Spherical Aberration: Understanding and Correcting the Blur in Optical Systems

Adetya Khade
4:30 MINS
July 11, 2025

In the quest to produce crisp, high-resolution images, optical designers often confront a set of imperfections known as aberrations. These deviations from ideal image formation degrade system performance and can limit applications in photography, astronomy, microscopy, and vision systems. Among them, spherical aberration is the most fundamental and frequently encountered.

This blog marks the beginning of our Optical Aberration Series, offering both theoretical insights and practical simulations to illustrate and solve real-world problems in optics. Here, we’ll delve into what spherical aberration is, why it occurs, how it affects performance, and how to mitigate it using tools like ANSYS Zemax OpticStudio.

What Is Spherical Aberration?

Spherical aberration occurs when rays of light passing through the outer zones of a spherical lens or mirror focus at different points than rays passing through the central zone. As a result, the optical system fails to bring all incoming light to a single sharp point, leading to a blurred image even when the object is perfectly on-axis.

This error stems from the geometry of spherical surfaces. Unlike a parabola, a sphere does not focus parallel rays to a single point. The discrepancy becomes more significant with faster (low f/#) systems and increases with aperture diameter.

Visualizing the Problem in Zemax

The signature of spherical aberration is a blurred focal spot, typically circular in shape, where marginal rays intersect either before or after the paraxial focus. This results in:

In practical terms, this means:

Identifying Spherical Aberration in Zemax

Using ANSYS Zemax OpticStudio in sequential mode, we can simulate spherical aberration with the following analysis tools:

For more information on when to use which MTF analysis refer this article: https://support.zemax.com/hc/en-us/articles/1500005575102-Methods-for-analyzing-MTF-in-OpticStudio

All these analysis tools quantify the degradation and serve as benchmarks before optimization.

Seidel Diagram for Surface-Wise Aberration Analysis

Zemax also provides a powerful tool called the Seidel Diagram, which allows designers to view individual aberration contributions at each optical surface. This diagram plots classical Seidel coefficients (including spherical aberration) across the system.

By using the Seidel Diagram in tandem with wavefront and ray-based analyses, you gain a complete understanding of where to make design changes for optimal correction.

Techniques to Reduce Spherical Aberration

Here, we will be using an F/4 system (Focal Length – 100 mm) to demonstrate various ways of effectively reducing spherical aberration. Spot diagram analysis is being used as a reference for performance analysis criteria in this section.

Techniques to mitigate this aberration:

1. Lens Bending

Adjusting the front and rear surface curvatures of a lens can minimize LSA. This reshapes the ray paths without altering focal power.

PCX Lens:

BCX Lens, both curvatures (lens bending) optimized:

For more information on singlet lens setup, analysis & optimization refer this article series: https://support.zemax.com/hc/en-us/articles/1500005576302-How-to-design-a-singlet-lens-Part-1-Setup

2. Aspheric Surfaces

Replacing a spherical surface with an aspheric profile corrects aberrations by aligning marginal and paraxial ray paths. Though effective, aspheres can increase fabrication complexity.

PCX lens with optimized conic constant:

For more information on aspheric surfaces in Zemax refer this article: https://support.zemax.com/hc/en-us/articles/31584981420691-Aspheric-Surfaces-Part-1-Introduction-to-Aspherical-Surfaces-in-Optical-Design

3. Optical Doublets

Combining two lenses with opposing aberration tendencies (e.g., a convex and concave pair) allows for mutual correction.

PCX lens upgraded to doublet lens & optimized:

4. Aperture Control

Reducing the aperture diameter limits peripheral rays that contribute most to the aberration, improving sharpness at the cost of brightness.

PCX lens with reduced entrance aperture:

Conclusion

Spherical aberration is among the most common defects in optical systems, but also one of the most manageable. With a strong understanding of its causes and effects—and the right simulation tools—designers can correct it efficiently.

Whether you’re engineering a telescope, microscope, camera lens, or sensor system, early detection and mitigation of spherical aberration lead to cleaner, sharper imaging results. ANSYS Zemax OpticStudio provides a powerful platform to identify, analyze, and optimize your design against these errors.