Colour fringing explained

Why fringing occurs – and how to fix it

People often talk about colour fringing but what is it? There are two main types of colour fringing. Lateral (or transverse) chromatic aberration and axial (or longitudinal) chromatic aberration.

Lateral chromatic aberration refers to a lens’s inability to focus different wavelengths or colours of light at the same point across the focal plane, or image sensor, perpendicular to the direction that the light is travelling. As an example, think of light passing through a prism and being split into the colours of the rainbow.

Lateral chromatic aberration is most noticeable towards the edges and corners of an image, around high-contrast lines. For example, dark branches against a bright sky near the edges of shots might have coloured fringes around them.

Axial colour fringing is common when using telephoto lenses and ‘fast’ lenses at wide apertures. In this case different colours of light are focused at different points ‘on axis’ along the path that the light is travelling through the lens.

It can be noticeable as a coloured band at the transition of focused and defocused areas in images. Narrowing the aperture increases the depth of field and makes the aberration less noticeable. Axial colour fringing may be referred to as ‘bokeh fringing’, because it’s often apparent when using wide apertures to make an object stand out against a blurred background. For example when blurring the background in portraiture.

There are various factors in cameras, lenses and software that aim to minimize colour fringing from images. Let’s take a look…

Good glass

Lateral chromatic aberrations around the edges and corners of images are due to different wavelengths of light being magnified by different amounts. Complimentary pairs of red and cyan, or blue and yellow fringes are the most common manifestation. Nikon’s ED (Extra-low Dispersion) glass can greatly reduce these aberrations.

Bokeh blues

In-camera and software-based corrections for lateral chromatic aberration tend to be extremely effective, often completely eliminating the appearance of fringing. Axial (or longitudinal) chromatic aberration is much more difficult to correct. Even when enabling the correction in Raw processing apps, you might see little reduction, as shown in this example.

Alternative apps

Adobe apps like Lightroom and Photoshop come with Adobe’s own ACR (Adobe Camera Raw) tool. Lens profiles can be applied with automatic corrections for vignetting and distortion. Even without using these profiles, an option is available for removing chromatic aberration, shown here with a before and after preview.

Clever cameras

Apart from the D60 and D3000, all Nikon DSLRS from the D3 and D300 onwards feature auto corrections for chromatic aberrations. Unlike with some other brands of camera, the corrections work with independently manufactured lenses as well as with Nikon’s own-brand lenses. The corrections can’t be switched off.

Purple patch

The lens fitted to your camera might not be responsible for all the colour fringing you see. In particular, purple fringing around highcontrast edges in a scene can be caused by micro-lenses fitted to the image sensor. These enable each photosite to gather more light.

Soft options

While images are automatically corrected in all current and recent Nikon DSLRS when shooting in JPEG quality mode, the correction is only ‘flagged’ when shooting in Raw mode. The correction is only actually applied later, when processing Raw files in Nikon apps like Capture NX-D and VIEWNX-I, and can be switched off.