A Question on Blue Sky Tonality Variation

[Kit-V]

I have noticed that in several of my images that have an expanse of clear blue sky there are variations in the shades of blue. Certain areas will be distinctly more saturated than others. This is distractingly obvious.

Although I cannot recall if the images were shot with a circular polarizing filter or not, might this be the culprit? I am trying to determine if this is a naturally-occuring phenomenon or just correctable user error.

Thank you kindly.

Kit

[Slobodan Blagojevic]

... a circular polarizing filter or not, might this be the culprit? I am trying to determine if this is a naturally-occuring phenomenon or just correctable user error...

Both.

Several factors at play: polarizer, wide-angle lens, user. The amount of polarization is not evenly distributed in the sky, it is usually the strongest at 90 degrees to the sun. A wide-angle lens just shows it more obviously, as it catches more of the sky. What a user can do? Select a less wide lens, point it toward the area in the sky with the maximum polarization, or, if the composition dictates otherwise, turn the filter back off the maximum polarizing effect. Alternatively, try various post-processing techniques to even out the sky.

[Kit-V]

Slobodan:  Your comments make sense to me. Indeed, the images to which I was referring were shot at 17mm. From the saturation of the sky (directly out of the camera), it appears that I was shooting with a polarizer.

It would be instructive for me to set up my tripod & shoot several images of the sky. First, shoot at various focal lengths without the polarizer. Second, at the lens' widest angle & with the polarizer attached, shoot several images varying the degree of polarization.

Thank you.

Kit

[Ellis Vener]

with a  camera -film or digital - with a 24x36mm size sensor, the widest focal length lens you can use and have "acceptably" even polarization across the sky with is a 28mm (you can go wider if you are shooting with the long side of the format oriented vertically). The sky has naturally occuring blue gradations, but the differences in color between blue tones in your photos can be exaggerated if you are using shooting JPEGS and using the sRGB color space.

[Tony Jay]

...The sky has naturally occuring blue gradations...

Yes, due to the effects of Rayleigh scattering of light - sky down on the horizon will look relatively pale while sky overhead is dark.
This effect is real and it is natural.

Tony Jay

[JeffKohn]

Yes, due to the effects of Rayleigh scattering of light - sky down on the horizon will look relatively pale while sky overhead is dark.
This effect is real and it is natural.

Tony Jay

Yep this not only occurs on vertical axis but also on the horizontal axis around sunrise/sunset.

[bill t.]

And the more-or-less natural blue gradation effect is particularly vulnerable to being exaggerated by a Curves or Levels adjustment in Normal rather than Luminosity blending mode.  But you definitely need for your horizon to be lighter and less saturated than higher parts of the sky.  The sky tone where it meets the horizon needs to have a similar range of contrast and hazing as the ground at that point.

[ErikKaffehr]

Hi,

A 17 mm would have a lot of vignetting, even if stopped down. Try lens develop->lens corrections->vignetting if you happen to use Lightroom. Ultrawides and polarizer are no, no.

Best regards
Erik

Slobodan:  Your comments make sense to me. Indeed, the images to which I was referring were shot at 17mm. From the saturation of the sky (directly out of the camera), it appears that I was shooting with a polarizer.

It would be instructive for me to set up my tripod & shoot several images of the sky. First, shoot at various focal lengths without the polarizer. Second, at the lens' widest angle & with the polarizer attached, shoot several images varying the degree of polarization.

Thank you.

Kit

[hjulenissen]

I recommend this book:
http://press.princeton.edu/titles/8369.html


It is a kind of science history book, tracing the efforts that went into understanding why the sky is blue throughout history. Lots of nice images in there as well.

-h