Chromatic aberration is a common lens aberration that appears as „color fringes“ or “purple fringing” along the borders of very dark or very bright parts of an image. But how does it occur and can it be avoided?
Dispersion and refraction of light
At first we will take a look at the dispersion of light by a prism. Dispersion is the separation of visible light into its different colors (wavelengths). This can be observed when light is passing through a prism.
Light that passes from one material to another, will be refracted (bent) at a boundary. In the illustration below, you can see that the blue rays of light are refracted stronger compared to the red rays of light.
The intensity of refraction depends on the optical density of the medium the light passes through as well as the wavelength. Shorter wavelengths (blue) will bend more than longer wavelengths (red) and material with a higher optical density will bend more compared to a lower optical density. The optical density is described by the index of refraction.
Here are some samples of refraction indices:
Dense Flint Glass
Keeping this in mind, you can imagine what happens when light passes through a lens to create an image on the sensor of the camera. Since the focal length of a lens is dependent on the refractive index, different wavelengths of light will be focused on different positions.
An ideal lens would somehow manage to compensate the different dispersion of the wavelengths in order to focus them in exactly the same point, see illustration below.
Unfortunately this ideal lens does not exist in reality. Instead, chromatic aberration is caused by lens dispersion, with different colors of light travelling at different speeds while passing through a lens. In an image it can be noticed as colored edges (red, yellow, green, blue, purple, magenta) around objects in high contrast situations.
There are mainly two different types of chromatic aberration – longitudinal and transverse
Longitudinal (axial) chromatic aberration
Longitudinal chromatic aberration occurs when different colors (wavelengths) focus at different points along the horizontal optical axis as a result of dispersion properties of the glass. In the illustration below only the green light is focused sharply on the image plane. The blue and red light have a so-called circle of confusion in the sensor plane and are not focused sharply. Stopping down the aperture helps reducing the effect of axial chromatic aberration.
Transverse (Lateral) chromatic aberration
Transverse (lateral) chromatic aberration occurs when different colors (wavelengths) coming at an angle focus at different positions along the same focal plane, as illustrated below. In contrast to longitudinal chromatic aberration, the transverse chromatic aberration only occurs towards the corners of a high contrast image and cannot be corrected by stopping down the aperture. Lateral chromatic aberration can only be corrected in post processing software.
An achromatic correction applies to wavelengths at both ends of the visible spectrum (red & blue). It is commonly achieved by the combination of a convex crown glass* element and concave flint glass* element – the so-called achromatic doublet.
*Crown glass has a lower refraction index than flint glass (see list above)
An apochromatic correction is designed to bring three wavelengths (usually red, green & blue) into focus in the same plane. This is usually achieved by combining three types of lens elements – the so-called apochromatic triplet – and is a lot more complicated and costly than designing an achromatic lens.
The abbreviation ED derives from Extra Low Dispersion Glass. Very often both, achromatic and apochromatic lenses, contain elements made of special low dispersion glass.
Primary and secondary spectrum chromatic aberration
Completely uncorrected lenses are called chromatic and show a so-called primary spectrum chromatic aberration where the far ends of the spectrum, red and blue, focus differently. But most photographic lenses have an achromatic design, which means that the red and blue end of the spectrum focus in the same point.
While blue and red wavelengths are focused in the same point, green wavelengths still focus in a different point. This is called secondary spectrum chromatic aberration and shows magenta/green artifacts instead of red/blue artifacts.
The type of chromatic aberration that you encounter most commonly is the secondary transverse chromatic aberration showing magenta and green lines along the borders of high contrast details towards the corners of an image.
This sample image of secondary spectrum chromatic aberration has been taken with an extreme wide-angle (fisheye) lens. You can clearly see the magenta artifacts towards the center and green artifacts towards the corners of each image detail.