What causes dot and onion-ring patterns inside out-of-focus highlights?
Asked 6/16/2018
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When testing several lenses, I noticed small dots and ring-like structures inside bright out-of-focus highlights (bokeh balls). I saw this on a Nikon 18-55 VR II, a Nikon 28-85mm f/3.5-4.5, and also on a Sigma 70mm Macro with no filter attached. What typically causes these artifacts inside bokeh highlights, and are they from filters, dust, or the lens design itself?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
7
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.
Originally by user37074. Source · Licensed CC BY-SA 4.0
user37074
8y ago
0
Generated from our catalog & community — verify before relying on it.
Two different effects can cause this.
Small dot-like marks near the center of a bokeh highlight are often from dust or dried water spots somewhere in the optical path. Those can create a diffraction pattern and may show up as tiny inclusions.
The ringed, textured pattern often called “onion-ring bokeh” is usually caused by aspherical lens elements. It comes from fine concentric or spiral tool marks left by the manufacturing/polishing process used to make molded or machined aspherical surfaces. This is a known lens-rendering characteristic, not usually a defect in normal use.
Since you also saw it on a lens with no filter, a filter is unlikely to be the main cause. Cleaning the front and rear elements is still worth trying to rule out dust or dried spots, but if the ring pattern remains, it is most likely inherent to the lens design/manufacture.
So in short: random dots can be dirt/water-related diffraction; uniform concentric rings are typically onion-ring bokeh from aspherical elements.
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