Aberration filter
The chromatic aberration (CA) decreases sharpness of images and causes color contours, lines, and spots which objects in reality do not have. It is caused by optical effects and processes on camera sensor.
There are two most visible types of CA that appear in digital cameras: first is purple fringing and second is lateral CA. Helicon Filter can help to fix them both.
Example of the purple/blue fringing (blooming):
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| The diadem got the purple contour which in reality it doesn't have |
The purple contour is removed |
Examples of the lateral CA:
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Original image The lateral CA usually appears on the sides and in the corners |
The right top corner (marked with red rectangle |
The lateral CA is removed, the letters have no contours |
The lateral CA is most visible on the contrast areas on the sides and in the corners of images. It looks like the red and blue parts of the image are shifted from or to the center of the image.
To get better understanding of CA, read the following articles: http://www.dpreview.com/learn/?/key=chromatic+aberration
Helicon Filter has two mechanisms for fixing CA:
1. Approach for fixing purple fringing: The program defines areas which have brightness higher than the brightness threshold. Then it desaturates the pixels of CA colors which are within radius from the bright areas.
2. Approach for fixing lateral CA: The program distorts red and blue color planes of the image to compensate the faults of the lens.
Recommended workflow
To fix the purple fringing:- Use the eyedroppers (Current CA color & Other CA color) to set the colors of the CA artifacts.
- Find the minimum radius which removes the most evident CA artifacts.
- Starting from a low threshold, find the maximum threshold, sufficient for removing all CA artifacts.
- Use CA map to check if other parts of the image were not affected.
To fix the lateral CA:
- Move the Lateral CA slider back and forth and find the value which compensates CA.
- If it doesn't help, then use the same approch as for fixing the purple fringing.
Current CA Color & Other CA Color
The eyedroppers let you define two exact colors of the CA artifacts by clicking with the mouse pointer on the image. Based on Color tolerance level, the program will look for similar colors and desaturate them. Zoom in to pick the exact colors.
CA Parameters
Radius - defines the distance from the bright overexposed areas which will be desaturated. Try to find the minimum radius, which is sufficient to remove all CA artifacts.
Brightness threshold - defines the bright areas. The program assumes that areas brighter than the threshold cause the blooming effect. Try to find the highest suitable threshold value to minimize artifacts in other parts of the image.
Color tolerance - defines precision of the color match. The zero level of color tolerance means that only exact colors defined by the current CA color and other CA color will be removed (desaturated). Very high color tolerance (100) will include all color in the processing.
CA Map
The CA Map switches to the special mode, where intensity of blue color corresponds to the degree of desaturation. In this mode you can check where your image is affected.
Lateral CA
Lateral CA slider changes the strength of the changes in blue and red color planes. Move it back and forth to find the value which gives the best result. Please note that this tool has no effect in the center of the image because lateral CA is most pronounced in the corners.
Lateral CA compensation should be applied before cropping.