Monday, 29 November 2010

PP Tips 14 - Removing Chromatic Aberration

This is part of a 'How To' series on post process work.

Many of today's cameras have routines 'built-in' to remove lens aberrations. This includes lens distortion correction and also chromatic aberration (CA). Typically, these are very effective routines.

A large majority of lenses will display CA. Certainly most of the lenses I have owned over the years have displayed CA, in differing severity, from the very cheap to the very expensive.

Most good image editors have CA correction routines. Some work well, some not so well.

The ones I will cover here are Photoshop, DxO Optics Pro, and also an action for use in Photoshop.

Firstly, what is CA ?

If you want a detailed technical explanation, I'm the wrong guy to ask. I'm not that smart. However, good news, have a look at this link for an explanation at Cambridge In Colour - Click here

More simply, and so we can see it's effects in an image, here is an example of an image with CA, at left, and after it is fixed, at right.

This is a 100% view of the image and is only showing the very top right hand corner.

Note : Clicking on any image will take you to a larger example of that image

As can be seen in the example above, the Chromatic Aberration manifests itself in the form of purple fringing (PF), but also with green fringing.

So, let's explore a few ways to reduce/remove the CA.

Example 1

The original image.

Example 2

Fixing the image using the PF Killer action in Photoshop.

Note : This is a simple action which you can download - Click here. It is the action at the bottom of the page. The original web site for this has disappeared

Once downloaded, simply copy it to the Presets/Actions folder for Photoshop. When Photoshop is opened, just load the action for use.

Here I am simply showing the before/after as if you are familiar with running actions in Photoshop, it will be intuitive.

As can be seen in the above example, PF Killer does a good job of removing the purple CA. However, it does not tackle the green fringing.

Example 3

Note : I open all my Raw and Jpeg images in Adobe Camera Raw (ACR). To understand how to set up Photoshop/Photoshop Elements to do this - Click here

Here, I have opened my image in ACR and selected the Lens Corrections pane. This is also pretty much the same as the adjustments which are available in Adobe Lightroom.

Example 4

As there is no preset profile for my camera/lens combination in ACR, I have selected the Manual tab. If your camera/lens combination does have an Adobe profile, then it should correct automatically.

With the Manual pane open, I have selected Defringe : All Edges, and adjusted the Red/Cyan Fringe and Blue/Yellow Fringe sliders.

For this particular image, I have used the values of -20 and +50 respectively. This will be different for any given camera/lens combination.

Example 5

The original image, at left, and the image after adjustment in ACR, at right.

Example 6

As can be seen, in the above example, ACR has done a good job of reducing the CA but has not removed it all, especially the purple CA. One way to help remove this, is to now run the PF Killer action.

Here, are the same images shown in the last example but with PF Killer now run on the image at right, and after the adjustments I made in ACR.

Example 7

If your camera/lens combination has a Lens Correction Module in DxO Optics Pro, then I have found this to be pretty much the best CA correction tool I have used.

Note : To check if your camera/lens combination is supported with a DxO Lens Correction Module - Click here to check DxO's availability page

The image as opened in DxO.

Example 8

In this case, my camera has a Lens Correction Module in DxO.

Here, DxO has automatically corrected the image. In the 'Detail' pane I have turned off the 'DxO Lens Softness' correction and turned on the 'Purple Fringing' correction.


Note : I also have DxO Lighting HDR Slight - Auto turned on. Hence the lighter image

Examples 9 through 13

These examples simply show the effects, on an image which has a predominance of purple and blue, and the effect which different CA removal techniques have on those colours. The image has purple fringing in the top left corner which I wish to remove.

The original image.

Note : Clicking on any image will take you through to a larger size

Fixed in ACR - Manual process as there is no lens correction profile for this camera/lens.

Fixed in DxO - Automatic.

Fixed with PF Killer. As can be seen, this is one of the draw backs with PFK. It is non-discriminate.

The same image as above, but what I have done here is to 'unlink' the correction mask layer and then use the Magic Eraser to restore the colours in the fishermans clothing. Again, a manual process.

Anyway, That's it. Hope you find it helpful.


Other posts in this series - Click here for the index page.


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