Chromatic Aberration question

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Marpel
Posts: 293
Joined: September 13th, 2009, 3:19 pm
What is the make/model of your primary camera?: Nikon D810
Location: Port Coquitlam, British Columbia

Chromatic Aberration question

Postby Marpel » September 23rd, 2017, 7:02 pm

I used Color>Chromatic Aberration on a recent image in an attempt to deal with some red/green fringing (Oregon sea-stack against a bright sky).

I was able to control the issue relatively well with Red Shift of -0.0387%.

However I later discovered, on close inspection, a bright line around the edge of the image. Because the line was so narrow and initially I almost missed it, I had to zoom in to see exactly what it was. It turned out to be a thin line along each edge although the top and bottom was thinner than the other edges. Rather than trying to clone alone those edges, I found I had to crop the image a single pixel along the top and bottom and 2 pixels off the left and right sides to get rid of it. Not a big deal but an extra step, nonetheless (and the size of the image then changes, making it incompatible with masks from other images).

Is this just a byproduct of this operation and, as I haven't been able to do some further testing, would these "lines" along the edges vary in width relative to the amount of shifting?

Marv

jsachs
Posts: 784
Joined: January 22nd, 2009, 11:03 pm

Re: Chromatic Aberration question

Postby jsachs » September 23rd, 2017, 7:19 pm

This is a normal side-effect of the transformation.

Chromatic aberration results from the refractive index of glass varying slightly with wavelength of light. This causes the focal length of the lens to depend on the color of the light which means there is a slight difference in magnification between different colors which causes radially symmetric color fringes around the edges of the image. The cure is to adjust the magnification of the red and blue channels with respect to the green channel. shrinking one of the channels leaves a fringe around the edge of the image where there is no data for that channels which is what you are seeing. You can just crop if off if you want.
Jonathan Sachs
Digital Light & Color


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