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This is a detail from a photo that I took with a Fujifilm XF55-200mm lens. The lens was zoomed all the way to 200mm, and the aperture was wide open—f4.8.

enter image description here

I wonder whether somebody knows the cause of the dark rings in the otherwise mostly-uniform, circular* bokeh.

My question partially duplicates another question, but that question was a two-parter, and nobody answered the part about the dark rings with the bright centers.

Additional info: I was using a lens hood—the one that came with the lens, though I would expect that any defects caused by the hood would show up at the shortest zoom setting, not the longest.

There were no filters or other accessories.

* I'm assuming that the ones that aren't perfectly circular are images of "points" of light that were not perfectly point-like.

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  • Look like an extreme form of onion ring bokeh. There is also quite a bit of chromatic aberration. That could be the result of both adding up. – Kai Mattern Jul 27 '20 at 13:04
  • @KaiMattern, I had not heard of "onion ring" before today, but based on what I just read, I'm thinking that this must be something else—maybe an Airy disk?—Seems to me that "onion ring," which is an artifact of the manufacturing process for the molds that are used to make aspheric elemnts—would always show as many fine rings. – Solomon Slow Jul 27 '20 at 13:23
  • True, but as the picture seems to be a very much cropped into, it might be showing up exaggerated. I have not hear of airy disc artifacts before. So we both learned something new. Cool! You might be on the right track there. – Kai Mattern Jul 27 '20 at 13:31
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    @KaiMattern, Re, "very much cropped," Yes. I did not re-scale the image. That crop is at full sensor resolution. – Solomon Slow Jul 27 '20 at 13:33
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    @xiota, Tried photographing a tiny flashlight, out-of focus, from across the basement at various different apertures, with and without the lens hood. I was unable to reproduce the effect. Though it now occurs to me that I had the flashlight in the center of the viewfinder, and the crop above is from the top edge of the frame. Maybe I will try again later. – Solomon Slow Jul 28 '20 at 11:43
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I'm assuming you are talking about the darker internal ring.

That appears to be different from the typical bright edge (nisen) bokeh caused by overcorrecting spherical aberrations. And it does not appear to be "onion ring" from the molding of the lens.

IMO, the dark inner ring is almost certainly due to an internal lens element/boundary. It could be a side effect of the lens design, but it could also be a defect and due to the imperfect bonding of two elements. I would compare it against a second version of the same lens.

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  • The dark ring contained within the bokeh balls is likely not produced by an internal element or other lens defect. The cause is likely transitory because OP states, "I was unable to reproduce the effect." – xiota Aug 5 '20 at 15:42
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Lenses suffer from defects called aberrations, there are 7 major types. One type is called chromatic aberration. Chromatic aberration is always present, the lens maker strives to mitigate but can’t eliminate. Each color of light, as it traverses the lens, is refracted at a slightly different angle. This results is, each color coming to a focus at a different distance from the lens. In other words each color has a slightly different focal length.

Now a positive lens (convex) and a negative lens (concave) have opposite chromatic aberration properties. The lens maker combines a strong positive with a weak negative in an attempt to mitigate. Again, the chromatic aberration is reduced but not eliminated.

The shorter frequencies of violet and blue come to a focus closer to lens. The warmer colors like orange and red come to a focus further downstream.

Thus each color has a different projection distance (focal length). The lens maker strives to cause the blue and red image to coincide. As you zoom the lens, the focal length increase and thus the aberrations worsen. At maximum zoom, the different projection distances become apparent. Each color will have a slightly different image size. This results in a color fringing surrounding the edges of objects. Which will be the outmost fringe color? This depends on the tiny residual error in chromatic aberration. In this case, blue becomes the larger of images.

images of a point sources images aa a blurry center surrounded by rings of light like a bull's eye target. These are out-of-focus point source images.

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  • The Airy disk (the bullseye ring pattern you describe) is due to diffraction, not chromatic aberration. Chromatic aberration produces a color fringe (most commonly violet toward the edges or corners of the frame), not a bullseye. Given the lens is wide open, this bokeh probably isn't an Airy disk, either; it's probably some combination of coma and spheric aberrations specific to the large element count in a zoom. – Zeiss Ikon Jul 27 '20 at 17:02
  • @ZeissIkon The brightest highlights in the example do shift very hard towards blue/violet at the edge. Without knowing how white balance was selected and what possible HSL may have been made, it is entirely possible that what was violet when it was projected on the sensor could have been rendered as blue/aqua. Fuji loves to do funky things with color processing for some of their "film" presets that might include pulling magenta and purple way back in some situations. – Michael C Jul 28 '20 at 9:01
  • @MichaelC chromatic aberration won't produce the dark ring inside the fringe -- though I suppose the magic performed before even RAW storage might do something to color that could. Still looks like idiosyncratic bokeh specific to a zoom to me. – Zeiss Ikon Jul 28 '20 at 11:19

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