Coma Aberration: Understanding Off-Axis Blur in Optical Systems with Ansys Zemax OpticStudio

In real optical systems, image quality is often limited not by diffraction alone but by optical aberrations arising from lens geometry, alignment, and field dependence.

Among third-order Seidel aberrations, coma aberration plays a crucial role in degrading off-axis image quality, producing comet-shaped blur patterns that are especially noticeable in imaging, astronomy, and automotive vision systems.

Modern optical design tools such as Ansys Zemax OpticStudio allow engineers to quantify, visualize, and correct coma efficiently during the design stage — reducing costly physical iterations.

This blog explains:

• What coma aberration is
• Why it occurs
• How it impacts imaging performance
• How Zemax helps analyze and minimize it

Section 01What is Coma Aberration?

Coma is an aberration affecting off-axis object points, where rays passing through different zones of a lens fail to converge at a single image point.

Instead of a sharp spot, the image appears:

• Asymmetric
• Tail-like (comet shape)
• Increasing with field angle

This distortion reduces spatial resolution, contrast, and measurement accuracy in imaging systems.

Section 02Physical Origin of Coma

Coma arises primarily due to:

• Variation in magnification across the pupil
• Lens shape deviations from ideal imaging conditions
• Improper aperture stop position
• High field angles in wide-FOV systems

Mathematically, coma is a third-order aberration proportional to h · y², where:

• h → field height
• y → pupil coordinate

Thus, coma increases rapidly for wide-angle optics.

Section 03Visualizing Coma Using Ansys Zemax OpticStudio

One of the strongest advantages of Zemax is its ability to directly visualize aberrations through multiple analysis tools.

1. Spot Diagram Analysis

In Zemax:

• On-axis field → nearly circular diffraction-limited spot
• Off-axis field → distinct comet-shaped spread

This provides immediate confirmation of coma presence.

2. Ray Fan Plot

The tangential and sagittal ray fans in Zemax reveal:

• Asymmetry in transverse ray error
• Field-dependent growth of aberration

This helps designers identify which surface contributes most.

3. Wavefront Map & Zernike Terms

Zemax expresses coma via Zernike polynomial coefficients:

• Primary coma (Z₇, Z₈)
• Secondary coma for higher-order systems

Monitoring these values during optimization enables quantitative coma reduction.

Section 04Impact of Coma in Real Applications

Astronomy & Telescopes

Stars near the edge of the field appear stretched into comets, reducing observation clarity.

Automotive Camera Optics

Edge-field blur degrades:

• Object detection
• Lane recognition
• ADAS reliability

Machine Vision

Measurement errors increase due to asymmetric point spread.

Hence, coma control is critical in wide-field imaging design.

Section 05Zemax-Driven Strategies to Reduce Coma

1. Stop Position Optimization

Zemax allows rapid evaluation of:

• Entrance pupil shift
• Stop relocation
• Resulting Seidel balance

Proper stop placement can significantly suppress coma.

Refer below example wherein a plano convex lens's performance is evaluated with aperture stop being on left side of the lens & with aperture stop being on right side:

Observe the geo radius values for off-axis field spot, you can see that spot size as well as coma spread is less when the aperture stop is placed appropriately, for the above case stop on right side of lens works fine.

2. Lens Shape Bending

Using Zemax optimization:

• Curvature distribution is tuned
• Marginal vs. paraxial ray focus is balanced
• Coma contribution per surface is minimized

Refer below example wherein one air-spaced doublet or 2 singlets are modelled & optimized, both the optical systems have same aperture size, focal length & lens material, but both are exhibiting different performances as per lens bending achieved through optimization iterations.

Observe the geo radius values for off-axis field spot, you can see that spot size as well as coma spread is less of OS 2 optical system compared to OS 1 despite having the same configurations & start design, OS 2's lens curvatures were optimized a bit more.

This is one of the most effective classical corrections.

3. Doublet based Optical Systems

Doublets or symmetric lens groups in Zemax:

• Introduce opposite coma contributions
• Enable aberration cancellation
• Maintain compact system length

Refer below example:

Compared to OS 2 system, OS 3 system is way better performing in terms of spot & coma spread. Also, the primary coma aberration (in waves) is quiet a low value. This tells us that a doublet lens pair is far superior to a singlet lens pair for coma correction.

4. Aspheric Surface Introduction

Zemax makes it straightforward to:

• Add aspheric surface & its coefficients directly or by using find best asphere tool
• Optimize higher-order terms
• Achieve near-diffraction-limited off-axis performance

This is widely used in modern camera optics.

Refer the below example:

OS 2 & OS 4 are same configuration optical system, but OS 4's performance in terms spot & coma spread is far less than the OS 2's performance.

Further if we look at the primary coma using full field aberration plot, it indicates that OS 4 system has less coma aberration (in waves) compared to OS 2

Section 06Why Zemax is Essential for Coma Analysis

Without simulation, coma correction requires trial-and-error prototyping.

Zemax enables:

• Fast aberration visualization
• Automated merit-function optimization
• Quantitative wavefront control
• Realistic manufacturing tolerance analysis

This dramatically reduces design cycle time and development cost.

Conclusion

Coma aberration is one of the most important factors affecting off-axis image quality in wide-field optical systems. Its presence can significantly degrade resolution and introduce distortions that impact applications ranging from astronomy to machine vision and automotive sensing.

Through advanced optical design software such as Ansys Zemax OpticStudio, engineers can perform detailed optical aberration analysis, identify the sources of coma, and implement systematic coma aberration correction strategies.

Mastering these techniques allows optical designers to achieve high image quality across the entire field of view while reducing development time and minimizing the need for physical prototyping.