CORRECTING CHROMATIC ABBERATIONS

Sunday, July 10, 2005

Chromatic abberation is a problem that annoyed me very often, especially when I was shooting birds after early morning hours. Here is a detailed description of the problem and the solution.

The problem

Sometimes you may see unusual colors on the edges in some pictures. The edges will have purple, red or cyan lines which are not present in the original object. This is likely to happen on heavily backlit objects or overexposed objects. Although this may not be evident in small sizes, it becomes obvious when viewed at 100% size. See the picture below. See the 100% crops that show purple lines.

A purple rumped sunbird

100 percent crops showing purple fringing

This is usually called color fringing, or purple fringing if the color induced is purple. The induced color is usually purple, sometimes cyan or red.

Why does it happen?

This happens due to an optical phenomenon called chromatic aberration. Refractive index of a glass is sensitive to wavelength of the light. This means that all colors do not focus on the same plane. Violets and Reds, which are on the edge of visible spectrum would get focused at different plane than others and hence create these colored edges.

How to avoid it?

Chromatic aberration is likely to happen when you shoot wide open. Stopping down the lens as much as possible can make the color fringing disappear. But this may not always be possible depending on the light conditions and the object you are shooting.

Correcting in Photoshop

There are several ways to fix the problem in Photoshop. Each one of it can help eliminate the stray colors. You can choose any one of these methods based on your comfort levels. I personally prefer to use Replace Colors option.

1. Using Hue/Saturation Command

a. Open Hue/Saturation command by clicking Ctrl+U or Image >> Adjustment >> Hue/Saturation. This opens the window below.



b. In the Edit drop down box, choose magenta.
c. Use the eyedropper tool and click on any point in the picture that needs to be fixed for chromatic abberation.
d. Reduce the saturation slider and lightness slider until the purple color fades. Doing this should remove the fringing.

2. Replace color Command

a. Open Replace Color command by clicking Image >> Adjustment >> Replace Color



b. Use the eyedropper tool and click on the area where the color needs to be changed. You see a preview of the area where the colors are going to be replaced. Use or and click on the area where colors have to be replaced until you get the desired area selected.
c. Create the color you want to replace the purple color with. This can be generated by adjusting hue, saturation and lightness sliders. Otherwise click on the box to open color picker and choose the color you want to replace with.
d. Once you have the desired color, click OK to replace the purple edges with this color. This should remove the purple fringing.

3. Color Replacement Tool

This is a crude method that requires replacing the colors by hand using a brush. But this could be useful if the image to fix has purple colored objects that should not be subjected to color replacement.




a. Select color replacement tool from toolbox.
b. You see some tools appearing on the top. Click on the brush; select the diameter of the brush to be almost the size of the purple lines.
c. In the picture, point to a place where you have the color to be replaced with. This would be usually adjacent to the border where you have color fringing. Now hold Alt key and click. Photoshop will take this portion of the image as the target color for replacement.
d. Move the brush on the area where purple colors have to be replaced.

OTHER OPTIONS

1. For those who shoot RAW, Adobe Camera Raw has commands that help eliminate chromatic aberration.
2. Panorama tools, a Photoshop plugin can be used to fix chromatic aberrations.
3. Color fringe reducer is another tool for removing CA and is available as photoshop plugin.
4. If you are shooting raw, photoshop can make it easier to correct CA. See here for details.

Below is the fixed image. This was dervied using Replace Color command which is my preferred way of fixing.



Despite many possibilities available to remove chromatic aberraion, it is better to take preventive measures to avoid this from happening. Remember that it occurs on overexposed and backlit objects. Closing down the lens as much as possible helps reduce the chances of chromatic aberration.