Chromatic Aberration?

[surfmanjoe]

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I shot pictures today accidentally leaving aperture at F2.8, some show blue fringing, I noticed many of them with this color fringing are NOT backlit, like the sample attached, the sky should blue. is that Chromatic Aberration?  

I had purple color fringing before and can be easily fixed in defringe in photoshop. But this one looks not easy to remove. I tried it removing CA and defringe, both no success. Does anyone know how to fix it or at least reduce it? Thanks in advance. 

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[Guest]

Top right looks purple to me. Typical subject for CA problems. Which lens did you use?

[surfmanjoe]

I am using M9P + ZM planar 50mm f2. 

[adan]

Short answer - CA is likely involved in producing the blue fringing. It is probably not the whole story.

Longer answer -

Overexposure of a blue sky (i.e. exposing for the dark trees as well) will accentuate any CA fringing present.

In addition, purple fringing that is bleeding into green pine needles may well have the most "violet" part of the spectrum negated by the green, leaving more of a pure blue.

Finally, the blue/violet/ultraviolet end of the spectrum is the most energetic, and thus has excess energy to flare and spill around or into the edges of things (and also burn your skin).

Blue photons carry an energy of about 3.1 eV (electron volts) whereas red photons carry about 1.8 eV - and thus can be used as a darkroom "safelight."

It is incumbent upon color photographers to study and "internalize" both the visible spectrum and the similar-but-different "color wheel." Really understanding those at a gut level can make it easier to see how neighboring (or opposite) colors interact if there is overlap or leakage (i.e. fringes from CA, or overexposure, light of one color falling on a subject of a different color, or other effects)..

Among other oddities, we can note that in the full spectrum ("white" sunlight - first image) - there is no such thing as "magenta." No magenta anywhere in that bar.

Magenta is a fictional, perceptual color created by the absence of green light, leaving only a mix of red and blue/violet light, which is what our brain interprets as "magenta".

The color wheel does display magenta, because it shows a fictional wrap-around of the color spectrum, and does consider the "complementary" or opposite colors, such as green, and its complement magenta. And also shows how the range of "purples," with the more magenta end of the range neutralized by mixture with green light, becomes bluer.

Stick your thumb over the second image below to cover the most magenta-violet of the "purples" (as though they had been absorbed by green needles) - and what is left are the violets and blue-violets.

images linked from wikimedia:

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[adan]

Oh, and yes, the Photoshop (or Camera Raw, or Lightroom) sliders for eliminating color fringes (Lens corrections > Color) are pre-set for green-purple fringes (the most common in modern lenses)

The HUE sliders can extended the purple into the blue or red, or or the green into the blue or yellow, to account for specific lenses, by dragging the little triangles at each end to include more of the spectrum to be detected and neutralized.

Although that can produce problems like truly red things acquiring "moth-eathen" edges. My 135 TE has CA on the red/cyan line, rather than purple/yellow-green, and I can't always use the sliders to full effect without "eating into" red traffic lights, stop signs, or similar subjects.