Image Quality Factors

Chromatic Aberration

Image Quality Factors
  1. Introduction
  2. The background of chromatic aberration
  3. Types of chromatic aberration
    1. Longitudinal (axial) chromatic aberration
    2. Lateral (transverse) chromatic aberration
  4. Correcting chromatic aberration
    1. Achromatic lens
    2. Apochromatic lens
    3. Primary and secondary spectrum chromatic aberration
  5. Conclusion
  6. References

Introduction

Lenses are designed so that light refracted at a lens meets at one focal point. Light, however, contains various wavelengths, which are refracted differently. So light rays of different wavelengths (red, blue, and green, for example) may not meet at a common point. When this happens, color fringes appear along the borders of very light or very dark parts of an image affecting the image quality.

The background of chromatic aberration

Chromatic aberration is caused by the dispersion of light best demonstrated by using a prism (in our case, a lens). Dispersion is the separation of visible light into its different wavelengths. The light that passes from one material to another, will be refracted or bent at the boundaries.

Dispersion of white light
Image 1: Light passing through a prism and being bent as it enters and again as it exits. The blue ray of light is refracted stronger compared to the green and red.

Image 1 depicts the blue ray of light is refracted stronger than that of the green and red. The intensity of the refraction depends on the wavelength as well as the optical density of the prism that the light passes through. Shorter wavelengths (blue) will bend more than longer wavelengths (red) and material with a higher optical density will bend more compared to a lower optical density. Optical densities are described by an index of refraction.

A few examples of optical densities:

  • Vacuum – 1.000
  • Air – 1.0003
  • Water – 1.3333
  • Crown Glass – 1.52
  • Dense Flint Glass – 1.66
  • Diamond – 2.417

Light passing through a lens is no different than light passing through any of the materials listed above. The focal length of a lens is dependent on the refractive index as different wavelengths will be focused on different positions as they pass through the lens into the sensor.

Ideally, a lens would be able to offset the various dispersions of the wavelengths by focusing them on the same point. This lens, however, does not technically exist as a single lens, and as a result, chromatic aberration is caused by lens dispersion. In other words, multiple colors of light will travel at different speeds while passing through a lens. This dispersion often leads to images with colored edges (red, yellow, green, blue, magenta, etc.) especially around objects in high contrast situations.

To compensate for the dispersion, optical systems consist of multiple lenses that are both convex and concave and made from different glass types with different dispersion levels. Lenses for which the dispersion is corrected for two wavelengths are called achromatic lenses while lenses that have been corrected for three wavelengths are called apochromatic lenses.

Ideal Lens
Image 2: A non-existing ideal single lens that points the wavelengths to the same point within the sensor.

Types of chromatic aberration

There are mainly two different types of chromatic aberration, longitudinal (axial) and lateral (transverse).

Longitudinal (axial) chromatic aberration

Longitudinal chromatic aberration occurs when different wavelengths are dispersed from the lens at different points along the horizontal optical axis (image 3). Wavelengths scattered across the axis are known as the circle of confusion and it leads to unintentional color fringes even in the center of the image. To reduce the effects of longitudinal chromatic aberration, one of the main strategies is stopping down the aperture as the aperture is responsible for the incoming light quantity.

Longitudinal CA
Image 3: The green light is focused sharply on the image plane while the red and blue are not focused sharply on the image plane.

Lateral (transverse) chromatic aberration

Lateral chromatic aberration occurs when different wavelengths enter the lens at an angle and then focus at different positions along the same focal plane. Compared to longitudinal chromatic aberration, lateral only develops towards the corners and gets visible at high contrast image structures. It cannot be fixed by stopping down the aperture. Lateral chromatic aberration can only be amended in post-processing software.

Lateral CA
Image 4: Angled wavelengths focusing at different positions along the same focal plane.

Correcting chromatic aberration

Even though chromatic aberration is difficult to correct and often impossible to fully eliminate, many lens solutions can lead to higher quality corrections.

Achromatic lens

An achromatic correction applies to wavelengths at both ends of the visible spectrum (red and blue). An achromatic lens contains, for example, an element of convex crown glass (lower on the refraction index) and an element of concave flint glass (higher on the refraction index). This combination creates what is known as an “achromatic doublet,” which can reduce the effects of chromatic aberration. In complex optical systems, these elements are often combined with extra-low dispersion glass (ED glass).

Apochromatic lens

Apochromatic correction is designed to bring three wavelengths (normally red, green, and blue) into focus on the same plane. To do this, three types of glass elements such as flint, crown, quartz, etc. are combined to create the “apochromatic triplet.” Unfortunately, combining three elements can be highly expensive and lead to other lens issues during production. As with achromats, a combination with ED glass can also be used for a more complex system.

Primary and secondary spectrum chromatic aberration

A lens that has not been corrected at all is known as a chromatic lens and will show a primary spectrum chromatic aberration. In other words, the far ends of the spectrum, i.e., the red and blue wavelengths, will focus differently.

When using an achromatic lens design, the red and blue spectrum will focus at the same point on the horizontal axis, but the green wavelength will continue to focus on a different point. This issue is known as secondary spectrum chromatic aberration and will portray magenta and green artifacts in the image as opposed to red and blue artifacts.

Secondary longitudinal
Secondary lateral
Image 5: Illustrations of secondary spectrum longitudinal (left) and secondary spectrum lateral (right) chromatic aberration.

Both longitudinal and lateral produce secondary spectrum chromatic aberration. Photographers mostly encounter lateral secondary chromatic aberration in their images. As a result, magenta and green colors will appear along the borders of high contrast details especially towards the corner of the image.

Chromatic aberration
Image 6: Angled wavelengths focusing at different positions along the same focal plane.

Conclusion

Chromatic aberration is caused by the dispersion of light. Dispersion is the separation of visible light into its different wavelengths. When this phenomenon occurs, color fringes will appear along the border of an image in either very light or dark parts of the image. These colors often have a damaging effect on the overall image quality of the camera.

Chromatic aberration is often impossible to fully eradicate from a lens, but to ensure that it is kept to a minimum, we recommend testing various lens solutions (e.g., achromatic or apochromatic lenses) and then implementing the right one for your system.