Chromatic aberration

The chromatic aberration ( gr χρωμα chroma, color ' and Latin however rare, wander '; in photography often abbreviated, CA ' ) is an aberration of optical lenses, arising from the fact that light of different wavelength or color differently refracted will. A distinction is made between the front lateral chromatic aberration, which particularly manifests itself at image edges in green and red or blue and yellow color fringing to light-dark transitions, and the longitudinal chromatic aberration in the form of different stains and behind the focal plane.

Cause

The degree of refraction ( refractive index) of a medium is dependent on the wavelength of light ( dispersion). This short-wavelength light (blue) is more than long-wave (red) broken and thus decomposes a previously appearing white light into its spectral colors in general. This effect is particularly visible in the prism.

Longitudinal chromatic aberration and lateral chromatic aberration

In the case of a converging lens, this leads to different focal lengths for different wavelengths, i.e., the blue component of the image is focused in front of the red component.

Both by their position on the optical axis and through its imaging plane in the image plane images shown of the three primary colors from each other. Therefore, differentiating with the chromatic aberration between a longitudinal chromatic aberration ( axial deviation, or longitudinal / axial chromatic aberration ) and a transverse chromatic aberration ( the image plane on, also: Color magnification error, or transverse / lateral chromatic aberration). Due to the longitudinal chromatic aberration caused different color edges in front and behind the focal plane. The lateral chromatic aberration produces color fringing on non radial contrast edges whose color depends on whether it is viewed from the image center to is a light-dark transition, or a dark-light transition. The lateral chromatic aberration is not visible in the center of the image and takes the edge of the image to go.

Countermeasures

This error can be corrected by combining lenses of glasses of different dispersion. If this the most divergent wavelengths or the primary colors red and blue together, it is called an achromatic correction or an achromatic lens, that is, the system has the same for both colors sectional width.

If, moreover, the primary color green merged with the other two, is an apochromatic correction. Thus, the lateral chromatic aberration is corrected. It is only at very high - available optical systems such as, for example, at a apochromatic lens - and correspondingly expensive. Such objectives are often not always lead, but the abbreviation " Apo " in the name.

Longitudinal chromatic aberration can be reduced by stopping down the lens. When lateral chromatic aberration, the ratio of aperture and extent of the error is less clear and varies from lens to lens.

An apochromatic lens, which should compensate for this, having a larger number (4-6 ) of individual lenses made of different glass types. They have equal focal lengths for the wavelengths (480 nanometers = cyan, 546 nanometer = yellow-green, 644 nm = red).

In digital photography can be lateral chromatic aberration by digital image processing to correct later by the different color channels of the image are scaled differently (about a RAW converter ).

Astronomy and distance vision

The atmosphere distorted color reproduction, since both the light absorption of the air and the light refraction of the wavelength dependent. Although the human visual system interprets the characteristic blue color of distant objects as depth information on optoelectronic sensors, they would, however, dissuade as a correction. Further, in the near horizon constellations and at atmospheric halo phenomena often occurs a vertical color fringe, astronomical because refraction is different for the red and blue light components.