Dealing with Chromatic Aberration

by Michael Clark

Chromatic Aberration is one of those topics that rarely gets discussed but is a huge concern when processing digital images. Chromatic Aberration is caused by a differing refractive index for each wavelength of light - hence red refracts at a different angle than say blue. This results in what is commonly called color fringing appearing in an image and especially in the corners of an image shot with a wide-angle lens.

In my experience, my fisheye almost always exhibits chromatic aberration in the corners, especially on high contrast edges. If I look close, I will normally see some amount of chromatic aberration in images shot with any lens - from my 17-35mm all the way up to my 300mmm. This isn't a defect in the lens build - it is just simple physics. If you wear glasses and look at any edge out of the corner of your eye then you can see that same color fringing as the light passes through your glasses. With high-resolution digital cameras these days the amount of CA is only exacerbated. It has always been there with film but we didn't seem to notice it as much due to the lower resolution of 35mm film.

Because chromatic aberration is so common I look for it in every image (just in case) when I am processing my selects. Lightroom, with the upgrade to 1.1 included some new chromatic aberration controls that are very effective. The Chromatic Aberration tool has been augmented so that you can choose to only work with "Highlight Edges" or "All Edges". You can also turn off the CA adjustment. It seems to be much more effective with these new controls. If you just want to remove fringing on the edges of your highlights then you would choose "Highlight Edges". For more extreme color fringing you would choose "All Edges". In my experience it just depends on the image - I use both settings equally.

There are two sliders, one for Red/Cyan fringes and one for Blue/Yellow fringing as in the screenshot below. It is rare that you only use one slider to take care of the fringing. Normally there are both R/C and B/Y going on in an image, sometimes on opposite sides of the image. Generally, you move the sliders in opposite directions for the best effect until the fringing is removed. If you use wide angle lenses often you'll want to make sure to look in the corners at 100% for color fringing. With fisheye lenses on a digital camera, as I said above, you are pretty much assured of fringing and will have to use this tool to remove it.

ca_1.jpg

The reality with using these sliders is this isn't a speedy process. First you'll have to zoom into 1:1 viewing mode so that you can see how moving the sliders is affecting the fringing. Then I normally start with the Red/Cyan slider since red fringing is the most visible. Adjusting the sliders so the fringing is gone will take some time. If you go to far with the sliders, you'll just introduce fringing of another color - and sometimes you have green fringing which is a combination of red and blue so you'll have to experiment with both sliders to take that out. There are no shortcuts.

Also a little side note, in Lightroom if you are looking at the full screen version of your image after you have removed the chromatic aberration you'll still see the fringing - it only goes away when you are zoomed into 1:1 view. When you export the images it will be removed but strangely it isn't taken out for the normal view.

That's it for this session. See you next week.

Adios, Michael Clark

2 Comments

Mike
2007-10-01 02:07:55
Interesting, but would have been even more interesting with some examples...
Scott
2007-10-02 16:01:51
Thanks for this guide. I agree with Mike that some examples would help. Also, it's likely something about the LR engine that the changes are only visible at 100%, just like noise and sharpening controls...