How can I improve Photoshop focus stacking when the nearest subject isn’t selected sharply?
Asked 2/2/2017
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I’m focus stacking a close-up flower image in Photoshop and the back/middle parts of the stack combine well, but the nearest petal is not being chosen as sharply as it appears in the source frame. The subject has low-contrast red-on-red areas, and Photoshop seems to prefer the middle/rear layers instead of the front-most focused layer.
I shot several manual-focus frames on a Nikon D5500 with a 24–120mm lens at 120mm, ISO 400, 1/125s, f/5.6, with AF and VR off. Reordering the layers before stacking didn’t seem to help.
Are there shooting or Photoshop techniques that can improve stack selection for the nearest plane, especially in low-contrast areas?
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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I don't know that much about focus stacking algorithms, but judging by your final image it looks like the problem has to do with contrast. The algorithm obviously has to pick out areas that are sharply focussed somehow, and we can see from the image that it's most successful where you have good contrast between the in-focus areas and the out-of-focus areas. For example, the petals against the blue background are relatively sharp except in the spots where they overlap -- the red-on-red areas are much blurrier even though they're obviously at the same distance as the rest of the petal:
I think the area you're asking about suffers the same problem -- there's just not enough contrast between the red petal and the red background in your first (leftmost) image, or between the sharp background and the blurry petal in your third (rightmost) image, for the algorithm to be able to discern which parts are in or out of focus.
Originally by user4262. Source · Licensed CC BY-SA 4.0
user4262
9y ago
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Yes. The main issue is likely low contrast in the front area, not simply layer order. Focus-stacking software generally identifies sharp regions by local contrast, so red-on-red petals with little tonal/color separation can be harder to detect than edges against the blue background.
What helps:
- Capture more focus slices, including one focused slightly closer than you think you need, so the nearest detail is clearly covered.
- Improve contrast at capture if possible: clearer separation between subject and background makes sharp areas easier for the algorithm to recognize.
- In post, manually refine the masks for the affected layer if the auto result misses the nearest petal.
- If available, rebuild or adjust the mask using a single color channel rather than the composite image; a channel with better separation can make the sharp front detail easier to isolate.
So: yes, an extra nearer-focused frame may help, but for this kind of low-contrast subject, some mask cleanup is often the practical fix.
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