jeudi 6 juin 2013

What Is Chromatic Aberration

By Brian Hart

Chromatic aberration sounds difficult, but it is really quite simple. It is seen in photos as magenta and blue-green fringes produced by the lenses. Chromatic aberration can be created in two ways: 1. The different colors do not focus on the same sensor plane. 2. The individual colors produce images of different size. Here we will take a deeper look at what chromatic aberration is and how to avoid or solve it.

Chromatic aberration is caused by the refractive index of glass, so let us first look at what refractive index is. Light changes its direction when it passes through a medium like the glass of the lenses. Light may hit the lens at 90 degrees, but leaves the lens at 80 degrees (just an example). Chromatic aberration arises because the different colors of light have different refractive indexes. Red might leave the lens at 81 degrees, while blue could leave at 79 degrees. This produces what is known as longitudinal chromatic aberration where you get thin magenta fringes. Since green is in-between red and blue it is used to focus the lens. Thus the red and blue are slightly out of focus which creates the magenta (red+blue) fringes

Transverse chromatic aberration arises when light does not reach the lens at 90 degrees, but from a different angle. In this case the different colors focus evenly, but not at the same spot. This causes the red image to be larger than the green and blue, and the blue the smallest of them all.This also creates colored fringes, but now both a magenta and a blue-green one. It is in the interest of lens manufacturers to avoid chromatic aberration, but since it is in the nature of light, it is hard to eliminate.

You get different kinds of fringes for each kind of chromatic aberration. Longitudinal aberration creates magenta fringes around objects and is spread uniformly throughout the image. Transverse aberration is absent at the center of the image, but grows in intensity towards the image corners. Longitudinal chromatic aberration is most pronounced in wide aperture lenses. It can be minimized by using a small aperture. Transverse chromatic aberration is most pronounced in telephoto lenses. However, lenses can be designed in many ways. The so called achromatic lenses are by far the most popular with minimal chromatic aberration. Superacromatic and apochromatic lenses almost eliminate color errors, but they are not common. Chromatic aberration can be seen on film, but is most pronounced on digital images. One explanation is that the sensors are more sensitive to ultraviolet and infrared light, which are at the outer edge of the spectrum where aberration is most pronounced.

Software can fix chromatic aberration. By sharpening the red and blue channels, one can somewhat correct longitudinal chromatic aberration; the green channel is used to focus the image and should be sharp. Transverse chromatic aberration is satisfactorily corrected by radially enlarging the blue channel image and radially reducing the red channel image.

A different kind of chromatic error is the dreaded purple fringe. It appears along hard contrast edges when photographing something against a hard back light, or when photographing a light source against a dark background.The purple fringe invades the dark area. Purple fringes are sensor errors, whilst chromatic aberrations are lens errors. Purple fringing is not a simple geometric error like transverse chromatic aberration, but is an overflow of light from the brightly illuminated sensor to its neighbors; hence it is very difficult to correct with software. Also the underlying color is usually eradicated. Software can thus reduce the color of the purple fringe to a grayish tone. At best the local color is not completely eradicated by the purple fringe and can be reconstructed.

About the Author:

Aucun commentaire:

Enregistrer un commentaire