Optics 101

Chromatic Aberration – The Definition of Chromatic Aberration

Introduction to Chromatic Aberration
Colored Fringes
Types of Chromatic Aberration
Longitudinal Chromatic Aberration
Lateral Chromatic Aberration
Differences between Longitudinal and Lateral Chromatic Aberration
Purple Fringing

Introduction to Chromatic Aberration

Chromatic aberration is also referred to as color fringing. Chromatic aberration occurs when a lens system is creating different focal lengths for different wavelengths of light. In other words, it is not creating identical focal plane for different light rays when the light rays are brought into focus.

Chromatic aberration arises due to the variation in the refractive index of every optical glass composition with wavelength. The refractive index in most cases is greater for shorter wavelengths such as blue light. Therefore the greatest refraction occurs for blue light, followed by green and red light. This observable fact is known as dispersion.

Colored Fringes

Colored fringes around an image is caused by the failure or inability of a lens to focus all colors to the same point. This leads to the image size and the focal point for each major wavelength group to appear slightly different. A green cast with a mixture of red and blue surrounding an image can occur when the focus is situated at the middle of the wavelength band.

Types of Chromatic Aberration

The two types of chromatic aberration in lenses are:

  • Longitudinal chromatic aberration
  • Lateral chromatic aberration

Longitudinal Chromatic Aberration

Focusing different wavelengths on different image plane is a result of longitudinal or axial chromatic aberration. Longitudinal chromatic aberration is also known as axial color. Longitudinal chromatic aberration is defined as the lens inability to bring different colors into focus in the same focal plane. In other words, focal length varies with color wavelength.

Lateral Chromatic Aberration

The second type of chromatic aberration is referred to as lateral chromatic aberration. Lateral or transverse chromatic aberration causes color fringing as a result of image magnification differs with color wavelength. A secondary chromatic aberration is also associated with lateral chromatic aberration. This secondary chromatic aberration causes difficulty in correcting blue, green and red together at the same time.

Differences between Longitudinal and Lateral Chromatic Aberration

Both Longitudinal and lateral chromatic aberration can produce colored edges. However longitudinal chromatic aberration causes colored fringes all around objects, whereas lateral chromatic aberration only affects tangential details.

Furthermore lateral chromatic aberration progressively worsens towards the corners of the image whereas longitudinal chromatic aberration can occur at any image position.

Purple Fringing

Purple fringing is a digital camera phenomenon that is generated by microlenses. Purple fringing is clearly visible throughout the entire image, which is different to normal chromatic aberration.

Source: AZoOptics

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