John Paul Caponigro's "Correcting Chromatic Aberration"

March 27, 2011

John Paul Caponigro's "Correcting Chromatic Aberration"
One optical difference between low-priced lenses and more expensive, professional lenses often is the degree of correction for chromatic aberrations…

What is chromatic aberration, and why does it occur?

Chromatic aberration is a phenomenon you can sometimes see in images when they’re viewed at high magnifications, such as 100% or more on-screen, or alternatively when large prints are made. Most commonly, it’s seen as one or more colored bands surrounding the contours of a subject, where light areas and dark ones intersect in an image.  Chromatic aberration is also sometimes loosely referred to as “color fringing”. 

Even when viewed at normal magnifications on your computer screen, or in modest-sized prints, chromatic aberrations can reduce your images’ overall contrast, and make them seem somewhat soft and reduce the visible fine details.

Chromatic aberrations can be caused by two separate sources:  your digital camera’s sensor, and also by your camera lens itself.  In some cases, they can combine to generate noticeable color fringing upon close inspection.  

When it originates in the lens, chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light.  Because each hue in the spectrum is not focused at a single point, colored halos or fringing appears.  There are two types of chromatic aberration:  longitudinal, where different wavelengths are focused at different distance from the lens, and lateral, where different wavelengths are focused at different positions on the focal plane.

One optical difference between low-priced lenses and more expensive, professional lenses often is the degree of correction for chromatic aberrations, especially when you’re looking at telephoto lenses or wide-angle zoom designs.  You’ll often see references in lens brochures or magazine articles to special types of optical glass, such as Low Dispersion, Ultra-low Dispersion, or even Fluorite.  These are among the types of materials used in higher-end lenses to minimize chromatic aberrations. 

How is chromatic aberration controlled?

If you’re looking for crisp, sharp pictures, it’s important to reduce chromatic aberrations whenever you can.  In the past, when photographers shot strictly film images, about the only thing that worked was using expensive, professional lenses.  Other in-camera techniques, such as shooting at smaller lens apertures and so on, have little or no impact on chromatic aberrations.  However, with today’s digital images, there are special software techniques that can reduce or in some cases even eliminate color fringing — and they’re not only easy to correct, but you can set up automatic corrections for entire batches of images taken with a particular lens.

Chromatic aberration is easily corrected with most RAW file converters, such as Adobe’s Camera Raw and Adobe Lightroom).  Recent versions of Canon’s dedicated RAW image processing software, Digital Photo Professional (version 3.2 or higher), have a new tool tab with special commands that will automatically remove chromatic aberration from your images, as well as sliders for ultra-precise manual control of color fringing.

If you shoot JPEG images in-camera, or you’ve scanned film originals to create a digital image, all is not lost.  Some image-editing software programs, notably Adobe Photoshop CS3, allow the user to bring up an image, and then manually adjust sliders until chromatic aberrations are reduced or eliminated in a magnified view on-screen.  It’s not automatic, like Canon’s DPP can be, but it can be just as effective if you take your time.  In Photoshop CS3, the feature is available in this pull-down menu:  Filter  > Distort > Lens Correction… > chromatic aberration sliders in on-screen toolbox.  Some other image-editing programs have similar tools to reduce chromatic aberrations as well.

Test your camera and lens(es) for chromatic aberrations

Some digital photographers deal with chromatic aberration as it comes, making manual corrections when they see it in an image.  However, if you run some tests on a specific camera with a specific lens, you can perform a correction in software, and then memorize that correction.  From there, any time you use that lens and camera, you can simply call up that previously-saved correction and apply it.  It may not seem like much time saved if you only have one image to correct, but what if you’ve just shot a wedding and you’ve got dozens or even hundreds?

(Keep in mind:  one outstanding reason for considering using Canon’s Digital Photo Professional software to process RAW files from a Canon EOS camera is that if you’re using a compatible Canon lens and camera*, all the lens corrections can be automatic — that is, the camera “tells” the software which lens, zoom setting, aperture, and even focusing distance you used in each picture you take.  From there, if you simply put a check in any of the available boxes in the Lens Aberration Correction tool palette, designed-in correction that’s already been tested by Canon’s engineers will be applied to your images automatically.  There are manual sliders if you want to either fine-tune automatic adjustment, or simply perform it yourself.)

 

Canon EOS Cameras Offering Correction

1Ds Mark III

1D Mark II N

5D

20D

1D Mark III

1Ds

40D         

Rebel XSi   

1Ds Mark II

1D

30D

Rebel XTi

 

 

Canon Lenses Offering Correction

EF 14mm f/2.8L**

EF 50mm f/1.4

EF 24-70mm f/2.8L

EF-S 18-55mm f/3.5-5.6

EF 14mm f/2.8L II

EF 85mm f/1.2L**

EF 28-70mm f/2.8L**

EF-S 17-55mm f/2.8 IS

EF 20mm f/2.8

EF 85mm f/1.2L II

EF 24-105mm f/4L IS

EF-S 60mm f/2.8 Macro

EF 24mm f/1.4L

EF 17-35mm f/2.8L**

EF 28-200mm f/3.5-5.6

EF-S 55-250mm f/4-5.6 IS

EF 28mm f/1.8

EF 16-35mm f/2.8L**

EF 28-300mm f/3.5-5.6L IS

 

EF 35mm f/1.4L

EF 16-35mm f/2.8L II

EF 100-400mm f/4.5-5.6L IS

 

EF 50mm f/ 1.2L

EF 17-40mm f/4L

EF-S 18-55mm f/3.5-5.6 IS

 

** Discontinued as of May 2008

Chromatic aberration produced by zoom lenses will vary depending on how far in or out they are zoomed;  they will require different settings for different focal lengths.  To test your camera(s) and lens combination(s) to determine a standard correction for chromatic aberration:

1.    Photograph a subject with high contrast contours (with zoom lenses, take test shots using at least two or three marked zoom settings, including the lens’s minimum and maximum zoom focal lengths)

2.    Open the image in your RAW converter, such as Adobe Camera Raw, or in an image-editing program that has a chromatic aberration correction tool if you shoot JPEG images.

3.    Zoom in on-screen (at least 200%)

4.    Go to Lens Corrections

5.    Use the Red/Cyan slider, and/or the Blue/Yellow fringe slider, to minimize visible color fringing in areas where light and dark parts of the scene come together. You can hold the Option key while using the sliders to view each component separately. If you’re using Canon’s DPP software with an image from a compatible camera and lens, you can just click the check-box in the Lens Corrections tool palette, and have the chromatic aberration automatically corrected.

These settings can easily be saved for future use.  In ACR, use the fly out menu (look for a tiny down-pointing arrow next to an icon with three small horizontal lines) to find Save Settings, and then use the pull down menu to go to Lens Corrections and check Chromatic Aberration.  In Lightroom, click the + (plus) sign in the presets palette and check Chromatic Aberration.

These settings can be easily applied to other images made with the same camera and lens combination.  In ACR, use the fly out menu, select “Load Settings”, and choose the specific settings you want to apply.  In Lightroom, check the preset.

Once you determine the ideal settings, you can use them for all exposures made with the same camera and lens combination, saving time and making the image-editing process easier.

Workflow is the lifeline of digital camera users who shoot lots of pictures, and steps that can save time at the computer can make a world of difference to both the working pro as well as the serious photo enthusiast.  A little initial testing will save a lot of time later.

 

John Paul Caponigro is an internationally respected fine artist, a member of the Photoshop Hall of Fame, author of Adobe Photoshop Masterclass, the DVD series R/Evolution, and a renowned Canon Explorer of Light. Learn more about him and his work at The Canon Digital Learning Center, or at www.johnpaulcaponigro.com

The CDLC contributors are compensated spokespersons and actual users of the Canon products that they promote.

All images are copyright JP Caponigro

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