Chromatic Aberrations are anathema in photography. They are recognizable even at modest enlargements in Photoshop (e.g. 50%) and may be resolved-to a great extent-in Adobe’s Camera RAW. I do not recommend using the PS Distort>Lens Correction Filter to resolve CA after demosaicing, particularly after any cropping or Transformation (including distortion correction).
This is a sneak peek at one of my judging criteria for PAPA’s Iron Photoshop Contest in Tampa, Florida next February. You can bet your bottom dollar that in any advanced category judging a mastery over all of the controls in Adobe Camera RAW will be considered…and rightfully so.
Without going to far into the birthing of it, Chromatic Aberration is not a Seidel Aberration (such as Spherical Aberration and Distortion). There are two species of CA: Longitudinal (“LCA”) and Transverse (“TCA”).
Here is a very good resource on CA. It’s one of the spatial and tonal resolution serial killers and is not accounted for in MTF (to the best of my admittedly limited knowledge-though I don’t understand why not since testing line pairs includes White). LCA is sometimes called Axial CA (that can confuse, because it occurs off-axis as well…but it does manifest along the lens axis), and TCA is sometimes called Lateral Color.
- LCA is nothing other than various colors focusing at different distances, and varies with the aperture (think “Depth of Field”). Canon’s L and Nikon’s ED glass have all but resolved LCA.
- TCA occurs when varying colors are magnified at different rates, manifests as color fringing in the periphery and is not mitigated by stopping down. TCA is common.
- Primary Chromatic Aberrations occur when the Red and Blue margins of the Red-Green-Blue Spectrum are incoincidental. If you notice Purple fringing, you’ll think to yourself, “Purple is a combination of Red and Blue!”
- Secondary Chromatic Aberrations occur in the CA that remains after the Primary CA’s have been corrected…meaning misbehavior by Green-you’ll notice Green-Magenta artifacting and think to yourself, “Green, and its complement Magenta are deviant! I sure wish I had an apochromatic lens!
Adobe Camera RAW (I’m currently using ACR 5.2) offers controls over Red/Cyan and Blue/Yellow fringing, and a mysterious “Defringe” control.
The Red/Cyan control corrects the size of the Red image relative to the remaining primaries (Cyan is Green and Blue). Differential size is a result of TCA, which is common and unaffected by stopping down-you’re stuck with it. Fortunately it’s localized in the periphery, so look for a high contrast edge in a corner to reference when you’re correcting it. Because TCA is dependent upon distance from the lens axis (center of the image), you don’t want to apply any therapy that is not both centered and weighted in the same measure as the disorder, or if the image has been resampled.
The Blue/Yellow control corrects the size of the Blue image relative to the remaining primaries (Yellow is Green and Red).
I cannot imagine a manifestation of TCA in which the most proper use of the controls wouldn’t be concurrent-in other words, use them both together. One adjusts the Red size, one the Blue, both of which are marginal in the visible spectrum (surrounding Green). The difference in magnification of the Red and Blue images are not necessarily equal, and that is the reason for separate controls.
Remember too that TCA and LCA can/do occur simultaneously, complicating diagnosis and therapy: Hold down your Alt Key while adjusting either slider to isolate that slider’s target color disorder (i.e. to hide the complementary disorder).
Concerning that mysterious Defringe control-it apparently identifies high contrast edges and desaturates them. This should be effective for the type and severity of chromatic diseases common with lower grade lensing/coatings, or a “quick fix” for those dedicated to speed and ease in their workflow.
“Say ‘Hello’ to My Little Friend…”
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