What causes a doubled edge on an out-of-focus subject near the frame edge?
Asked 6/14/2025
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In a photo made with a Nikon Z50 and Sigma 56mm f/1.4 at f/1.4, one flower shows a strange doubled/outlined edge and an overall harsh look. It was windy, so motion blur was my first thought, but the dark edge and the way the doubling appears make me wonder if this is actually a bokeh/optical effect from shallow depth of field, especially near the edge of the frame. What causes this kind of doubled outline, and how can it be reduced?
Originally by noearchimede. Source · Licensed CC BY-SA 4.0
noearchimede
1y ago
2 Answers
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I would say the vertical doubling of the flower is an example of nisen bokeh.
The primary cause of it is undercorrected/overcorrected spherical aberrations. If it is due to overcorrection it will occur in the background blur; and if it is due to undercorrection it will occur in the foreground blur. It causes a doubling of the image, but only in areas of partial defocus. It also tends to be worse at wider apertures and at the periphery as that is where SA is the greatest.
If a lens is overcorrected it will have hard edged/ringed "soap bubble" bokeh for defocused point sources in the background. Nisen bokeh is essentially just the edge of a linear source. If a lens is overcorrected in the background, it is necessarily undercorrected in the foreground, and the bokeh characteristics will be opposite in those regions (i.e. hard edged vs no edge; nisen FG vs BG).
This is an example of the undercorrected Sigma 50mm f/1.4, and you can see how the hard edge in the bokeh can shift to one side creating something of a double image; and how the bokeh characteristics reverse FG/BG.
The other possibility is that it is due to having a cheap filter installed. That can cause light reflected from the first lens element to be reflected back by the filter as a fainter second offset image slightly out of focus. But in that case I would expect it to be more pervasive.
Originally by Steven Kersting. Source · Licensed CC BY-SA 4.0
Steven Kersting
11mo ago
0
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This is most likely nisen bokeh (often described as “nervous” bokeh), not simple motion blur. It happens in partially out-of-focus areas when the lens’s spherical aberration is not fully corrected. That can create a double edge/outlined look on blurred subjects, especially:
- at wide apertures like f/1.4
- in the foreground or background blur depending on the lens’s aberration balance
- toward the edges of the frame, where aberrations are often stronger
That dark or bright rim is consistent with this kind of bokeh rendering. Wind may have contributed a little, but the doubled contour itself points more to lens blur character than to pure subject motion.
To reduce it:
- stop down a bit (for example from f/1.4 to a smaller aperture)
- recompose so the problematic subject isn’t near the frame edge
- keep important subjects closer to the plane of focus
- if needed, use a lens known for smoother bokeh rendering
If a similar effect appeared from processing rather than optics, it could resemble posterization, but from your description this sounds like a lens-bokeh artifact.
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