A Complete Guide to Optical Defects in Camera Lenses: Causes of Axial/Chromatic Aberration, Spherical Aberration, Coma, and Astigmatism, and Improvement by Stopping Down the Aperture

1. Physical causes of the five major optical monochromatic aberrations (Seidel Aberrations)

Due to the limitations of the refractive physical properties of an ideal spherical lens, light cannot perfectly converge on a pure point, resulting in five classic monochromatic aberrations:

1. Spherical Aberration:
   -Cause: The focus of the light refracted through the edge of the lens is closer to the lens than the focus of the light refracted through the center of the lens, resulting in a blurred circle before and after the focus.

• Symptoms: When the large aperture is fully opened, the entire screen has a layer of white haze (low-contrast soft focus).
• Improvement: Shrink the aperture 1-2 stops and disappear significantly, or use aspherical lenses to correct it.

2. Coma:

• Cause: The magnification is not equal when the off-axis oblique incident beam passes through different lens areas.
• Characterization: The point light source at the edge of starry sky photography or night scene is elongated and torn into a radial divergent tail similar to a "comet" or "little swallow flying", destroying the edge star point shape.

3. Astigmatism and Field Curvature:

• Astigmatism: The focus points in the sagittal direction (Sagittal) and the meridional direction (Meridional) are not on the same focal plane, resulting in the cross lines on the edge of the image being unable to be in focus at the same time.
• Curvature of field: The best focusing surface of the lens is not a flat surface but a curved spherical bowl-shaped bottom, causing the edges to become out of focus and blurry when the center of the picture is in focus.

2. Two branches of chromatic aberration: axial chromatic aberration vs. magnification chromatic aberration

Light of different wavelengths (red, green, blue) has different refractive index in optical glass (dispersion phenomenon):

1. Axial chromatic aberration (Longitudinal/Axial CA) - "Out-of-focus purple-green edge":

• Physical mechanism: Red light has a long wavelength and a small refractive index and falls behind the sensor; blue-violet light has a large refractive index and falls in front of the sensor.
• Symptoms: A purple/magenta diffuse edge appears on the high-contrast edge in front of the focal plane, while a green edge appears on the edge behind the focal plane.
• Elimination method: The aperture must be stopped down (from f/1.4 to f/2.8 or f/4, the convergence will be large); the advanced achromatic lens (APO - Apochromat) uses fluorite (Fluorite) and extra-low dispersion lens (ED) to converge the three wavelengths to the same point.

2. Lateral/Transverse CA—"Edge Red and Blue Edge":

• Physical mechanism: Due to the different magnifications on the focal plane after refraction of off-axis rays, the sizes of red light and blue light images are different.
• Symptoms: Only at the extreme edges and four polar angles of the screen, bright red-cyan or blue-yellow subtle color edges appear at the edges of high-gloss objects.
• Features: Stopping down the aperture does nothing to improve this! However, since it is a purely geometric scaling error, checking "Remove Chromatic Aberration" in the camera or Lightroom can achieve almost 100% perfect lossless digital correction.

3. Quantitative table of aperture contraction improvement amplitude

Understanding the sensitivity of different aberrations to aperture contraction can extract the best resolution with the most scientific aperture in real shooting:

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