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I was doing a lens review an two of my lenses showed these rather unsightly inclusions in their bokeh. There are two different lenses.

EDIT: The third lens I have tested also seems to have this artifact. The third lens had NO FILTER ON. So I will give the lenses a finer clean and try again.

1) Nikon 18-55 VR II- the most noticeable. 18-55 VR II

2) Nikon 28-85 f3.5-4.5- more subtle. enter image description here

3) Sigma 70mm Macro- No filter. enter image description here

HelloWorld
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    Is there a flat filter attached to the front of the lens? – Michael C Jun 17 '18 at 02:48
  • I assume the examples are crops from a larger frame. Does the location of the artifact within each circle change with overall position? – mattdm Jun 17 '18 at 03:17
  • Both lenses had UV filters attached. @MichaelClark – HelloWorld Jun 17 '18 at 08:49
  • @Chai I bet that is the one where the front of the lens rotates for focusing and/or zooming, isn't it? – Michael C Jun 17 '18 at 11:42
  • @MichaelClark- spot on Michael, however what are the implications of the rotation? – HelloWorld Jun 17 '18 at 11:45
  • It means the problem is on the front part that is rotating. Take off the UV filters and try again. If you insist, for no rational reason, to continue using UV filters then spend what you need to get high quality ones. – Michael C Jun 17 '18 at 11:49
  • @MichaelClark- first off only the second one has a rotating element. I don't know if that was your guess. Secondly, the filters are high quality ones judging by the brand. Secondly I didn't need them at night obviously, I just didn't about taking them off. – HelloWorld Jun 17 '18 at 11:53
  • @mattdm- On second thought I'm not sure I have understood you, can you rephrase please? When you say 'overall position' what are you referring to? Focal length or while focusing it manually? – HelloWorld Jun 17 '18 at 12:02
  • @Chai Please clarify. Are you saying the lens with the front rotating element does or does not reposition the smaller artifact within the bokeh when the front of the lens is rotated? Does the dot that is at "10 o'clock" within each bokeh ball in the first image move to "2 o'clock" when the front of the lens is rotated 120° clockwise? – Michael C Jun 17 '18 at 12:27
  • Does it change with aperture? Do they stay in the same place when you swap lenses? Looks like crud on the sensor to me. – BobT Jun 17 '18 at 19:51
  • @BobT-it's the same artifact at different focal lengths (=different apertures). – HelloWorld Jun 18 '18 at 11:11

2 Answers2

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Probably the dots are dried water specs.

Definitely the unsightly rings are from the glass polishing process.

Zeiss Batis 2.8/135 Zeiss Batis 2.8/135

See this Imaging-Resource article: "The end of onion-ring bokeh? Panasonic beats the curse of aspheric lenses":

These days, most aspheric lens molds are created using a process called Single-Point Diamond Turning (SPDT), where a minute, incredibly sharp diamond tool is used to turn the desired profile on a nano-precision lathe. While SPDT can generate very precise profiles, the machines used to do this turning have a finite mechanical resolution, so the profiles generated will have very tiny steps in them. As the diamond cutting tool advances across the mold surface, these minute steps form either a spiral or a series concentric rings. Depending on where the aspheric element is in the optical formula of the lens as a whole, this spiral/ring surface pattern can cause the characteristic appearance of onion-ring bokeh. Basically, you're seeing an image of the sub-microscopic ridges on the surface of the aspheric element.

Onion Mold

Different onions, same taste

Rob
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  • @Rob- great answer. However in the second image I am more concerned with the 'inner' onion rings. Did you notice those? What could cause them? – HelloWorld Jun 17 '18 at 08:58
  • @Chai - That is explained in the quoted (blue) portion of the answer. – Rob Jun 17 '18 at 11:26
  • I don't believe it does. The way I understand it, it speaks of the overall onion effect on the periphery of the lens. Not the ones that are 3-4mms across. – HelloWorld Jun 17 '18 at 11:47
  • I've updated the image, to draw your attentions to the ones I'm talking about – HelloWorld Jun 17 '18 at 11:53
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    @Chai - You can see in the link provided for Imaging-Resource or in this link for Toshiba's press "G-3 High-Precision Optical Glass Heating and Molding Technology" that it's press formed, there are no gates (last paragraph) ... – Rob Jun 17 '18 at 12:38
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    and use of a ring ejector makes sense. The tiny specs I have answered about in the first sentence - I only have a guess on that part and nothing else to offer. You could examine your lens under a powerful light with a magnifying glass, rotate the lens while pointed at a black/white paper and see if the dot spins. I have nothing more to offer, no other guess. – Rob Jun 17 '18 at 12:42
  • If you are talking about the "cats eye bokeh" in the image of the girl, that's simply because the front of the lens tube is vignetting the edge rays. "Cat's eye" bokeh is the result of using a very wide aperture lens where the entire entrance pupil is not visible from the angle of the widest light rays in the field of view. How can I take pictures with extreme bokeh with an in-focus subject nearby? and What causes this 'bokeh crop' effect? – Michael C Jun 17 '18 at 12:54
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    @Chai - You could try asking at Engineering.SE. Be certain to mention that you have already asked the question here (provide the URL for your question), you are welcome to ask if anyone agrees with my answer (use my answer's 'share' for the URL). Maybe someone there has experience manufacturing lenses. – Rob Jun 17 '18 at 13:24
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In the center, this should be the diffraction pattern from dust particles (so called "Airy disk") but not the 'onion-ring'.

enter image description here

Simply speaking, a point is not be projected as a sharp point but the 'airy disk' on CMOS, because of the wave character of light.

In addition, the simulation of a whole bokeh and the example similar to yours (from Hasselblad 80mm 2.8 CF T* Bokeh, Bokeh control):

enter image description here enter image description here

A typical “onion-ring” bokeh is resulting from the aspherical elements inside your lens, which is known as a kind of processing defect resulting from aspherical elements grinding. The pattern is more uniform than the diffraction pattern.

Spherical lens is a combination of sphere, it converges the light but perfect convergence can not be achieved by ideal spherical surfaces.(spherical aberration) Spherical lens

Ref:

Understanding bokeh

非球面镜头，真是那么神奇又昂贵么？

jianjie
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