Dan Margulis Applied Color Theory

A "Why Is the Sky Blue?" Question

A couple of "why is the sky blue" questions
Posted by: Mike Russell
Tue Jan 22, 2008 4:31 am (PST)

Curious minds want to know:

1) Why can the eye accommodate color casts, even mixed lighting situations in natural scenes so effortlessly, but not when confronted with an uncorrected photograph with a cast. As an example, why do shadows appear blue on a photograph, but not to the eye when viewing the original scene?

2) Is there an explanation based on chemistry, physics, or optics for why cyan is the weakest ink?

Mike Russell - www.curvemeister.com
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Re: A couple of "why is the sky blue" questions
Posted by: Howard Smith
Tue Jan 22, 2008 8:35 am (PST)

Mike, it's not a matter of the eye being fooled by two different views of the scene, but a matter of the eye not being fooled at all. The brain can re-interpret the faint blues in the shadows but not in the photograph because of the difference in intensity of the color being viewed in two different ways. The natural colors recorded by the camera are altered by the camera, just as they would be altered in our interpretation of the original scene if the scene was lighted with light that had a strong blue component to it. If the blues were enhanced by lighting in the natural scene, the eye would not be able to ignore the cast. It's a matter of relativity. Because the blue cast shows up in an image recorded by a camera, we naturally think it was recording something that was in the scene but ignored by our vision. In fact it is strengthened by the photographic process (or by the printing) and does not represent natural lighting.

I might add, since someone is sure to bring this up, that the eyes can accommodate to a cast whether it's in real life or in a photo. But the stronger the difference in the cast between two different views (photo vs. natural), the harder it is to accommodate for the stronger one.

Howard Smith
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Re: A couple of "why is the sky blue" questions
Posted by: "RJay Hansen"
Tue Jan 22, 2008 8:35 am (PST)

I seem to recall having read somewhere that a more spectrally pure (is that the right terminology?) cyan ink could be produced but it would be cost prohibitive... which would make it a chemistry issue I suppose. I may be entirely off base on this though.

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Re: A couple of "why is the sky blue" questions
Posted by: "William Pratt"
Tue Jan 22, 2008 9:09 am (PST)

I'm not going to offer a general explanation, but just a couple of observations:

First, people are amazing when it comes to seeing what they expect to see. In real life situations, expectations almost always have an influence on people's perceptions.

Second, once you are aware of types of casts generated by differents sorts of lights, you can also start to see the casts more easily in real life situations. For example, interiors of subway stations in NY now look greenish to me because of the flourescent lighting. At sunset, I can pretty well make out the bluish tones on the shadow side of my white dogs, and the warm tones on the lit side.

Third, the ability to adapt to a cast will depend on a person's field of vision and on the light source. If you could make a person view a print so that it occupied his entire field of vision, then the adaption would more likely kick in when viewing the print. This happens pretty readily when viewing a monitor. It's much easier to adapt to a cast on a monitor, because people tend to get immersed in the monitor's light.

Duffy Pratt
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Re: A couple of "why is the sky blue" questions
Posted by: Henry Davis
Tue Jan 22, 2008 9:53 am (PST)

As to 1) I think that Dan has elaborated on how amazingly adaptive our viewing system is, but there is also an assumption that film captures images exactly as they are perceived. Film is only an approximation, a compromise that fails in providing the true experience.

We are not "within" the photograph, thus we are not adapting to the actual scene. When we look at a photograph, we are looking at an object that represents some scene that is removed from its actual and proper place of existence. We can only adapt to the scene we are within. Perhaps this is a good thing - imagine if photography could transpose us to another state of existence. Once we were caught by a television, we would be trapped forever!

The photograph begs us to believe that it is the reality that it pretends to be, but we know that it is only a representation. We see pictures of the Grand Canyon and ask ourselves if it really looks like this. Then we visit the actual place and find that it is even bigger, and more "real" than we expected. Of course it's more real!

As to 2) I'm not so sure that this is a bad thing. If cyan were a purer, less contaminated ink, would this be better? (This is just a piggyback question to add to yours).

Henry Davis
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Re: A couple of "why is the sky blue" questions
Posted by: Ron Kelly
Tue Jan 22, 2008 11:01 am (PST)

Mike:

I don't have any answers to offer, but while we're on the subject of curious differences between the human perceptual system and the camera I have a question to add: why can the visual system so easily compensate for geometrical anomalies in real time, but the camera/print is locked into a fixed perspective?

I'm talking about rectangular objects, for example, like buildings. As I walk down the street and I gaze skyward from a long or moderate distance, there's no "distortion" as my rational brain knows (and my visual system confirms) that the buildings are vertical. However, even a moderate incline of camera gives keystoning that is painfully obvious.

As you approach a tall building very closely, when you look up you see the recession of the upper floors in the distance, as you would expect. The ground floor is now fifty times closer to you, and the top floors recede in the distance; the size difference is undeniable to the visual system. This, too, seems "normal".

In the middle distance, however, there's no conflict. The mind seems to compensate seamlessly between these two extremes, without a catastrophic moment of adjustment. How can this
be so?

Ron Kelly
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Re: A couple of "why is the sky blue" questions
Posted by: J Walton
Tue Jan 22, 2008 11:59 am (PST)

On Jan 22, 2008 10:48 AM, Ron Kelly wrote:

I'm talking about rectangular objects, for example, like buildings.
As I walk down the street and I gaze skyward from a long or moderate distance,
there's no "distortion" as my rational brain knows (and my visual system confirms) that the
buildings are vertical. However, even a moderate incline of camera gives keystoning that is painfully obvious.

That depends on the camera and how it is set up. It's possible to minimize or even eliminate that distortion on certain cameras, so I don't think it's something peculiar to our visual system but rather something with the way many cameras work.

--
J Walton
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Re: A couple of "why is the sky blue" questions
Posted by: "David Barrack"
Tue Jan 22, 2008 11:59 am (PST)

I would recommend subscribing to "Scientific Mind", from Scientific American. It's an ongoing dialogue about the human mind and touches things like this.

David

David Barrack Photography and Web Productions
www.davidbarrack.com
+1.404.805.0888
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Re: A couple of  "why is the sky blue" questions
Posted by: "Fred D Yocum"
Wed Jan 23, 2008 6:03 am (PST)

I have had first hand experience that the mind seems to do a remarkable job of bringing together what we see and what we know. When I studied painting with a student of a student of student of Monet, I had to be forced to 'see' the actual colours I was viewing. Only by slapping pure blue paint into the shadows on the canvas could I begin to see the amount of blue which was actually there in the scene I was painting. We know that earth is brown, our dog is white, grass is green and mentally smooth out the colour extremes. Because the camera is one step removed from reality, we see what our mind ignores, and it looks wrong.

At the moment, I am working with a photograph of a figure in a hallway lit by warm light -- everything in the scene is coloured by the strong peach/auburn colour bouncing off the walls. It is very evocative. Should I pull the scene all the way back, making the flesh tones more natural or acknowledge what makes the scene so powerful, the harmonies created by the strong dominance of one set of colours. Or somewhere in between?

Fred D
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Re: [colortheory] Re: A couple of  "why is the sky blue" questio
Posted by: Ron Kelly
Wed Jan 23, 2008 3:39 pm (PST)

Fred:

I have worked hard at color correction for many years now. One area that I would say I'm weak in is preserving a "cast".

Why? I don't know. It's strictly an aesthetic choice. However, even knowing that, I can't resist over compensating for strong casts like the one you mention. When I print them, they don't look colored so much as off-color. I am aware, however, that there are many pictures that I'm not portraying well.

How do I get over this?

Ron Kelly
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Re: A couple of "why is the sky blue" questions
Posted by: John Naman
Wed Jan 23, 2008 3:39 pm (PST)

Henry's note about not being "within" the photograph is important. An Adobe technical paper on Rendering the Print: the Art of Photography by Karl Lang discusses visual perception: the eye "moves around very fast focusing and identifying various parts. As we do this the iris will adjust the light levels for each part of the scene so we can see into the shadows and find detail in the bright areas. We may note that one area is dark or another really bright; however, at the time we scan each element, the photoreceptors are receiving an optimal exposure." The "adaptation" to color cast comes about as a result of preprocessing, i.e., the body's own version of camera raw and HDR. William Pratt has started noticing casts because he is looking directly at shadows, NOT the entire scene. One test might be to look through a paper towel tube or even a telephoto into shadows and see if you don't see different colors than when you view the entire scene "wide angle" without a camera or tube.

But the short answer is, eyes merge and blend together a hundred different micro-photos while cameras grab the whole scene. If photographs could be more 3-D, the eye might have to focus in and out and shift around more, thus better approximating the way the photographer viewed the scene. Hope this helps,

John Naman, PhD
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A couple of "why is the sky blue" questions
Posted by: "Ray St.Arnaud" r
Wed Jan 23, 2008 3:39 pm (PST)

As an example, why do shadows appear
blue on a photograph, but not to the eye when viewing the original scene?

The shadows are blue. The camera records what is there. In the case of a 3D object in nature, there are 2 light sources, the sun and the sky. On the shadow side of an object, the fill color is sky blue.

Our eye adapts to this sensation, the camera cannot. The long used correction for cameras was to shoot through a pale yellow filter, neutralizing the blue sky fill light.

This is most controversial with snow shadows which appear blue in photographs. Should they be changed to gray, or left as blue?

Regards
Ray

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I see the transition from “art prints centric” to “art book centric” as simply an extension of making art. You should think of these books, just like the prints, as an alternate form of art work, rather than as books, a commodity. -Raymond St Arnaud-
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Raymond St Arnaud, Victoria, B.C., Canada
Photography and Digital Imaging
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Re: A couple of "why is the sky blue" questions
Posted by: Dan Margulis
Sun Feb 3, 2008 8:41 pm (PST)

Before I left for vacation, Mike Russell wrote,

1) Why can the eye accommodate color casts, even mixed lighting
situations in natural scenes so effortlessly, but not when confronted with an
uncorrected photograph with a cast.

I am sorry to have missed this thread because it relates to a lot of the issues that we commonly face in color correction.

The human visual system is amazingly complex. It can apparently cope with any number of casts simultaneously. It rejects cross-lighting (which is why we have to correct such images when they arise). Also, we ignore mini-casts caused by reflections from colored objects--I show two examples of that in the first two chapters of PP5E. One is of a woman wearing a purple blouse, which casts a purple reflection on her chin that the camera sees but we don't. The other is of a black ape in the jungle. The camera sees the green reflections off the leaves, we don't.

In researching the new workflow I was surprised by how often these mild casts occur. The workflow calls for eliminating them even if they aren't noticeable in the original, on the theory that they may become objectionable as we try to intensify colors. Even forgetting those images that had an apparent color problem, more than half of all the images I tested had *some* kind of trivial color issue worth correcting.

As an example, why do shadows appear
blue on a photograph, but not to the eye when viewing the original scene?

This is actually a different issue. Leonardo da Vinci noted this effect--that humans neutralize shadows even where the lighting suggests that they must be colored. When I demonstrate the new workflow, the examples that draw the most oohs and aahs extend that principle. I believe that the neutralization effect starts earlier than the shadows and that humans generally see quartertone-darkness areas as being more saturated than a camera does and three-quartertones as less saturated. Accordingly, my methods exaggerate colors in lighter areas and go to some lengths to retard them in the darker half of the image.

2) Is there an explanation based on chemistry, physics, or optics for why
cyan is the weakest ink?

I know the historic reason and believe that it still applies. Cyan colorants have historically been extremely expensive. Any painting from about the twelfth to sixteenth centuries that features vivid blues is almost certainly church-supported, because the pigments were based on the very costly lapis lazuli gemstone. Only the Vatican and similar repositories of wealth could afford to allow their artists to use it.

I found an interesting extension this year while museum-hopping. From time to time we see recommendations that those who are preparing files for CMYK shouldn't use sRGB as a working space because--in theory--a large number of colors are printable on press yet outside of the sRGB gamut. I've pointed out that the argument is fallacious because these colors are mostly deep, rich cyans, which basically don't exist in nature. I've seen naturally occurring yellows that are printable yet outside of the sRGB gamut, but we have such little perception of detail in bright yellows that it doesn't matter. And, as of the last time we had one of these piefights a bit over a year ago, I said that in the last five years I'd been looking for either cyan or magenta objects that might meet this description, and had yet to find a single one.

I have now. In the 18th century, the Sevres factory in France produced the most opulent porcelain that the world had seen before or since. Not surprising, since their patron was King Louis XV, to whom cost was no object. Inlaid gold, intricate artwork, you name it. I may have done the conversion badly but as nearly as I can tell, if you were a contemporary Frenchman wishing to buy a dinner service for twelve of Sevres porcelain, it would cost well over a million dollars in today's money.

One of the best ways to make the product as expensive as possible--which seems to have been the whole idea--was to make lavish use of the most costly colorants. So, many patterns would not be described as having a white base at all, but rather are predominantly an eyepopping color that is misnamed "celestial blue". It's really a cyan, but deeper and purer even than 100c0m0y in good printing conditions.

I will admit to having felt, at various points in my career, that the art director was deliberately trying to make the job cost as much as humanly possible. These art directors, I now know, are pikers in comparison to French royalty.

So, in answer to the question, I suspect that a better cyan colorant is possible, but the price would be such that we would could only get away with it for a little while, and then the guillotines would be brought out, and we would find ourselves back in the equality of mediocrity.

Dan Margulis
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Re: A couple of "why is the sky blue" questions
Posted by: "Mike Russell" r
Mon Feb 4, 2008 4:14 pm (PST)

Thanks, Dan, that was interesting and entertaining reading and much appreciated. Thanks also to everyone else who responded. I am frankly pleased at the interesting and varied set of responses to my humble questions.

The subject of blue shadows, and how and whether to always remove them is particularly interesting to me. In each of curvemeister classes there are one or two students refuse to remove blue casts in shadows, even in the context of an exercise and when there is no aesthetic motivation for the cast.

I am grateful to everyone who responded, and I am still in doubt as to the causes of the failure of the eye's automated color correction in the context of a print. Several of the responses in this thread have provided food for thought.

Mike Russell - www.curvemeister.com