What is the cause of inclusions in my bokeh?

Question

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.

2) Nikon 28-85 f3.5-4.5- more subtle.

3) Sigma 70mm Macro- No filter.

Answer 1

In the center, this should be the diffraction pattern from dust particles (so called "Airy disk") but not the 'onion-ring'.

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):

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)

Ref:

Understanding bokeh

非球面镜头，真是那么神奇又昂贵么？

Answer 2

Probably the dots are dried water specs.

Definitely the unsightly rings are from the glass polishing process.

^{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.