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In this photograph, which is a 4-layer stack of just the central area, I left the front & back soft intentionally.

It has quite a strong area of what I can only call 'vertical chromatic aberration' at the back right, where the least in-focus area is.
Reading other questions makes me think the term for this is Longitudinal (axial) chromatic aberration. Please correct me if I'm wrong.

So, I have two linked questions, one objective, the other subordinate question [I'm afraid] more subjective. The second couldn't stand alone, so I'm sneaking it in & hope I don't bring down the wrath of the opinion-based close voters.

  1. What caused this & can it be fixed?
    Is it caused by my admittedly cheap lens just not being up to the task, or it it because the backlight is actually being diffracted as it hits the petal edge?
    Photoshop's defringing doesn't seem to want to touch it because it's not lateral. Is there a suitable tool in Photoshop I've not yet found?
    I could paint it out by hand, but I'm not certain I actually want to, which brings me to the subjective question...

  2. Should I just leave it as it is? Does it work? Is it 'art' or a 'mistake'?
    ...or more objectively - do other photographers use this as a technique in itself, or do they take great pains to avoid it, even in intentionally soft-focus areas?
    Is it, in effect 'colour bokeh'?

Additional information from comments, which helped towards the accepted answer...

How was the image sharpened? What happens to this effect if the overall sharpening settings are reduced?

It was sharpened with a high-pass overlay, but the softer edges are painted out of the high-pass manually, so there is no sharpening at the problem area. 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.

enter image description here

Full size jpg

Nikon D5500 Tamron 70-300mm f4-5.6
300mm ISO 100 1/200 @f5.6

From similar questions - predominantly Photographic techniques to avoid chromatic aberration?
I can't avoid the contrast between foreground & background, the composition is dependant on the juxtaposition.
This is already a crop of a larger shot, but the aberration appears unchanged as the lens centre is approached.
Stopping down for this particular style leaves the front & back too sharp & also starts to kill the vignette background & make the actual shapes too visible; I do stop down if I need sharp right the way through, but there's something about focus-stacking with the lens wide open that I particularly like.

Is the only real answer, "Get a dedicated macro lens." ?

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  • \$\begingroup\$ I don't think it's too bad, it's up to you if you want to "fix" it. I probably wouldn't have noticed it as a "defect" if you hadn't mentioned it. Because the background is so even, it would be pretty easy in photoshop to select the petal edge and blend it with the background to make a little more of a sharper edge. \$\endgroup\$
    – MikeD
    Commented Mar 4, 2017 at 11:57
  • \$\begingroup\$ How was the image sharpened? What happens to this effect if the overall sharpening settings are reduced? \$\endgroup\$
    – Michael C
    Commented Mar 5, 2017 at 0:30
  • \$\begingroup\$ @MichaelClark - it was sharpened with a high-pass overlay, but the softer edges are painted out of the high-pass manually, so there is no sharpening at the problem area. 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. \$\endgroup\$
    – Tetsujin
    Commented Mar 5, 2017 at 7:47

3 Answers 3

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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.

enter image description here

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  • \$\begingroup\$ Thank you. Subjectively, it had become the only part of the shot I was seeing, so I painted it out & left only the tiniest hint it was ever there. I do like your re-crop, but this is part of a series, all approx 4:3 against plain or vignette backgrounds, so I 'need' to keep the overall feel. Should I post another answer, showing the result, even though this is [& will remain] the accepted answer & what finally prompted my 'fix'? \$\endgroup\$
    – Tetsujin
    Commented Mar 6, 2017 at 9:34
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  1. It's probably not diffraction. One of the problems I and others run into with focus stacking is that (depending on the lens) magnification can vary slightly depending on focus distance. This is especially true if the lens front element is designed to move as you change focus. One way around this is to put the camera or subject on a moveable platform so you don't change the lens settings, but move the plane of focus mechanically.

  2. I don't find it objectionable.

A dedicated macro lens is optimized for close focusing but probably won't help with this particular problem. For instance, the newest Nikon 'micro' apparently changes magnification as you focus closer or further away from the lens. According to several sources, this is true for most modern macro lenses. Check out What are the pros and cons of using macro rail vs lens focus? and What is the benefit of an internal focus lens? for more detailed info.

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  • \$\begingroup\$ Using a focus rail to move the camera or subject has a similar effect to varying the focus distance. As the camera moves closer the field of view is narrowed, as it moves away the FoV is widened. The end result is very close to the same as changing the magnification ratio slightly via altering focusing. \$\endgroup\$
    – Michael C
    Commented Mar 5, 2017 at 0:28
  • \$\begingroup\$ The Tamron does zoom slightly when I focus-stack; I always have to crop the centre out of the resulting stack to avoid bad fill guesswork from Photoshop. This particular fringing effect, though, is in every layer, lessening as it comes towards focus. [& thank you for 2. :) \$\endgroup\$
    – Tetsujin
    Commented Mar 5, 2017 at 7:56
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Based on the accepted answer from Michael, which fully answered my question of
"what is it?" - bokeh
I thought I would post what I actually ended up doing to fix the issue.

I painted it out by hand in Photoshop.
It had got so it was the only aspect of the picture I could see, completely distracting me from the rest of the image - so it just had to go.
I took the opportunity to fix another couple of hard edges whilst I was editing.

This is the result...

enter image description here

Full size jpg

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