Chromatic Aberration

Vincent Bockaert, 123di.com

Chromatic Aberration in a Single Lens

Chromatic aberration or "color fringing" is caused by the camera lens not focusing different wavelengths of light onto the exact same focal plane (the focal length for different wavelengths is different) and/or by the lens magnifying different wavelengths differently. These types of chromatic aberration are referred to as "Longitudinal Chromatic Aberration" and "Lateral Chromatic Aberration" respectively and can occur concurrently. The amount of chromatic aberration depends on the dispersion of the glass.

Longitudinal or Axial Chromatic Aberration
Focal length varies with color wavelength
Lateral or Transverse Chromatic Aberration
Magnification varies with color wavelength

Chromatic aberration is visible as color fringing around contrasty edges and occurs more frequently around the edges of the image frame in wide angle shots.

Example of cyan and red fringing

Achromatic / Apochromatic Doublets

Special lens systems (achromatic or apochromatic doublets) using two or more pieces of glass with different refractive indexes can reduce or eliminate this problem. However, not even these lens systems are completely perfect and still can lead to visible chromatic aberrations, especially at full wide angle.

"Purple Fringing" and Microlenses

Although the above chromatic aberrations can be purple in color under certain circumstances, "Purple Fringing" usually refers to a typical digital camera phenomenon that is caused by the microlenses. In simplified terms purple fringing is "chromatic aberration at microlens level". As a consequence, purple fringing is visible throughout the image frame, unlike normal chromatic aberration. Edges of contrasty subjects suffer most, especially if the light comes from behind them, as shown in the example below. Blooming tends to increase the visibility of purple fringing.

Example of purple fringing
This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
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