![]() To deal with these issues Chromatic aberration is also expressed in percentage of the distance from the image center to the Region of Interest (ROI), corrected for the angle of the ROI with respect to the center. ![]() The chromatic aberration in most lenses is roughly proportional to the distance from the image center. The result depends strongly on the measurement location.It penalizes cameras with high pixel counts.(shown reversed: displayed levels always increaseĪlthough area in pixels is a good measure of perceptual CA, it has some shortcomings. It is proportional to the optical CA (the actual chromatic aberration of the lens, unless it has been corrected during raw conversion), but it is less visually significant.Ĭhromatic Aberration figure (relatively high CA) The distance between the crossings (the centers of the transitions) is also shown. It is displayed in magenta in the figure on the right. CA defined by this equation is called the area chromatic aberration. Since x has dimensions of distance in pixels and Edge is dimensionless, CA (area) has units of pixels- units of distance even though it is an area. Imatest analyzes the edge and produces a number that indicates the severity of the lateral chromatic aberration. The black-to-white edge to the right side of this rectangle has equally vivid green fringing. Red fringing, the result of lateral CA, is clearly visible. The thumbnail on the right is from a 12 megapixel compact digital camera with fairly high chromatic aberration. SFRplus charts, which are available in a wide variety of sizes and media, are much better for measuring CA. The obsolete ISO 12233:2000 chart (shown on the right) has a very limited number of appropriate edges. Because typical Imatest SFR edges have angles in the range of 3 to 7 degrees with respect to vertical and horizontal, the best edges for measuring CA are near-vertical edges on the left and right of the image. Tangential curves and radial lines (which differ by 90 degrees) are shown in burgundy and blue in the Test chart illustration, above. Lateral chromatic aberration is best measured on a tangential (or nearly tangential) edge near the sides or corners of an image. It tends to be far more visible than longitudinal CA. Lateral Chromatic Aberration is the color fringing that occurs because the magnification of the image differs with wavelength.It can be measured by acquiring SFRplus, eSFR ISO, or Checkerboard images at multiple distances, analyzing them as a batch, then running the resultant JSON file in the FocusField postprocessor. It cannot be measured from a single image by Imatest it causes a degradation of MTF response- by differing amounts for different colors. Longitudinal Chromatic Aberration causes different wavelengths to focus on different image planes.The two types of chromatic aberration are illustrated above. In this case, the chart will be displayed in pale colors and CA in % will be omitted. ![]() Pixel shift ( not CA) is best measured on near-vertical and horizontal edges near the center of the image where CA is minimum.ĬA cannot be measured reliably if the center of the region of interest (ROI) is less than 30% of the distance from the center to the corners. Imatest applies a correction to edges that are not exactly tangential. It is not visible on radial edges such as A. Measurement tips- Lateral chromatic aberration is best measured using (nearly) tangential edges near the sides or corners of the image, for example, edge B (above). Lateral and Longitudinal CA Tangential and radial lines In many designs intended for digital cameras, CA is no longer a major consideration because it can be corrected in the digital domain if the correction is applied before demosaicing (as long as it’s not too extreme). But it remains a problem in several lens types, most notably ultrawide lenses, long telephoto lenses, and extreme zooms. It is accomplished by combining glass elements with different dispersion properties. Minimizing chromatic aberration is one of the traditional goals of lens design. It is sometimes confused with another effect, which we call pixel shift- a color channel offset that is typically uniform over the sensor and can be caused by physical misalignment of multi-chip sensors or demosaicing errors. It appears as color fringing, most visibly on tangential edges near the boundaries of the image. (Others include coma, astigmatism, spherical aberration, and curvature of field.) It occurs because the index of refraction of glass varies with the wavelength of light, i.e., glass bends different colors by different amounts. Chromatic aberration (CA) is one of several aberrations that degrade lens performance.
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