Chromatic Aberration - Guillermo Algora - Visual Effects Compositor

Guillermo Algora
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CHROMATIC ABERRATION
Chromatic aberration occurs when different wavelengths of light are refracted differently by a lens, causing colors to focus at different points. This is due to, for example, the refractive index of glass which is wavelength dependent, with higher refraction for shorter wavelengths (blue) compared to longer wavelengths (red), resulting in visible color fringing at the edges of elements in the image, especially noticeable in areas of high contrast and generally is more pronounced around whites (as it contains all the wavelengths) or highly saturated colors (as the contrast between the colored element and the surroundings is higher).

The resulting c
olor patterns are greatly influenced by the wavelengths of the light involved as well as by the lens design, for example, such as achromatic lenses that are specifically designed to focus two specific colors simultaneously (usually red and blue) or apochromatic lenses which are designed to bring three colors into focus in the same plane. Therefore, although we will explore the "basic" scenario, chromatic aberration can vary greatly visually.

There are two types of chromatic aberration, both can degrade image quality and reduce sharpness, but they can also be used creatively for artistic effects or to enhance the realism of a composition.
Table of Contents:
1. Longitudinal (axial).
2. Lateral (transverse).
1. Longitudinal (axial):

Also known as axial chromatic aberration, it occurs when different wavelengths of light are focused at different distances along the optical axis: the short wavelengths (blue) focusing nearer to the lens (due to the higher index of refraction) and long wavelengths (red) further away, while green being in between the two.

This visually results in blur of colors in front of and behind the actual focal point, depending on which wavelength of color is actually in focus.
Nonetheless, while its color pattern depends heavily on the specific lens design and wavelengths of light involved, a common visual result of longitudinal chromatic aberration is the red and blue planes being defocused (assuming that the green plane is in focus, as it is generally the most sensitive wavelength in film/sensors), showing magenta color on the outer edge and green color on the inner edge of the fringe.

This phenomenon is an axial optical error (i.e. along the optical axis) and therefore affects all parts of the image roughly equally. It is often most apparent in parts of the image that are somewhat out of focus and most noticeable at or near full aperture (due to the narrower depth of field), in ultra-fast lenses (due to extremely wide apertures) and in long focal lengths (due to magnification). It can be reduced by stopping down the lens (narrower aperture), which increases depth of field so that though the different wavelengths focus at different distances, they are still in acceptable focus.
Longitudinal CA Diagram
Longitudinal CA Representation
2. Lateral (transverse):

Also called transverse chromatic aberration, this type occurs when different wavelengths of light are focused at different positions along the focal plane (i.e. perpendicular to the optical axis) as the magnification of a lens (i.e. focal length) varies with wavelength (varying along the radii).

The most prominent visual characteristic of lateral chromatic aberration is the presence of colored fringes at the edges of high-contrast elements. Most commonly, with reddish color on the outer edge and violet/blue color on the inner edge of the fringe.

It is an abaxial optical error (i.e. off the optical axis) and therefore does not occur in the center of the image, while it increases towards the periphery. Contrary to longitudinal chromatic aberration, it is not affect by stopping down the lens (narrower aperture), as aperture changes do not have an impact on focal length (being the focal length difference between colors the root cause). This phenomenon is often observed in wide-angle lenses, because they are more prone to distortion and light entering at obtuse angles (i.e. abaxial light).
Lateral CA Diagram
Lateral CA Representation
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