color has no direct link to coating quality. It's a corollary.

QuoteOriginally posted by ripit Quote
Maybe getting perfect coatings with no hue is nearly impossible and only the rare sample will achieve it, so they intentionally sway the color cast to a color that will give the most pleasing image. Maybe that purple hue gives the least negative effect so its what many better manufactures try for
They do not sway the color in any way, nope. They design the coating to perform the way they want, and that yields some color cast in the reflection because no coating is perfect, but they don't care about the color, itself.

A coating's job is to help light pass through. Without a coating, any interface reflects roughly 4% of the incident light. For a single lens (think loupe) that means 2 interfaces, 8% loss. For a classic 50mm, which is a doublet, that means 3 interfaces, 12% loss. That light is reflected back the way it came (I'm neglecting multiple reflections, to keep it simple). A coating will limit the reflection of one wavelength (or color). Multi-layers coatings, if well done, can let a brand range pass through more or less flatly, Here's an example of the response of a bandpass filter that I pulled randomly from the web :



That's a pretty good one, actually, Quite flat. Not perfect, though. Here's another one:



That's a high-pass filter, different behaviour and response.

Those would have different colours, but could be used in similar applications.

Filters and coatings are a very complex world, trying to reduce them to colors when seen with the naked eye does not tell you much about them.
02-09-2012, 12:18 PM   #18
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QuoteOriginally posted by bdery Quote
That's just Pentax's recipe. You can take my word for it or not, your choice. What I'm telling you is that color has no direct link to coating quality. It's a corollary.
I'm not talking about only Pentax lenses, or even primarily Pentax lenses. Nor I'm I contradicting you -- if the link is direct or indirect, it still exists. Are you telling me that there are clear-looking lenses that have coatings as good as the purplish coatings? Or light yellowish orange? If so, please name them. I'm not talking theory -- I'm talking actual lenses. If you have some examples of real lenses with excellent multi-coatings in odd colors, please list them.

All the modern multi-coats are dark -- basically purplish, but depending on the reflecting light they can also appear dark green or magenta. But they are dark, right? (Now I am talking theory.) The color of the reflection is dependent on the properties of the coating -- it must be. After all, when you look THROUGH the lens they look clear with no color cast (if they are any good) -- they aren't supposed to change the basic light qualities but some lenses are known for different color rendition than others, and surely the coating is a part of that. So all else being equal, I would still assume that a nice dark purplish coat is better than a light blue or amber.

P.S. I was just looking at an article about coatings that said that some single-coats do appear light bluish, so I guess they are out there.
02-10-2012, 06:35 AM   #19
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QuoteOriginally posted by vonBaloney Quote
Are you telling me that there are clear-looking lenses that have coatings as good as the purplish coatings? Or light yellowish orange? If so, please name them. I'm not talking theory -- I'm talking actual lenses. If you have some examples of real lenses with excellent multi-coatings in odd colors, please list them.
I don't have experience with enough different photographic lenses to list all examples, sorry.

A perfect coating would make the lens look transparent. which is the case when you view the lens more or less straight.

Also remember that in the present case we are looking at the back of the lens, which is not the way light travel through the lens. The back coating is designed to improve light transmission from a glass component to air, not from air to a glass component.

The Vivitar s1 70-20 is a 1978 lens. Its coating looks quite like that of the A50 f1,4 and K135 f2,5 and FA50 macro. And Takumar baoynet 135 f2,8. Those are quite different lenses, with different glass types probably, and different perceived levels of quality. So you can't determine much by the colour of their coatings.

QuoteOriginally posted by vonBaloney Quote
All the modern multi-coats are dark -- basically purplish, but depending on the reflecting light they can also appear dark green or magenta. But they are dark, right? (Now I am talking theory.)
I don't understand what you mean by "dark".

QuoteOriginally posted by vonBaloney Quote
The color of the reflection is dependent on the properties of the coating -- it must be
QuoteOriginally posted by vonBaloney Quote
when you look THROUGH the lens they look clear with no color cast
Indeed. You begin to see colours when you look at the filter from an angle. And that's not its normal use case, so any behaviour can occur in that case.

If a coating looks like it's a particular colour, it means it reflects more light at that wavelength, so in essence it's flawed at that wavelength. Since all the photo coatings are designed to let visible light through, it is to be expected that they will look somewhat the same. I confirm that this does not mean the MUST be, however.

QuoteOriginally posted by vonBaloney Quote
some lenses are known for different color rendition than others, and surely the coating is a part of that
A part, though most of those differences come from the choices of glass types for the various glass components.

QuoteOriginally posted by vonBaloney Quote
So all else being equal, I would still assume that a nice dark purplish coat is better than a light blue or amber.
QuoteOriginally posted by vonBaloney Quote
I was just looking at an article about coatings that said that some single-coats do appear light bluish, so I guess they are out there.
They will appear bluish if they are made in a way that lets them reflect blue light. You can design a coating to do pretty much anything you want.

02-10-2012, 08:18 AM   #20
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QuoteOriginally posted by bdery Quote
I don't have experience with enough different photographic lenses to list all examples, sorry.
Or any, apparently.

QuoteQuote:
A perfect coating would make the lens look transparent. which is the case when you view the lens more or less straight.
Not talking about looking through the lens, but looking at the coatings, i.e. rotating in around in your hand at an angle under a light -- looking mainly at the reflections of the lights themselves -- single coats usually will return one color, multi-coats often several.

QuoteQuote:
I don't understand what you mean by "dark".
Dark, as opposed to light -- the better coatings are usually more opaque (at an angle) -- they look like filters almost -- you'd think they'd block light by their appearance. I'm surprised you don't know what I mean -- maybe you've never looked at older lenses with single coats? They look different. This is what I'm talking about theory vs practical experience. If you pick up a couple of lenses in the thrift store you don't know much about, how do you make an educated guess as to the quality of the coating? Better coatings look darker, usually purplish to appearance, and when you start looking at the reflections you'll see green and magenta separately, whereas a single coat usually will only reflect one color, often amberish. And some lenses just look like old window glass, with basically neutral/white reflections. Can you be *sure* because after all, a coating *could* be made to look like anything? Of course not, but that's WHAT MOST LENSES ARE REALLY LIKE, so it is good to know.
02-13-2012, 06:31 AM   #21
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QuoteOriginally posted by vonBaloney Quote
Or any, apparently.
This is bordering on rude, and does not bring anything to the discussion.

QuoteOriginally posted by vonBaloney Quote
maybe you've never looked at older lenses with single coats?
I did, thank you very much. I even mentioned it before.

I have tried to provide a crash course on some lens design elements, if you're not interested in them, or if you don't trust my training and experience (about which I'm not interested to brag), then feel free to move along.

I'm only trying to impress that it's dangerous to assume colours are a reliable quality assertion system.

Have a nice day.
02-13-2012, 09:57 AM   #22
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QuoteOriginally posted by bdery Quote
This is bordering on rude, and does not bring anything to the discussion.
As is your condescending and dismissive attitude. What did you expect?
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