Basic question on effects of a filter in B&W

[Pál_K]

This question is about being able to see distant objects through atmospheric haze, such as making a photo of a city from atop a distant mountain (which is exactly what I was doing last week).

In B&W I know that, compared to an image made with no filter, if one were to use yellow, orange or red filters, each filter would allow for successively more resolution (i.e. visibility) of the distant object (e.g. buildings).

But what is really happening? My thought is that, unfiltered, too much blue light obscures the distant details and by using yellow, orange, or red, more of the blue (hazy?) light is blocked, thus allowing the details to strike the film. Is that what is happening?

But what about situations where even looking out over vast distances (20 to 25 miles) from atop a mountain, my eyes see the haze obscuring the city and land beyond but, from other photos I’ve seen, the use of a red filter “cuts through” this haze. How is it possible that the red filter renders something clearer and more visible than I can see with my eyes - or are my eyes over-sensitive to blue just like B&W film? If I look through the filter with my eyes I don’t see any increased detail through haze (Edit: not really sure about this last part).

Thank you in advance for your replies.

 

[Rob-F]

First we have to distinguish between the haze that occurs when the atmosphere is clear to the eye, yet haze appears in the photograph. This happens when ultraviolet light (outside the visible spectrum) causes some partial fogging of the film. For that, a UVa filter generally is sufficient. But when there is haze visible to the eye, it's because there is particulate matter in the air between us and those distant mountains or that distant city. These particles scatter the light and reduce contrast. That's harder to get rid of, because it's really there--between us and the subject. We can try to take advantage of the fact that the longer wavelengths, closer to the red end of the spectrum, are somewhat less susceptible to being scattered by the particles. So a wratten no. 6 yellow filter gets rid of wavelengths shorter than 450 millimicrons, and that's a start. A wratten no. 16, which is fairly orange, knocks out everything shorter than about 515, and that's better. Use a red filter of around no. 29, and all that's left of visible light is the range from 600 to 700 millimicrons, and that's better still for cutting through the haze. The final step is to use infrared film, and a filter that blocks almost all visible light, which my Kodak filter book lists as a wratten no. 70.

So that's the theory, at least. The idea is that eliminating shorter wavelengths reduces the scattering of light.

Edit: I had meant to add that in actual practice, the results can be disappointing!

 

[markjwyatt]

Doesn't the "K" filter basically combine UV blocking and the effect of the yellow filter (i.e., beyond just a plain yellow filter)? They used to have an "aero filter" which was yellow,and may be similar to the "K".

 

[Rob-F]

Doesn't the "K" filter basically combine UV blocking and the effect of the yellow filter (i.e., beyond just a plain yellow filter)? They used to have an "aero filter" which was yellow,and may be similar to the "K".

Yes, a yellow filter will not only block the shorter blue wavelengths, but the even-shorter UV ones as well. The K1, K2, and K3 will all block ultraviolet light.

 

[Ambro51]

Films made for traffic surveillance use see more into the red range of the spectrum, and do a good job of “clearing the air”, Agfa ASP 400 is an example, if a film is “near infrared” capable that’s what I’m describing. You don’t use a filter, these are high speed films with excellent definition.

 

[David Hughes]

I know, vaguely, what Wratten filters are bu what are K filters? I thought they were electronic but as film has also been mentioned I'm baffled; although it would be nearer the truth to say I don't want to go on a wild goose chase when I can just ask and hope someone will explain...

Regards, David

 

[Rob-F]

I know, vaguely, what Wratten filters are bu what are K filters? I thought they were electronic but as film has also been mentioned I'm baffled; although it would be nearer the truth to say I don't want to go on a wild goose chase when I can just ask and hope someone will explain...

Regards, David

I took Mar Wyatt's question to be referring to the "K" prefix of Kodak's designation for its series of yellow filters, including K1, K2, and K3. Those three were called "cloud filters" for their ability to bring out the clouds by darkening the blue sky. Why did George Eastman choose "K" ? Who knows? He liked to make up names for things. He also called his filters "Wratten." So a K2 cloud filter was also known as "Wratten no. 6," for example.

 

[Benjamin Marks]

I think you also get an apparent sharpening effect by increasing contrast, although that's going to have an effect on your negative too. For instance, if you have a red filter on, your blues (like the sky) aren't getting to the film. Skies are therefore darker, clouds "pop" more. So, you may be able to see distant objects more clearly, but the picture is going to have a very different overall tone. I think everyone who grew learning with pan-chromatic film becomes fascinated with this effect at one time or another. But my overall palette is much more in the middle of the spectral response curves these days (compared to when I was younger), and subtle differences or gradations of grey are more interesting to me than the Pow-Wham-Zap! of a high contrast landscape.

 

[Pál_K]

... But when there is haze visible to the eye, it's because there is particulate matter in the air between us and those distant mountains or that distant city. These particles scatter the light and reduce contrast. That's harder to get rid of, because it's really there--between us and the subject. We can try to take advantage of the fact that the longer wavelengths, closer to the red end of the spectrum, are somewhat less susceptible to being scattered by the particles. So a wratten no. 6 yellow filter gets rid of wavelengths shorter than 450 millimicrons, and that's a start. A wratten no. 16, which is fairly orange, knocks out everything shorter than about 515, and that's better. Use a red filter of around no. 29, and all that's left of visible light is the range from 600 to 700 millimicrons, and that's better still for cutting through the haze. …

Thanks - that’s the situation I was thinking of. I wasn’t sure that what is happening is that the longer wavelengths of light are actually reaching the film and thereby create an image that I can’t see with my eyes.

 

[willie_901]

...

In B&W I know that, compared to an image made with no filter, if one were to use yellow, orange or red filters, each filter would allow for successively more resolution (i.e. visibility) of the distant object (e.g. buildings).

But what is really happening? ...
Thank you in advance for your replies.

Diffuse sky radiation occurs because atmospheric gas molecules, water molecules, non-atmospheric gases (natural and unnatural) and particles suspended in aerosols in the atmosphere absorb sunlight and then eventually emit energy as scattered light. The energy (wave length) is preserved but the direction (wave propagation vector) changes. The motion of the atmospheric content responsible for light scattering is Brownian, so the light is diffuse.

The size of the atmospheric content strongly affects the scattered light intensities which changes the scattered light wavelength intensity distribution. When the size of the molecules are smaller than the sun light wave lengths Rayleigh scattering occurs.


However, the size of the atmospheric content determines the scattered light intensities and the wave-length intensity distribution can be different than the distribution in sunlight.




Scattered UV light intensities from atmospheric gas molecules are higher. We can't see the UV light but the film emulsion granules respond. There are a fixed number of light sensitive granules per cubic mm of film emulsion. The scattered UV light means less granules are available for direct light from objects of interest. This reduces contrast. Perceived sharpness is highly dependent on contrast. Contrast is further reduced because the scattered light is diffuse. Light reflected or absorbed and then emitted by fixed objects is directional (higher contrast). A UV lens filter minimizes these effects because the light from objects of interest affects more film granules.

What about yellow, orange or red lens filters?

Water molecules and, or aerosol particles in the atmospheric also scatter sunlight. When these are present at high levels, scattered light from longer wavelengths become important. This explains how come sunset skies appear yellow to red in color. As the sun approaches the horizon the sunlight passes through lower levels in the atmosphere. At lower levels the atmosphere has a high particulate concentration and the intensity of diffuse light from higher wavelengths increases. So, high concentrations of aerosol particles when the sun is high in the sky can decrease contrast as much or more than the scattered light from atmospheric gases.

If the haze between you and the distant cities contained molecules and particles larger than atmospheric gas molecules, their scattered light would decrease contrast. Adding yellow, orange and red filters increases the relative contribution of direct light from the subject (the city) which in turn increases their perceived sharpness. In fact you might be able to estimate relative particle size concentrations in the haze by estimating the change in perceived sharpness as you change the order of adding the yellow, orange or red lens filters.

 

[Rob-F]

Many thanks to Willie_901 for a thorough and rigorous treatment of this subject. Thanks especially for the Scattering vs. particle size plot. I was not aware that the worst scattering occurs when the particle size is in the 200-250nm range. It enhances our grasp of how the choice of filter color affects the image. As the wavelength increases, a given particle becomes a smaller and smaller fraction of a wavelength, and it's reasonable that at some point the particle size becomes negligible with respect to a wavelength. So the plot showing the 200-250nm particle size is the worst offender, really provides theoretical support for the practical results of those who have found that yellow filters help, orange is better, and red is best, as follows. For wavelengths in the UV region, 200 or 250nm would be just about comparable to a full wavelength. Near the violet end of the visible spectrum, a particle that size would be close to 1/2 wavelength, still quite significant. As we progress to longer wavelengths, yellow, orange, and red, the particle size becomes smaller ad smaller with respect to the wavelength, until when we reach red, the particle is now more like 1/3 to 1/4 wavelength. At still longer wavelengths, the particle size eventually becomes negligible with respect to a wavelength, so presumably has no effect at all. This might happen when the particle is perhaps 1/5 of a wavelength or smaller. But by then the light will be outside the range of visible light. Now we are in the infra-red range: 700nm and longer. This suggests that infra-red film, (or a sensor not covered with an IR filter, like the Leica M8) together with a strong filter that blocks nearly all visible light, would be the ideal way to photograph through haze.

Now it's time to test theory against reality. Does anyone know of this method, infrared film, being used successfully to shoot through haze?

 

[Pál_K]

…
Adding yellow, orange and red filters increases the relative contribution of direct light from the subject (the city) which in turn increases their perceived sharpness…

(emphasis mine)

This explanation adds to my understanding of the “cutting through” effect that had been told to me long ago.

Although I was using a UV filter, it apparently was not enough. I’m tempted to try both yellow and orange when I go back there next May.

I stated earlier that I don’t see any improvement when I look through a filter with my eyes - I should retract that, as I’ve never performed that experiment at any great distance with atmospheric haze.

 

[Retro-Grouch]

To further complicate things, remember also that a polarizer is very effective in cutting haze, eliminating some of the light that gets scattered by atmospheric particles. A polarizer in conjunction with a red filter can be extremely effective, in addition to giving a nearly black sky (depending on the direction in which its pointed). This is certainly dramatic, giving you a "full Ansel", but usually too artificial looking for my taste. It's a combination best used sparingly.

 

[Rob-F]

To further complicate things, remember also that a polarizer is very effective in cutting haze, eliminating some of the light that gets scattered by atmospheric particles. A polarizer in conjunction with a red filter can be extremely effective, in addition to giving a nearly black sky (depending on the direction in which its pointed). This is certainly dramatic, giving you a "full Ansel", but usually too artificial looking for my taste. It's a combination best used sparingly.

Gee, I hope I didn't complicate things! 😱 I agree that strong filtration can change the character of a scene in a way that is not necessarily good. In fact, when a hollywood film crew needs to shoot a black and white night scene, but the schedule does not allow staying until night time, they will shoot "day for night" by doing three things: 1, they backlight the scene; 2, they underexpose; and 3, they use a red filter.

 

[David Hughes]

I took Mar Wyatt's question to be referring to the "K" prefix of Kodak's designation for its series of yellow filters, including K1, K2, and K3. Those three were called "cloud filters" for their ability to bring out the clouds by darkening the blue sky. Why did George Eastman choose "K" ? Who knows? He liked to make up names for things. He also called his filters "Wratten." So a K2 cloud filter was also known as "Wratten no. 6," for example.

Thanks; I've little use for filters and haven't used them since they stopped making Agfa's Dia Direct B&W slide film.

Thanks again.
Regards, David

 

[ColSebastianMoran]

I majored in physics. Last week, my daughter asked, "Dad, explain why is the sky blue?"

Of course, the answer is the same as the answer to "Why is the sunset red?" And, it's the answer to the question posed by the OP.

Shorter wavelengths of light are scattered more by the content of the atmosphere. Eliminate the shorter visible wavelengths (violet, indigo, blue, even green) along with UV (even shorter wavelength) and you are left with the less scattered red light.

Or, use the "Dehaze" slider in LR which amazes me, but it's probably not good enough for your 25mile shot of city from mountain top.