5

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

  • 1
    Is there a flat filter attached to the front of the lens? – Michael C Jun 17 '18 at 2:48
  • 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 3:17
  • Both lenses had UV filters attached. @MichaelClark – Chai Jun 17 '18 at 8:49
  • @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? – Chai Jun 17 '18 at 11:45
3

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?

7

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- great answer. However in the second image I am more concerned with the 'inner' onion rings. Did you notice those? What could cause them? – Chai Jun 17 '18 at 8:58
  • @Chai - That is explained in the quoted (blue) portion of the answer. – Rob Jun 17 '18 at 11:26
  • 1
    @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
  • 1
    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
  • 1
    @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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