There are three main types of colour fringing:

• Lateral chromatic aberration. This is the result of the lens focal length differing depending on the wavelength of incoming light. It is seen mainly in the corners and can be readily corrected, either by the camera (in JPEG mode) or by the RAW conversion software. Better lenses show less lateral CA but in the world of digital it's not the problem it once was.

• Longitudinal chromatic aberration otherwise known as axial colour. This is the result of different wavelengths coming into focus at different distances, resulting in out of focus highlights having a magenta tint in front of the plane of focus and a green tint behind. This is much harder to correct as the camera/RAW software doesn't know what is in front or behind the plane of focus. Better lenses exhibit less axial colour, and it disappears stopping down but fast lenses all show some degree of axial colour wide open.

• Purple fringing. This is the result of axial colour in the infra-red spectrum being picked up as the blue and to some extend red dyes used in the sensor CFA both pass IR resulting in a purple glow around highlights. It can be removed in software and in theory can be prevented by using an IR cut filter on the lens, though I have never tried this.

Unfortunately ultra-fast lenses like the EF 50mm f/1.2L are going to exhibit all of these aberrations at f/1.2. So if you're shooting for shallow depth of field you're going to have to remove them in software.