You may have heard photographers talk about “Chromatic Aberration” like it’s a disease. Today, we’ll give a explanation of what this term means, and share an advanced technique for removing it from photos, should the need arise.
A function of light conditions or using a lens incorrectly, chromatic aberration can ruin a photograph and reduce the amount of detail captured by the camera. While lenses and cameras are developed with technology to reduce these glitches, the simple fact of the matter is they still pop up under the right (or wrong?) conditions. Keep reading to learn more, including how you can fix them with Photoshop or GIMP.
Defining Chromatic Aberration in Photography
Here’s the image we found that’s a good example of chromatic aberration. Parts of the image appear soft, which could be partly from the lens being slightly out of focus, except that the aberration is almost certainly causing a loss of detail resolution. Notice the blue halo on the left hand side of the image and scattered throughout in various areas.
Lenses can change the speed light moves through it, and this is a function the frequency of that light. As the light travels through this lens, the different wavelengths (colors) move at different speeds and fall in different places on the sensor. Sometimes using a lens in a way it was not designed can cause an aberration—like this shot, using a lens that was possibly inappropriate such a extreme closeup. As a result, the red, green, and blue converge on different points and create image channels that don’t line up.
There are more technical descriptions of what a chromatic aberration is, but for our purposes, we’ll be focusing on this simpler explanation and how it relates to image files.
In this animation, it becomes more clear what all this means. You can see the image shift as it cycles through the red green and blue color channels. Each channel may be rendered reasonably sharply, but because they combine to create an image, the image quality is damaged. It is possible to repair that damage, and here’s a proper method, using an understanding of what a chromatic aberration is.
Fixing A Chromatic Aberration the Advanced Way
Open the image afflicted with aberration. The appropriate way to fix an aberration is to simply adjust and align the channels to fit to one another. This can be a challenge, as light tends to speed up at different points in the lens, creating more distortion is some areas than others.
A note to readers: Our demonstration is done in Photoshop. But for those readers using other programs, like freeware GIMP, can also use this method, as we’ll be using techniques common to powerful image editing software. Readers working from RAW photographs have a quicker, more automatic method for reducing aberrations already existing inside camera raw. There’s also a specific kind of aberration sometimes called “purple fringe” that isn’t what we’re adjusting today. More information on all of that later.
Begin your work by making a copy of your image. We’ll be doing the majority of our adjustments on this copy, but we’ll also need our background image when we’re done.
We’ll start by adjusting our green channel to fit our red channel. The frequency of the light speeds up as light moves from red to green to blue, so we’ll adjust green and blue to fit to slower frequency red. Select your green channel and make sure that your blue channel is turned off so you can see how your green and red channels overlap.
With your green channel selected, press Ctrl + A to select all, then press Ctrl + T to do a free transform on that green channel.
Carefully transform the channel to fit around the outside edges. When you’re done, press enter, then Ctrl + D to get rid of the marquee. Don’t worry if part of your image still appear have some aberration. We will fix these later.
Same adjustment goes for the blue. Turn off the green channel, then select and turn on the blue channel as shown.
You’ll make a similar adjustment to your blue channel—select all with Ctrl + A, then transform the channel with Ctrl + T. When you’re done and your image fits around the outside edges, press enter to commit to the transformation.
When you return to the combined RGB channels, you’ll find that this has repaired much of the obvious aberration in the photo. It’s probably impossible to perfectly repair aberration on every photo, so it’s not a perfect repair. Parts of the image are now somewhat blurry, so let’s take a moment to repair those.
Repairing Blurs Caused by Shifting Channels
You can easily spend an unnecessary amount of time tweaking out aberrations, but for the sake of demonstration, we’ll just eliminate the ones causing blur in the main focal point.
Create a copy of your newly tweaked layer.
Select the green channel and nudge it to fit in your focal point areas. You’ll have to select all with Ctrl + A before you can nudge the image in the green channel. Repeat with the blue channel, worrying only about your focal points.
Mask out the areas outside of your focal point. Use your brush tool to blend them together. When you’re done, group your two copies and create a layer mask for your group. Use it to revert any areas you care to to your original image.
Create a small crop to remove the edges affected by the channels you’ve transformed.
Repeat this process as many times as you feel you need to to perfectly fix aberrations to the level you need. It may take multiple adjustments to channels to get the image perfect, or you may be satisfied with one simple adjustment.
Feeling a little bit more confident about how to deal with this sort of problem? Confused about the complexity of this how to? Have a better, simpler method for combatting chromatic aberration? Tell us about it in the comments, or send your suggestions to firstname.lastname@example.org.
Image Credits: Daisy by Ivan T, Creative Commons.
- Published 03/12/12