I must admit to being a Facebook junkie…but purely for professional purposes of course. The ability to see new landscape images from so many talented photographers around the world, often on the same day the image was captured, is incredibly inspiring and just an amazing way to connect with a worldwide community.
But one thing that surprises me is how many good, and sometimes very good, photographers fail to correct chromatic aberration (CA) in their images. It surprises me because it is such an easy thing to correct, and because CA can turn an otherwise excellent image into an eyesore.
Chromatic aberration arises when a lens fails to focus the different wavelengths of incoming light onto the same plane (the camera sensor), resulting in unsightly coloured fringes in an image. These fringes also makes images appear soft, and additional sharpening to offset the problem only results in sharper colour fringes. It is a common flaw of cheaper lenses, but can also occur with very expensive top-end lenses when the conditions are right…or wrong.
Lens manufacturers add additional glass elements to their lenses to try and correct for CA, and some lenses use super-low dispersion glass to help avoid the problem. But most lenses will still display at least small amounts of CA, especially in high contrast situations and when a wide aperture is used. In extreme cases, the fringing will be visible across the whole image, but often it is only noticeable as a problem in the corners of the image.
I’ve read some articles that recommend photographers can limit CA by avoiding high contrast situations and wide apertures – that’s okay if you’re happy for your creative options to be severely restricted, but if like me you want to keep your options as wide open as possible, you need to accept that CA is a fact of life and learn how to deal with the problem post-capture.
I did a quick survey of various photo-editing software and every one I looked at either had the means to deal with CA built in, or it was available as a plug-in. There is some variation between programs as to how to undertake the task so you might need to do a little research depending on the particular program you use.
My preference is to correct CA on the RAW file – if you don’t shoot RAW files I’d recommend you do, but that’s another topic in itself. Whether your file is a RAW, jpg or tiff, the first thing to do is to find the CA adjustment in the program you’re using (try the Help files, or search online if you can’t find it). Then zoom right in (200% – 400%) to the corners of the image to see if you can find any fringing. Pay particular attention to any high contrast edges, like between a dark tree trunk and the sky, or a well-lit blade of grass and deep shadowy background. Taking the blade of grass as an example, what you will often see is a reddish fringe on one edge of the grass and a greenish fringe on the opposite edge.
To fix the problem, there will be either sliders, an eye-dropper tool or an auto-fix button (or a combination of these). The eye-dropper and auto-fix buttons usually do a good job, but you can fine tune the results using the sliders if necessary. As you move the sliders up and down, the colour fringes will increase and decrease, so it’s a simple matter of finding a setting that removes, or at least minimises the fringe. Through trial and error, I have found a setting for each of my lens/camera combinations that removes most of the CA with a single click.
All very simple, although you will occasionally encounter situations where the CA proves more resistant. Sometimes with highly contrasty images, such as those taken into a rising or setting sun, a setting that removes the CA from the corners of your image might leave some residual fringing in other parts of the image. This is rarely a big problem, but the more pedantic among us have a couple of options available to deal with the issue.
The first is to process your image twice using two different CA settings, and then blend the two versions together using layers and masks so that the whole image is CA-free. Another method is to selectively target problem colour fringes with the hue-saturation control – if you have obvious red/green fringing in a part of your image, you can add a hue-saturation layer, reduce the saturation of reds and greens until the fringing disappears, invert the layer mask, and then carefully paint the desaturation effect back into the mask over the offending areas.
That’s it in a nutshell! There is plenty more information about chromatic aberration you can read online if you are so inclined, but the important thing is to be aware of it and make sure its removal becomes a standard part of your workflow. If we all pull together we can create a CA-free world and that has to be a good thing!