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Optical Aberrations Explained: From Spherical to Chromatic Aberration

An ideal optical system achieves perfect imaging for paraxial objects with narrow beams: object/image points correspond one-to-one, object/image planes maintain correspondence, and transverse magnification remains constant across the object plane. However, real optical systems involve non-paraxial points and rays, causing deviations between actual and ideal images—known as optical aberrations. These fall into two categories: monochromatic aberrations (affecting non-paraxial points/rays) and chromatic aberrations (caused by wavelength-dependent refractive indices in lenses).

1. Spherical Aberration

Cause

Occurs due to asymmetric refraction, especially in large-aperture beams. When monochromatic light passes through a thin lens, non-axial rays intersect the optical axis at different points, forming an extended spot instead of a single focal point.

Impact

Creates a blurred circle around the image point. The smallest/highest-intensity blur circle (called the circle of least confusion) lies between the paraxial (Q′P) and marginal (Q′M) focal planes.

Reduction Methods

• Lens optimization (e.g., bending surface curvatures)

• Lens pairing: Combining positive/negative lenses to cancel aberration at specific heights.

2. Coma

Cause & Manifestation

Results from large-aperture beams imaging off-axis points. Forms an asymmetrical, comet-shaped flare (resembling a comet’s tail). Example: Tilting a magnifying glass under sunlight disperses the focus into a comet-like pattern.

3. Astigmatism & Field Curvature

Astigmatism

Caused by asymmetric refraction of off-axis rays. Refracted beams become astigmatic, projecting elliptical spots on the image plane. Spot geometry changes as the screen moves toward the lens.

Field Curvature

Even after correcting spherical aberration/coma, the image plane remains curved (a sagittal surface), preventing flat-field focus. Astigmatism severity varies with off-axis distance.

4. Distortion

Types & Cause

Non-uniform transverse magnification distorts shapes:

• Barrel distortion: Magnification decreases with off-axis distance (edges squeeze inward).

• Pincushion distortion: Magnification increases with off-axis distance (edges bulge outward).

Correction

Use a symmetrical aperture stop centered on the optical axis to align spot centers with ideal image points.

5. Chromatic Aberration

Cause & Manifestation

Different wavelengths refract at different angles due to dispersion, causing separate focal points for each color. In polychromatic light, focal points appear at different positions along the optical axis.

 Reduction

Combine lenses of different materials (e.g., crown/flint glass) to offset dispersion. This method can be integrated with spherical aberration correction techniques.

Key Terminology:

• Spherical Aberration

• Coma

• Astigmatism

• Distortion

• Chromatic Aberration

• Circle of Least Confusion

• Lens Bending