Chromatic aberration
| The term chromatic aberration covers a number of different optical phenomena, all of which manifest as unwanted colored fringes round picture elements, particularly noticeable around high-contrast transitions in relatively neutral picture areas. Lateral chromatic aberration is produced when optical distortion results in object points coming to a focus at different points in the image field, depending on the wavelength (= color) of the light. The result is a focused image spot that is displaced between the three (Red, Green, Blue) color channels. With ever-higher sensor resolution, lateral chromatic aberration is becoming increasingly visible, in turn making it more and more important to precisely address distortion for each color plane. Longitudinal chromatic aberration occurs when the lens brings light from the object to a focus in different image planes according to its wavelength. The result is an image spot that is less sharp (= larger) from one color channel to another; to further complicate matters, there may also be differential astigmatism! Both of these produce unwanted color fringes that are lens and setting dependent.
(Sony F828)
So-called ‘purple fringing’ is essentially due to lens longitudinal chromatic aberration, causing the blue or/and red channel to be slightly defocused with respect to the green channel. When only the blue is out of focus, fringes appear blue. When both red and blue are defocused, fringes appear purple. Purple fringing is more visible close to backlit parts of the image, where the contrast is maximal.
"Coma" describes the phenomenon where highlight areas produce a ‘halo’ effect that may be different between the three color channels. Other causes of unwanted color artifacts may include, among other things, the particular demosaicing algorithm used in certain equipment. DxO Optics Pro handles chromatic aberration in two ways:
The first correction takes place as part of DxO’s distortion and sharpening algorithms, where the calibrated corrections are applied to the three color channels independently, which is a good solution for the equipment-dependent errors, where camera/lens support is available. A second level of correction, which is available for all images (irrespective of having the appropriate DxO Correction Module), makes it possible to also correct for those phenomena that are picture-content dependent. This works basically by suppressing differences between the color channels where these are associated with large luminance transitions in the image. |