What is Chromatic Aberration?

March 23, 2018  •  Leave a Comment

Have you ever shot an image only to have those pesky blue, green, or purple halos around the edges of some elements in the image?  This is called “Chromatic Aberration”.   This happens because the wavelengths of color do not all converge at the focal point in the image.  Your image is made up of a combination of colors and tones.  It’s this combination that gives us our pictorial view of the world around us.  The lens attempts to capture this light through the shape of the glass in the lens and movement of the glass element to converge the light together for a proper focal point.  Sometimes specific colors do not converge at the focal point.  Therefore, that color of light appears as a halo on edges.

Take a look at the diagram below.  Notice how the light is converging at a focal point.  The expectation is that all of the colors will converge at this point.

Courtesy of PhotographyLife.com

 

When this happens you won’t see the fringing, or halos, around the edges.  Below is another diagram showing what actually happens when the light rays converge at a different focal point.

Courtesy of PhotographyLife.com

 

See how the colors are converging at different points?  In this diagram the blue and red rays are converging at different focal points and will cause the fringing to occur.  This fringing will be off of the sharp edges and be somewhat out of focus.  This type of chromatic aberration is called “Longitudinal Aberration” because it occurs horizontally along the Optical Axis.

The second type of chromatic aberration is called “Lateral Aberration” because it moves along the focal point.  Take a look at the image below to see how the light rays move along the focal plane.  When this occurs you will only see the fringing along the corners, never in the middle of the image.  This typically occurs with fisheye, wide angle, or low quality lenses.

Courtesy of PhotographyLife.com

 

So now that we know what it is how do we prevent it?  Well the good news is that usually stopping down the lens will help with longitudinal aberration.  It’s much more common in fast aperture prime lenses than slower ones.  The bad news is that there is really no answer to controlling lateral aberration.  As indicated above, this type of aberration usually occurs with fisheye, wide angle, or low quality lenses.  The best thing you can do with this is to learn how to remove it in postproduction.

That being said, once you discover a particular lens that is prone to this problem there are a few of things you can do that may help.  First, avoid high contrast scenes with that lens.  These types of scenes are more subject to fringing and may include white backgrounds, bright sunrises, and backlight subjects.  If you see it occurring in a particular lens that you are using, you may want to recompose or change lenses.

The second thing you may do, as alluded to above, is to change your focal length.  If you are using a zoom lens, the fringing usually occurs at the shortest and longest focal lengths.  Therefore, if you have to shoot at a short or longer focal length you may want to change to a prime lens.

Finally, once you determine which lenses are prone to lateral chromatic aberration you may want to shoot with your subject more in the middle of the frame.  I know you were taught to compose in camera, but since lateral aberration does not occur in the middle of the frame you can avoid the problem and crop to a more pleasing composition in postproduction.  Not what I like to do, but it’s an option to avoid having to remove the fringing afterward.

Even high-end expensive lenses are prone to some chromatic aberration.  Many lens manufacturers have used certain techniques with special optics to reduce this somewhat, but still cannot totally eliminate it.  However, many of today’s cameras have included processing that can reduce or eliminate some of this issue.

I hope this has helped with the understanding of what this issue is and ways to reduce or eliminate it.  The reality is that I own high-end expensive lenses and still have to deal with this on occasion.  I would suggest understanding how to recognize which lenses may be prone to it and how to use either Lightroom or Photoshop to remove it.

 

Bob Woodfin
bob@bobwoodfin.com

 

To see more of my work visit:
http://www.bobwoodfin.com/

https://www.facebook.com/BobWoodfinPhotos/
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