What is Causing This?

[Wirefox]

I would like to pick your brains if I may. What is causing the deepening of the blue in the sky in the left of the image. I was using CPL and lens hood.

Cheers

Steve

1/100 sec, ISO 100, f/9, Pattern metering

[pwnage101]

I don't have experience with CPLs, but I have studied topics on light scattering. Any direction 90 degrees from the sun will look more blue due to Rayleigh scattering. Anything more or less than 90 degrees will look more pale (and bright) since it consists of a more broad spectrum of wavelengths.

From looking at the direction of the shadows I can deduce that the sun is almost directly camera left from the left edge of the lens' FOV, which means the sky towards the left of the picture should be more saturated.

[Dave Humphries]

Hi Steve,

Yep, Troy is correct.

I see from the EXIF it was a 28mm lens, so you are looking (by my guesstimate) at getting towards 90 degrees FOV horizontally, which explains why the CPL has a lot of effect on the left hand sode and so little effect on the right hand side of the image.

This is exactly why Colin hates them!

[ScoutR]

Vewy interseting! I believe this is another example of the same thing?
1/250; f5; ISO 200; 18mm; CPL; No lens hood; Sun behind me and to the right



Wendy

[Colin Southern]

This is exactly why Colin hates them!

You beat me to it!

[pwnage101]

Not quite. In your picture, wendy, the gradient is vertical (likely to have a slight horizontal component, but that's not what i'm talking about - this makes you half right). This is not entirely caused by Rayleigh scattering, but rather the fact that the increased number of particles lower to the horizon increases the probability of collision with the sun's rays, and the reflection of the sun's rays, and the reflection of the reflection, and so on.

draw a circle within a circle. now draw a tangent line anywhere on the inner circle. you will notice the length of the tangent contained in the shell is longer than that of the perpendicular to the tangent.

i feel so smart yay physics and probability!

[pwnage101]

Hi Steve,

Yep, Troy is correct.

I see from the EXIF it was a 28mm lens, so you are looking (by my guesstimate) at getting towards 90 degrees FOV horizontally, which explains why the CPL has a lot of effect on the left hand sode and so little effect on the right hand side of the image.

This is exactly why Colin hates them!

Oh, by the way, it's not nearly 90 degrees I know this because i've been considering a 11mm capable lens for a long time, and on cropped sensor it's about 90 degrees horizontal. I just can't bring myself to throw down $500 USD for a lens!

[Wirefox]

Excellent replies Troy. I understand what is happening now. The thing that still has me puzzled is that I have read that CPLs perform best when the lens is 90 degrees to the incoming sunlight. I assume this is referring to the CPLs ability to 'see through' reflection rather than it's incidental use in darkening blue skies.

Steve