Chromatic Aberration experiment in LAB!

[captain_j_hook]

Hi again!

I haven't logged in for ages... I've been really busy with work. Yesterday I was struggling with the umpteenth green-magenta chromatic aberration and how to remove it ultimately when Filters>Lens Correction is not enough.

I didn't know what to do and I was thinking: since it's just a issue of channels not corresponding, why not try to make them coincide in the channel palette? So I was messing about in RGB, seeing two channels per time, selecting one, cmd+t and trying to move it to make it coincide with the other one. It kind of worked a bit but then it was creating another chromatic aberration (red-blue) in some other parts of the image and anyway it wasn't decisive.

So I converted the image to Lab and noticed that OF COURSE! that magenta-green shift was all in a channel and it was represented as very sharp borders, very dark on one side and very bright on the other side. So I did a very flattening curve to make them similar and grey and -surprise! chromatic aberration was GONE!

Of course the a channel had been messed up so green and red lost a lot of snap so I went back to RGB and tried to put some color back with an rgb curve in color mode and a selective color adj layer. had to mask it but after this it was all good and my image was free of chromatic aberration without moving a single pixel actually!

Have you ever tried something like this? Any comments/ improvements?

[mistermonday]

Re: Chromatic Aberration experiment in LAB!

Welcome back, Captain. I think you are heading in the wrong direction. Chromatic Aberration (CA) results from an optical effect and it is non linear. Realigning the channels linearly will not get rid of it. I have seen many tricks people use to desaturate the edges, blur edges, color mix, etc, and all those techniques degrade the image, even if we try to hide the damage.
1. The first defense against CA is to use a high quality lens. The lens often costs 50 - 100% more than a cheap lens but the optics and mechanics go much further to preventing or minimizing CA.

2. If at all possible, set your camera to shoot RAW. It is much easier to completely eliminate CA from a RAW file. The best tool I have found is the Automatic CA Removal option on the lens tab in Adobe Camera RAW. DON'T use the Lens Correction Filter inside of Photoshop because it does not work anywhere near as good as Adobe Camera RAW. You need the ACR version that was released at CS6 or later (any version 2 years ago or later). I have made many comparisons of both ACR and PS Lens Correction and the results are 100% consistent. The conversion algorithms are different.

3. You need to get rid of CA before doing any processing of an image or saving it as jpg. As soon as the modify the contrast of those edges by conversion to jpg or adjusting contrast or brightness or anything else that modifies the pixel values alters the relationship between the channels and you no longer have chromatic aberration, you only have what appears to be chromatic aberration. Now it will be difficult if not impossible to get rid of it effectively.

Cheers, Murray

[captain_j_hook]

Re: Chromatic Aberration experiment in LAB!

Hi Murray, thanks for your message!

I'd like to say few things.

1) The pictures I get from my clients are always shot with high-end lenses and cameras (Canon and Hasselblad)

2) Of course I always get raw files =) but sometimes CR and LR tools are not enough. That's where I came from.

3) I worked on CA before adjusting any contrast with a very flat image (a 16 bit tiff exported from LR).

[marcchristopher]

Re: Chromatic Aberration experiment in LAB!

Interesting thread.

I, too, haven't logged in for ages, but have been lurking around for a few days now. Would like to contribute more in the future.

Anyway... I had to work on lots of images with a high amount of CA recently. Of course, I received the RAW files (Canon), but the photographer had used a medium-quality zoom lens. Exposure wasn't optimal, on many of the images the sky was completely blown and on many images a high (!) amount of CA was visible. No reshoot possible, as the images had been shot outdoor on different occasions over a few weeks.

Now, manually correcting chromatic aberrations in hundreds (!) of images isn't exactly funny. So, I tried to remove most of it during RAW conversion, but wasn't overly successful. (By the way: I tend not to use ACR any more. Capture One seems to yield a better image quality. Even RawTherapee and Darktable do a much better job - especially in reducing CA.)

So, I had to try a few ideas in Photoshop. Lab color seemed promising at first, but unfortunately I wasn't able to come up with a procedure that made my life any easier. So, I ended up using a Hue/Sat layer, with Magenta (or whatever) dialed down, and masked the relevant parts. Definitely not a great or quick solution, but it got the job done.

I still believe there's got to be an easier solution in Lab mode. Would be interested, if anybody comes up with an idea.

[skoobey]

Re: Chromatic Aberration experiment in LAB!

Welcome back, Captain. I think you are heading in the wrong direction. Chromatic Aberration (CA) results from an optical effect and it is non linear. Realigning the channels linearly will not get rid of it. I have seen many tricks people use to desaturate the edges, blur edges, color mix, etc, and all those techniques degrade the image, even if we try to hide the damage.
1. The first defense against CA is to use a high quality lens. The lens often costs 50 - 100% more than a cheap lens but the optics and mechanics go much further to preventing or minimizing CA.
Cheers, Murray

You are freaking kidding. It also has to do with the absence of anti aliasing filters on HIGH END cameras. I've been working on some images that were shot with pretty pricey lenses and the aberration was still there.

2. If at all possible, set your camera to shoot RAW. It is much easier to completely eliminate CA from a RAW file. The best tool I have found is the Automatic CA Removal option on the lens tab in Adobe Camera RAW. DON'T use the Lens Correction Filter inside of Photoshop because it does not work anywhere near as good as Adobe Camera RAW. You need the ACR version that was released at CS6 or later (any version 2 years ago or later). I have made many comparisons of both ACR and PS Lens Correction and the results are 100% consistent. The conversion algorithms are different.

Cheers, Murray

You better shoot RAW if you're paying someone to retouch your image.

3. You need to get rid of CA before doing any processing of an image or saving it as jpg. As soon as the modify the contrast of those edges by conversion to jpg or adjusting contrast or brightness or anything else that modifies the pixel values alters the relationship between the channels and you no longer have chromatic aberration, you only have what appears to be chromatic aberration. Now it will be difficult if not impossible to get rid of it effectively.

Cheers, Murray

I don't know, i found lightroom to do a great job removing these even after the retouching has been all done and image saved as a tiff(it might mess up some purples and greens all over, but you can mask it out with broad strokes).

So, how do I deal with the CA?
1.Do a single fix, or multiple fixes in RAW processor(different strengths and radii to affect everything).
2.Clone in color mode.
3.Use Hue/saturation adjustments and do broad masks.

[unimatrix001]

Re: Chromatic Aberration experiment in LAB!

Convert image to lab mode. convert for smart filter. Do your sharpening and color adjustments using smart filters. Then open the smart filter for the original image and convert it to rgb mode inside lab use lens correction filter to get the best you can. Save or update (if you like) the image. Now back in lab mode and still in smart filter mode choose noise - median choose an amount to blur out all the CA that is left click ok to accept and then reopen the filter and set blending mode to color. this should remove any CA that is left after the lens correction done in rgb mode.

[ChrisJones75]

Re: Chromatic Aberration experiment in LAB!

In Photoshop, firstly make a stamp of the layers so you have one flat layer with which to try this on. Then:

Apply a gaussian blur of 3%
Then fade the effect > cmd + F and leave on 100%, just set to colour mode in the drop down

For strong CA apply the steps more than once.