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Chromatic Aberration (CA) typically shows up on a photograph as blue or purple near an edge that is high in contrast. It is caused by the lens having a refracting index that is different for varying wavelengths of light. What this means is that the speed of the light traveling through the lens does not stay constant across the whole image. Changes in speed of light through the lens causes it to shift toward blue or purple.
Here is a shot that I took a while back that has a lot of chromatic aberration. It may not be very visible in this small picture. So take a look at the next three 100% crops to see the details
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| The neck shows blue on the left side and purple on the right | This wing is looking pretty bad! | Even the water has high amounts of chromatic aberration. |
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What can be done to fix this? The best solution is use a better lens! But before you throw your lens in the trash, here is a method using Adobe Photoshop that can usually take care of the problem.
First we need to zoom in on the area where the CA appears the heaviest. I typically zoom in to about 400% to get a good view of it. Then in the layers box, click on the little half black, half white circle at the bottom, then select Hue/Saturation from the drop down menu.
In the dialog box, under “Edit” select either “Blues”, or “Magenta” depending on the color that you want to fix. For this example, I’ll use the blues to get rid of the CA on the back of the gooses neck.
It should already be selected, but if not, click the eye dropper tool in the dialog box. Of the three, pick the one on the left. Then click on an area that shows the CA really well. This will refine the color selection in the dialog box.
Next, move the Saturation slider to the left until the CA disappears. Be careful though, because this can remove color from areas of the photograph that you did not want to disappear. You may need to zoom out to take a look.
Finally, if everything looks alright. Go ahead and flatten the image.
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