What causes colored fringes in out-of-focus areas of a focus-stacked flower photo, and can they be fixed?
Asked 3/4/2017
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I made a 4-frame focus stack of a flower, intentionally leaving the front and back soft. In the least in-focus area near the back edge, there’s a strong colored fringe that looks like “vertical chromatic aberration.” Each source frame shows the same effect, and it becomes weaker as that area gets closer to focus. If I had stacked more frames to bring that area into focus, the effect would likely disappear.
The image was made with a Nikon D5500 and Tamron 70-300mm at 300mm, f/5.6. Photoshop’s defringe tools don’t seem to help.
What is this effect: chromatic aberration, diffraction, focus-stacking misalignment, or just colored bokeh from the out-of-focus transition? Can it be corrected in post, or is the practical fix to change shooting technique or retouch it manually?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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Each layer of the stack shows the same effect, lessening as that area gets closer to focus. Had I done 2 or 3 more layers to get the entire thing in focus, as I normally do, it would have disappeared; so it's an effect of the out-of-focus itself.
Then it is neither diffraction nor chromatic aberration. Both would manifest themselves even when the edge in question was in focus.
Is it, in effect 'colour bokeh'?
It appears to my eyes to be bokeh. The color is the result of blending the pink/fushia of the flower with the yellow/green of the background.
Should I just leave it as it is? Does it work? Is it 'art' or a 'mistake'?
Subjectively, I find it a bit distracting and would probably prefer a version that did not include it. But that's not to say that the image as presented is a total waste - It's still a very nice picture.
Again, subjectively, perhaps cropping the image to a square or 5:4 aspect ratio to eliminate the entire area of yellow/green might result in a stronger image.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
9y ago
0
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Based on the behavior you described, this is most likely not diffraction or chromatic aberration. Since it weakens as the area approaches focus and would disappear if that region were included in the stack, it’s better described as out-of-focus bokeh color: the blurred edge is mixing the flower’s magenta tones with the green/yellow background.
Focus stacking can also introduce small scale/alignment differences because some lenses change magnification slightly as focus distance changes, which can make edge artifacts more noticeable.
Why Photoshop defringe doesn’t help: that tool is mainly for true lateral CA, not color created by blur and blending.
Practical fixes:
- include more focus slices so that area is in focus
- reduce stacking/alignment artifacts if present
- manually retouch/paint the edge in post if it’s distracting
- if possible, change technique so focus is shifted mechanically rather than by refocusing the lens
Whether to leave it is subjective, but several photographers would find it a bit distracting rather than an intentional effect.
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