How to Avoid Spherical Aberration Defects in Optical Lenses?

Spherical aberration is one of the common aberrations in optical systems, and its formation mechanism is closely related to the spherical structure of the lens.

1.The nature of spherical aberration

Spherical aberration is a type of image blur caused by the fact that when a lens surface is spherical, light rays entering at different heights (different distances from the optical axis) converge at different points, thus affecting image sharpness.

Paraxial rays → focus at a farther position (closer to the theoretical focal point)

Marginal rays → focus at a closer position (before the theoretical focal point)

This difference in focal position can easily lead to a phenomenon where the center is sharp but the edges are blurry.

2.Physical explanation of the formation mechanism

(1)Difference in refraction angle. For a spherical surface, the radius of curvature varies with position; the further from the center, the faster the surface normal changes, resulting in a larger refraction angle.

(2)The accumulation of higher-order aberration terms. For large-aperture lenses, the impact of this difference is magnified.

(3)The difference between a single lens and multiple lenses: A single lens can hardly correct spherical aberration, while multiple lenses can compensate for it through a combination of different curvatures and different glass refractive indices.

Schematic diagram of spherical aberration

3.The dangers of spherical aberration

Spherical aberration directly degrades the image quality of a lens, specifically manifested in the following ways:

(1)Point spread: An ideal "point source" (such as a star or a pixel) will become a "blurred light spot," with a halo of varying brightness at its edges;

(2)Image quality degradation: Overall image sharpness is reduced, and details (such as text and textures) become blurry, especially when the lens aperture is fully open (when edge light is more prominent);

(3)Reduced contrast: Diffuse light spots can lead to a reduction in the contrast between light and dark areas of the image, resulting in loss of detail in dark areas and overexposure in bright areas.

4.Design methods to avoid spherical aberration

To avoid spherical aberration problems, the main methods used in lens manufacturing are as follows:

(1)Optical design phase

①Using aspherical lenses

Aspherical lenses, whose curvature varies with the radius, allow all light rays to converge to a single point, thus eliminating spherical aberration at its source.

Advantages: A single aspherical lens can compensate for the aberrations of multiple spherical lenses.

Disadvantages: High manufacturing difficulty and extremely strict tolerance requirements.

②Multi-lens combination optimization

By using a combination of lenses with different refractive indices and curvatures (commonly found in achromatic designs), ray tracing optimization allows the marginal rays and central rays to focus at the same point.

③Refractive index distribution lens

By using a gradual distribution of refractive index within the material, the light path is pre-compensated inside the lens.

Design methods to avoid spherical aberration

(2)Manufacturing and process stages

①High-precision grinding and polishing 

The accuracy of the spherical surface must be within the nanometer-level surface shape error range; otherwise, aberrations will be amplified.

②Molded aspherical lenses

Suitable for mass production, but mold temperature control and stress release are crucial.

③Injection molded aspherical plastic lenses

Low cost, but limited by material thermal expansion and aging, it is mostly used in consumer-grade devices.

(3)Client-side assistance

①Reduce the aperture (f/8 or smaller)

Although reducing the aperture is not a complete solution, decreasing the effective light path can mitigate the effects of stray light at the edges.

②Software algorithm compensation

Some cameras perform sharpening compensation during JPEG/RAW processing based on the lens model.

5.Special considerations for 4K/high-resolution lenses

How to manufacture lenses with 4K and higher resolution, where the tolerance for spherical aberration is extremely low:

Small pixel size → any minute aberration will lead to a decrease in MTF (Modulation Transfer Function);

Full-frame or large-format sensors → require higher image quality at the edges;

High-speed aperture requirement → Spherical aberration is more difficult to suppress at large apertures.

Therefore, high-end designs often utilize:

Multiple aspherical elements combined with low-dispersion glass; MTF optimization targets are set across the entire field of view and aperture range; tolerance analysis rigorously controls surface form errors and assembly deviations.