Chromatic aberration is the colour fringing you sometimes see around object outlines near the edges of the picture. With most consumer lenses it’s a fact of life, and as soon as you know what you’re looking for you can see the signs in many of the pictures you take.
Some camera makers ‘tune out’ chromatic aberration as the picture is processed by the camera. Nikon does this with its current D-SLRs – though it only works with in-camera JPEGs, since RAW files are saved before this processing takes place.
Alternatively, you can use software to remove chromatic aberration. DxO Optics Pro will do this automatically as part of its lens corrections, and Lightroom and Capture One do the same.
The Aperture chromatic aberration tool is less sophisticated. It doesn’t have lens correction profiles, but it does offer manual chromatic aberration correction, and it is wise to fix any colour fringing issues right at the beginning – especially if you’re going to open and edit your pictures in an external editor – because contrast and colour adjustments will only tend to make the fringing look worse.
I’m using this delicate snow scene as an example of what chromatic aberration looks like, why it’s so important to fix it, and how to use the chromatic aberration correction tools in Aperture.
01 So what’s the problem?
This snow scene looks fine as it is, but I know that the trees at the edge of the frame are showing some bad colour fringing. You might not see it at this magnification, but if I print the image out, or view it on a large, high-resolution screen, it does become apparent.
02 100% view
You can view images at 100% magnification simply by hitting the Z key. This still isn’t quite close enough here, though, because I’m working on a Mac with a high-resolution Retina display, where the individual pixels are tiny.
03 Zooming in further
If you need to zoom in further, use the View menu or the ‘command=’ (command-equals) shortcut.
04 300% view
Now I’ve increased the zoom setting to 300%, the problem is obvious. I’ve used the pop-up navigator palette to pan to an area on the far right of the screen, where the tree trunks have a purplish fringe on one edge and a greenish tinge on the other.
This is typical of chromatic aberration – you actually get two fringes along object edges, not one. There’s an outer fringe of one colour and an inner fringe of its complementary colour.