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PhotoShop Hints – Chromatic Aberration

I use two lenses extensively:  a Canon 17-40mm zoom, and a Canon 100-400mm zoom.  The wide angle zoom has much more chromatic aberration than I like, so I regularly use PhotoShop’s Lens Correction tool to correct my images.

The Problem

Chromatic aberration occurs when the various colors of light from an object are focused at different places on the detector. All lenses have some amount of chromatic aberration, with some lenses having more than others.  Also, chromatic aberration increases with distance from the center of the image, so you will notice this problem more at the corners of an image.  Chromatic aberration can be seen most easily with distant, highly detailed subjects such as tree branches.

Correcting Chromatic Aberration in PhotoShop

I typically perform any Lens Correction functions first before making any other modifications to an image.  This is because these Lens Correction adjustments apply changes assuming radial symmetry from the image center.  Do NOT try to apply the Lens Corrections to a cropped image, because the adjustments will not be applied correctly because of this radial symmetry assumption.

Here is a common image that shows chromatic aberration in the distant trees.  I’ve identified two areas where you can easily see the chromatic aberration (green rectangle and blue oval).  I’ll use the area in the oval for the rest of this discussion.

Original Image

Blackwater-Falls-SP---Original

The chromatic aberration in this image is easily seen under magnification.  Notice the tall tree … it has magenta “fringes” to the left side of the trees, and green “fringes” on the right side.  This is classic chromatic aberration.

Enlargement of Original Image

Blue---Original-Annotated

FILTER > LENS CORRECTION

With the image in PhotoShop, select Filter > Lens Correction.

The initial Lens Correction Screen allows you to select camera MAKE, MODEL and LENS used.  Notice in this case it already knows what lens was used from the metadata.  Unfortunately, this lens has NOT been characterized by PhotoShop, so no lens profile exists for it.

Initial Lens Correction screen menu

Lens Correction Menu 1

I next enter the correct data for my camera MODEL and I select a lens profile that is close to what I was using (in this case a 16-35mm zoom).Check the Geometric, Chromatic, and Vignette options to have PhotoShop apply the lens profile information to the image.

Camera MODEL and LENS Profile choices made on Auto Correction tab.

Lens Correction Menu 2

My first step is to correct the Geometric distortions using the Transform functions.  However, since this image has no rectilinear objects like buildings, this step isn’t as important at the moment.

Next is correction of the chromatic aberration.

My process for correcting this follows:

Since chromatic aberration increases with distance from the center of the lens, I select a corner of the image which has fine detail (high frequency objects like tree limbs or roof shingles).  I ZOOM the image (zoom selection tool is at the bottom left corner of screen) to a large enough size (I often use 200% or more) so that I can easily see the chromatic aberration.  (Notice that the tree edges show heavy green/blue/magenta fringing).

Enlarged portion of image shows color fringes easily

Blue---before-CA-adjustment

Next select the CUSTOM tab.

I next adjust the Chromatic Aberration (CA) sliders one at a time to obtain the most “neutral” image I can, in which I’ve minimized the color-pair fringing that a particular slider controls.  I generally work the sliders top to bottom (Red/Cyan … Green/Magenta … Blue/Yellow).  The goal is to obtain the most NEUTRAL image possible.

Chromatic Aberration sliders on the Custom tab are used to reduce color fringes

Lens Correction Menu 3

NOTE:  essentially what this function does is align the RED, GREEN and BLUE pixels maps with each other based on the chromatic aberration map of the lens you used … remember that a typical digital camera sensor uses a BAYER pixel pattern in which an “aggregate pixel” is typically formed by four single-color pixels (2 green, 1 red, and 1 blue pixel).  This Chromatic Aberration function adjusts the single-color pixel planes to map correctly.

I’ve noticed that the best settings for Chromatic Aberration result in a linear arrangement of the three Chromatic Aberration sliders.  I assume this is because the chromatic aberration of the lens is constant and the actual pixel map is fixed, hence the Chromatic Aberration adjustments will always end up in a similar relationship for any given lens (at a given focal length setting).  I find this LINEAR alignment of the sliders helpful, because it gives me a double-check on the corrections I’ve selected by eye.

Selecting OK allows PhotoShop to apply the selected adjustments to the image.

Corrected Chromatic Aberration

Blue---before-CA-adjustment

Blackwater_Falls_SP-4443---After-CA-adjustment

Now continue your image editing to express you artistic needs.

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Mark Bobb

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