Posted by Basil Greber on February 20, 2007 at 2:58 PM
Hi all
I'm having trouble with an image shot with the Tokina 12-24/4 DX on a Nikon D100. At first, it appeared like chromatic aberrations (which isn't unexpected with that lens) - a mountain in front of a bright sunset had a red contour around it, about 2 px wide.
But as I understand it, CA can be corrected by shifting the color channels relative to each other. This didn't work at all, and a look onto the blue channel revealed the cause: There is a fat black line around the mountain. Obviously, shifting the channels (or trying to correct it in Adobe Lightroom) can't cure something that's happening inside one channel - all I got was an additional green line around the red one. How did this possibly happen? In camera sharpening (yes, I was shooting JPGs)? Some kind of sensor artefact?
See the offending channel here as a 100 % crop:
http://n.ethz.ch/student/bgreber/npnposts/CA.jpg
Thanks for any interpretations, and for advice on how to avoid this.
It does look like a sharpening arifact. It could also be a diffraction effect caused by the sunlight bending over the ridge crest. Aside from a night-long cloning fest, I don't see how you could eliminate it. Maybe one of the gurus can shed some more light for you than I.
Can you post the color version on the same page as your grey-scale sample? That might help to identify what's happening.
-PB
Posted by Basil Greber on February 20, 2007 at 3:20 PM
Thanks. Resized full frame is here, with a 100% crop inserted:
Discussion
2 repliesI'm having trouble with an image shot with the Tokina 12-24/4 DX on a Nikon D100. At first, it appeared like chromatic aberrations (which isn't unexpected with that lens) - a mountain in front of a bright sunset had a red contour around it, about 2 px wide.
But as I understand it, CA can be corrected by shifting the color channels relative to each other. This didn't work at all, and a look onto the blue channel revealed the cause: There is a fat black line around the mountain. Obviously, shifting the channels (or trying to correct it in Adobe Lightroom) can't cure something that's happening inside one channel - all I got was an additional green line around the red one. How did this possibly happen? In camera sharpening (yes, I was shooting JPGs)? Some kind of sensor artefact?
See the offending channel here as a 100 % crop:
http://n.ethz.ch/student/bgreber/npnposts/CA.jpg
Thanks for any interpretations, and for advice on how to avoid this.
Basil
It does look like a sharpening arifact. It could also be a diffraction effect caused by the sunlight bending over the ridge crest. Aside from a night-long cloning fest, I don't see how you could eliminate it. Maybe one of the gurus can shed some more light for you than I.
Can you post the color version on the same page as your grey-scale sample? That might help to identify what's happening.
-PB
http://n.ethz.ch/student/bgreber/npnposts/CA2.jpg
Basil