Photo Science: why does chromatic aberration occur?
The purpose of a lens is to refract the light – turning the straight path of the rays through an angle and towards the sensor.
Unfortunately, the various wavelengths of light are refracted by slightly different amounts, meaning that the path of red light is turned through a different angle than blue light, which is a different amount again from green light.
Different colours are then brought to focus at different points, so this creates the colour fringing.
Lens manufacturers go to great lengths to minimise the effect of this unavoidable law of physics. Several lens elements are used in combination, so the aberration caused by one can be minimised by another.
There are two types of chromatic aberration. Traverse (or lateral) chromatic aberration, which creates colour fringing, is caused by the fact that image magnification varies with wavelength.
Longitudinal (or axial) chromatic aberration is caused by different wavelengths being focused at different distances.
This creates image softness across the whole of the image, and can be reduced by avoiding the widest apertures to maximise the depth of focus.