Correcting Chromatic Aberration in Adobe Camera Raw 5.6 - a few suggestions

pathfinderpathfinder Drive By Digital Shooterwestern IndianaPosts: 14,434Super Moderators moderator
edited June 17, 2011 in Grad School
Proposition: In addition to Capture Sharpening in Adobe Camera RAW ( hereafter called ACR ) or LR2, images can be improved by the correction of chromatic aberration.

I told rutt, I would write up a brief description of how I do chromatic aberration correction in Adobe Camera Raw or Lightroom2's RAW engine. Here we go.

My basic workflow in ACR is to choose my camera profile first, if my exposure seems within + or - 1/2-3/4 stop or so. For my Canon DSLRs, I rather like the Camera Standard profile, but that is a matter mainly of taste. (My Panasonic GF-1 only has a single profile available in ACR, unfortunately.)

Then I will adjust my Exposure slider and my Black sliders to set my range of tones. I will re-examine my choice of White Balance at this time and adjust it as seems appropriate, either by eye, or using the eyedropper on a known neutral or a photographed grey target. I will use the Recovery slider if I have highlights that are blown, and I will use Fill light to bring out detail in the shadowed lower quarter tones of the image. I will leave the Curve either Linear, or a Medium Contrast setting.

A few comments about my Capture Sharpening

For Capture Sharpening one MUST look at the image at 100%, and hold down the option key as they move the Amount slider - the image will become monochrome, and one can dial up the Amount to achieve a sharper image - I usually end up between 50 and 80, but sometimes use more or less depending on the image frequency - how much very fine detail is present in the image - leaves versus stark graphical images.. I rarely change the radius from 1.0 unless I really need to make the image seem sharper. I really prefer to delete unsharp images, rather than to try to salvage them in Photoshop. (High Pass sharpening in PS will help images look sharper ) Detail I usually set at 25-28 or so. The Masking slider I make strong use of. While holding down the option key, and clicking on the Masking Slider, I can see the black and white mask being created, real time, as I adjust the slider, dictating what portion of the image will receive the sharpening and what portion will be masked out. Low frequency images will use a fairly high value of the Masking slider, sometimes 50-80 or even higher for simple images. High Frequency images will require much lower masking settings, between 20-40 or so. The beauty is the real time mask lets you see precisely where the sharpening will be performed and where it will not. Sharpening really needs to be done in conjunction with Noise Reduction, but I have not developed great affection for the Noise Reduction panel in ACR yet. )( I am hoping this is better in LR3 ) If the image is very noisy, and needs significant sharpening, then I do use the Luminance and Color sliders in the Noise Reduction panel in the Detail panel under Sharpening. One trick I have been thinking about with regard to sharpening, is whether you could sharpen in different manners in ACR for different Smart Objects and then blend them in PS. Just a thought so far, but I think it might be a useful one.

I usually skip the HSL/Greyscale panel and the Toning panels at this time, as they are not always a routine part of my RAW processing in ACR.


Chromatic aberration correcting step by step

Chromatic aberration refers to the fact that different frequencies ( colors ) of light are focused to different focal points/planes by lenses. Better quality lenses, generally, have better chromatic aberration corrections, but most lenses can sometimes display chromatic aberration, especially in the corners of the image near the edge of the image circle.

Typical chromatic aberration consists of bands of Red and Cyan light on opposite edges of straight objects near the edge of the image circle. One can also see Blue/Yellow fringes on opposite edges of grass, tree, limbs, hair, etc , although Blue/Yellow is usually less common than Red/Cyan. Many lenses demonstrate only Red/Cyan, without significant Blue/Yellow CA. My 24 T&S lens is like this. My Tamron 28-300 Di VC lens can display both or neither, depending on the focal length, aperture, and the distance from the film plane.

Chromatic aberration can best be found by examining your RAW file in ACR, AFTER Capture Sharpening, by examining the image at 200-400%. You will not usually notice it at 100% unless your have learned to identify it at higher powers first.


791519120_zQQE4-XL.jpg
I will make available this Raw image of a Moto Guzzi, shot with a 40D and a Tamron 28-300 travel zoom, found here - http://public.me.com/path_finder . The file name is Motorcycle wheel CA VBRally-7795.CR2 You can download the file to compare your results to mine. There are other RAW files in there shot with a Tamron 200-500 lens that are remarkably free of CA for an inexpensive travel/zoom lens. Examined carefully they do exhibit more CA, I feel, than the file shot with an EOS 500 L, the anhinga, but not much. ( I am retaining all rights to these images; they are just so that you can see the CA in them )

The Tamron 28-300 lens will display CA at times, depending on aperture, focal length, focus distance, and lighting. I am not disparaging this lens, as sometimes I do not see any apparent CA with it at all, and I like it when traveling very light. I have numerous 16x19 inch and larger prints, on paper, with it, despite its lack of aristocratic origin. When I rode to the Aerostitch Rally in Duluth, August 2008, I only took one lens on my bike, for my 40D, the small, light, Tamron 28-300 f3.5-6.3 IF XR Di Vc .

My Canon EOS 24f2.8 L T&S L pretty consistently demonstrates some Red/Cyan CA, so do not think L glass is immune to CA, but it is easily correctable.

Most of my images, shot with a large variety of different lenses, from consumer to pro lenses, will demonstrate some CA if carefully searched, so a brief part of my workflow in Raw processing each image is to look at it after Capture Sharpening in the Lens Corrections panel.

I enlarge the image to 200% ,300% or 400%, whichever best displays the CA, by hitting the Command and Plus keys at the same time, until the image is sufficiently enlarged. I then use the mouse and hold down the space bar to move the image around in the ACR window so that I can see the corners. I actually prefer this step in LR2 version of the Raw engine, because I do not have to hold down the spacebar, and I can see a small thumb view of the image in the NAVigation panel on the left side to see where I am in the image easier. There is no Nav view panel in ACR.

I move the image around until I see the typical red and cyan highlighted edges of an object. These may be along vertical margins, horizontal margins, or irregular margins. If present, I click the the Defringe - All Edges box routinely in all images, found in the Lens Corrections panel, by pressing the little Lens icon in ACR, the third icon from the right.. No more than that may be required. If there is still residual Red/Cyan CA, then I begin to slide the Red/Cyan slider carefully to the right or left to minimize their presence. The trick is to watch both side of a sharp object, which will be red on one side and cyan on the opposite side. As you move the slider, at some point the colors will swap sides of the object, which means you have passed through the null point. SO back up a bit until you eliminate the Red/Cyan bars on both sides at the same time. Sometimes you can make them disappear entirely, and sometimes you can just diminish them, but only have them pop up somewhere else as the slider is moved back and forth. If you hold down the Option key while moving the slider, the image will change to a display of the Red and Cyan tones, omitting the Blue/Yellow ones. This can sometimes be quite helpful to display the null point, but I usually do not find it much better than without depressing the Option key. In the upper right of ACR viewing window, is a Preview box with the arrow checked. Turning this preview box off and on while you are adjusting the Red/Cyan slider can be helpful to see how the image compares with and without the CA correction. If you see Blue/Yellow CA, it can be minimized with the Blue/Yellow slider in the same way, and you can hold down the option key to help display the blue/yellow fringing as you work. These corrections do not always correct absolutely all the CA as the correction for CA along vertical elements may require different slider positions than horizontal elements. But you can usually diminish the CA by 80-90% or better, depending on the lens and its optical quality. Sometimes the CA involves both Red/Cyan and Blue/Yellow in the same area in the image, and you see green or a purple fringing. Careful adjustment of both sliders can usually help diminish this. Some purple fringing of very strongly backlit branches, for instance, is not really CA but a form of blooming of pixels due to the light intensity, and will not respond to the CA sliders.

790932121_iB9Ls-X2.jpg
Here is the lower right corner of the ACR viewing window, with the Red/Cyan fringing quite prominent along the blades of grass at 300% - You can also see some blue tinge to the inner edge of the wheel rim. The Preview button is unchecked ( in the upper right corner ) so that the CA can be seen, despite already having the sliders adjusted to what I feel are more appropriate locations.


790932218_rhzP3-X2.jpg
Here is the view of ACR with the Preview box checked so that you can see clearly the effect of the chromatic aberration correction sliders in ACR. Notice the decrease in the blue tone along the wheel rim, and the diminished color along the grass compared to the first image. One needs to quickly explore all four corners while adjusting the sliders to decide on a final choice for the slider positions. It is important to compare the corrected image to the final image to evaluate the success of your editing. You may say my final image still has some residual CA remaining. That is not the issue; the issue is how it compares to the uncorrected image, and in this simple example I think we will agree that the corrected image has much less apparent CA.

I chose this image because it does demonstrate the presence of Red/Cyan and Blue/Yellow CA, that is significantly improved by checking a box, and adjusting a couple sliders in ACR. You may have some images that do not demonstrate any CA at all . That's fine; just move on to your next step in your workflow.

You will likely find, with certain prime lenses, you need almost the same slider positions each time, for each image. This is typical for my 24 T&S. Most zooms are less predictable in their needed slider corrections.

Identifying the CA and correcting it is a bit of an acquired skill. I found I had to just play with the sliders, turning the preview button on and off, and watching for the red/cyan to switch from one side of an object to the other to identify the null point to kind of get a feel for what was going on. With experience it usually takes less than 30 seconds to correct most images. A small amount of work to significantly improve the technical quality of your important images.

I look forward to hearing from other's experience with correcting chromatic aberration in ACR or LR2.
Pathfinder - www.pathfinder.smugmug.com

Moderator of the Technique Forum and Finishing School on Dgrin

Comments

  • Jason HermannJason Hermann Jason Hermann Posts: 220Registered Users Major grins
    edited November 17, 2010
    Really great read, and info!!

    Best,
    Jay
  • pathfinderpathfinder Drive By Digital Shooter western IndianaPosts: 14,434Super Moderators moderator
    edited November 25, 2010
    This was written 10 months ago in February 2010. You no longer have to make the adjustments yourself with sliders now if your ACR 6.+ or LR 3+ has a profile for your lens. You can just select Profile and choose your lens - whether EOS, Nikon, Sigma, Tamron, Leica and a few others - and the software will do it for you. It really does make some lenses better than they would otherwise be. I can see a significant improvement in the files from my Sigma 150-500 OS.
    Pathfinder - www.pathfinder.smugmug.com

    Moderator of the Technique Forum and Finishing School on Dgrin
  • flashfilterflashfilter Beginner grinner Posts: 6Registered Users Beginner grinner
    edited June 17, 2011
    I am also enjoying the automation. I am not too much a whiz when it comes to computing so having the ability to automate this process has really improved my shots, especially when it comes to detailed landscapes. This is one of the reasons i am trying to shed all of my older equipment except for one or two cams, I just wish i could get listed in one of these online review site for photography. Have you guys ever seen the myriad of reviews on Angie's List?
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