Aperture aperture and bokeh balls (Bokeh Balls) edge color fringing (cat-eye bokeh/onion rings) physical causes and aesthetic control

1. Geometric optics of Bokeh and Circle of Confusion

When the subject is outside the depth of field of the lens, the light from the point light source cannot converge to a point on the focal plane, but diverges to form a circular blurred spot, which is called a circle of confusion or a bokeh ball:

1. Characteristics of ideal bokeh spots:

• Perfect round shape, uniform internal brightness and no hard edges;
• The blur is soft and natural, and the out-of-focus transition melts like butter.

2. Two types of common inferior bokeh defects:

• Onion Rings: Circles of concentric interference textures like tree rings or onion cross-sections appear inside the bokeh spot;
• Cat's Eye Bokeh: The light spot at the edge of the picture is no longer a perfect circle, but is compressed and distorted into an olive shape, lemon shape or cat's eye shape, and even causes a strong out-of-focus swirling feeling (Swirly Bokeh).

2. The underlying physical causes of onion rings and cat-eye bokeh

1. The origin of onion ring manufacturing: ultra-precision turning tool marks on aspherical lens molds:

• In order to eliminate spherical aberration, modern large-aperture lenses use a large number of molded glass aspherical elements;
• When manufacturing super-hard molds for aspheric lenses, a single-point diamond turning machine (SPDT - Single Point Diamond Turning) is used for sub-micron rotational cutting;
• The micron-level spiral cutting marks left by the turning tool on the surface of the mold are directly printed on the glass lens during molding;
• When the out-of-focus point light source penetrates these microscopic ring-shaped concavities and convexes, the light undergoes Fresnel diffraction interference, projecting dazzling "onion growth rings" inside the bokeh ball.

2. The physical origin of cat-eye bokeh: Mechanical Vignetting of the lens barrel:

• In the center of the picture: the light passes vertically through all the circular lenses and aperture holes of the front and rear groups of the lens, and is projected on the sensor as a perfect circle;
• But at the four polar edges of the picture: light enters at a very large tilt angle, and the beam is physically blocked and cut by the metal filter aperture ring in the front of the lens and the edge of the rear lens barrel**!
• The beam cross section is cut from a circle into an olive shape by the edges of the front and rear lens barrels in both directions. This is the purely mechanical cause of edge cat-eye bokeh.

3. Practical taming strategies for bokeh aesthetics

1. Eliminate onion rings and cat eyes: shrink the aperture 1 stop (f/1.4 $\to$ f/2.0):

• Slightly stop down the aperture from wide open to f/2.0 or f/2.8;
• At this time, the physical aperture aperture becomes smaller and becomes the only bottleneck limiting the light beam, and the light is no longer blocked by the mechanical edges of the front and rear lens barrels;
• The light spot on the edge instantly returns from the squeezed flat cat's eye to a round regular polygon/circle, and the outer edge of the onion ring greatly converges in color.

2. Selection of lenses with top grinding technology:

• Sony XA (super aspherical) lenses and Canon ground aspherical lenses use nanoscale magnetorheological polishing (MRF) technology to reduce the mold surface roughness to less than 1 nanometer, completely eliminating onion rings from the physical source.

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