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Imatest Documentation horizontal and vertical axes, respectively. The sign may be different from the sign in the percentage measurement, depending on the measurement quadrant. Average pixel levels in the dark and light areas. Clipping can occur if they are too close to 0 or 255. Because Chromatic Aberration cannot be measured accurately near the image center, the chart is rendered in pale colors with the Region of Interest (ROI) is less than 30% of the distance from the center to the corner. Demosaicing Chromatic Aberration in percentage of distance from the image center Severity of chromatic aberration Severity 0-0.04 Insignificant 0.04-0.08 Low. Hard to see unless you look for it. 0.08-0.15 Moderate. Somewhat visible at high print magnifications. over 0.15 Strong. Highly visible at high print magnifications. Severity of chromatic aberration (old table) Chromatic Aberration in pixels Severity 0-0.5 Insignificant 0.5-1 Low. Not visible unless you look for it. 1-1.5 Moderate. Somewhat visible at high print magnifications. over 1.5 Severe. Highly visible at high print magnifications. Purple fringing is not chromatic aberration, though it is often mistaken for it. It's a saturation phenomenon in the sensor, also known as "bloom," caused by the overflow of electrons from highly saturated pixel sites to nearby unsaturated sites. It tends to be worst in cameras with tiny pixels (e.g., 8+ megapixel compact digital cameras). It has everything to do with the sensor and nothing to do with the lens. Demosaicing is the process of converting Bayer RAW images, which have one color per pixel (RGRGRG...; GBGBGB...), to standard images, which have three colors per pixel. In the process of demosaicing, missing detail for each channel is inferred from detail in other channels. This is especially significant for Red and Blue pixels, which are half as common as Green. Demosaicing algorithms can be very mathematically sophisticated, but all of them can perform poorly in the presence of lateral CA, where detail is shifted from its expected location. Demosaicing explains the shifted edges shown in the above example. Lateral CA cannot be reliably corrected after demosaicing, but it can be corrected to near-perfection prior to demosaicing. Correction coefficients can be calculated with Imatest Master, which can analyze Bayer RAW files created by converting manufacturer's Camera RAW files. Details are in the page on RAW files. Here is an example illustrating the same region for a RAW and demosaiced file. Canon EOS-20D, 17-85mm IS lens, 53mm, f/11. 38 of 451
Imatest Documentation Chromatic aberration before demosaicing: easy to correct using a different magnification for each color ( (1-0.00023)x for red; 1.000622x for blue; 1 for green). Chromatic aberration after demosaicing: difficult to correct. R-G and B-G are the CA correction coefficients. They are the spacing between the Red and Green and Blue and Green crossings, respectively, expressed as percentage of the center-to-corner distance. This mesurement is relatively independent of the location of the measurement. Corrected chromatic aberration measurements In Imatest, edge profiles are measured along horizontal or vertical lines. The blue line (x) on the right is an example. But chromatic aberration takes place along radial lines-- lines from the center of the image to the region of measurement (shown in red on the right). Unless this line is vertical or horizontal, there will be a measurement error that must be corrected. The correction is illustrated on the right for a near-vertical edge, where the profile, and hence CA, is measured horizontally. In 39 of 451
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