Lens chromatic aberration purple fringing and green fringing (axial chromatic aberration LoCA vs lateral chromatic aberration TCA): physical causes and post-processing lossless removal
Deeply understand the physical essence of refractive index dispersion Abbe number, rigorously distinguish the 'magnification lateral chromatic aberration' within the focal plane from the 'axial chromatic aberration' caused by defocused depth of field, and master the logic of optical ED lenses and the practical use of post-processing color fringe removal tools.
1. The physical nature of light wave dispersion: Abbe number and focus separation
Complex white light contains monochromatic light of different wavelengths (red, green, and blue). According to Snell's law of refraction, when light passes through an optical glass convex lens, the short-wavelength blue-violet light has a large refractive index and is violently deflected; the long-wavelength red light has a small refractive index and is gently deflected**. This means that the focus of light spots of different wavelengths cannot fall on the same physical point in space, resulting in color separation, which is called chromatic aberration.
2. Comparison of the two major families of axial chromatic aberration (LoCA) vs. transverse chromatic aberration (TCA)
| Category of chromatic aberration | Spatial occurrence area | Visual expression form | Is the aperture improved after closing two stops | Lens optical solution | One-click repair capability of post-processing software |
|---|---|---|---|---|---|
| Transverse Chromatic Aberration (TCA / Lateral CA) | The edges of the screen and the dead corners (no corners at all) | The edges of branches and buildings show symmetrical red-cyan or blue-yellow bright edges | No improvement at all (almost irrelevant to the aperture size) | Added aspherical lenses and asymmetrical low-dispersion lens designs | 100% perfect one-click elimination (Lightroom Check [Shift Chromatic Aberration] to automatically align wavelengths within seconds) |
| Axis Chromatic Aberration (LoCA / Longitudinal CA) | Full-frame bokeh transition area (appears in both center and edge) | The front-focus blur edge has a magenta-purple edge, and the post-focus blur edge has a cyan edge! | Significantly improved (the dispersion converges sharply after closing down the aperture to f/4) | Must use expensive fluorite (Fluorite) or ultra-low dispersion ED lenses | Extremely difficult to repair automatically (need to use the [Manual Defringe] straw to manually absorb) |
3. Actual post-combat non-destructive removal of purple and green edges with large aperture and large light ratio
When shooting portraits with backlit hair or mottled tree shadows in the sun, the out-of-focus light spots are often coated with a dazzling, disgusting green edge (post-focus axial chromatic aberration):
- Expand the [Optics] panel in Lightroom/Camera Raw;
- First check [Remove Chromatic Aberration]. This action can instantly eliminate 90% of edge horizontal and horizontal chromatic aberration;
- For the stubborn purple edges of out-of-focus hair and green edges of background leaves, switch to the [Manual Defringe] panel;
- Click the straw icon, zoom in to 200% on the screen, aim at the most dazzling purple circle on the model’s hair, and tap lightly to absorb the hue;
- Moderately fine-tune the [Purple Amount] slider to between 3-5. You will see that the original eye-catching purple edges are instantly neutralized by the algorithm and restored to natural hair color. At the same time, the true color of facial clothes is not damaged at all.
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