Chromatic Aberration

Introduction

Chromatic aberration is an optical lens related problem. Light is electromagnetic radiation and is the only part of the electromagnetic spectrum that is visible to the human eye. A lens is an optical device designed in such a way that the light that falls on it bends while it passes through it. The light rays that reach the lens may converge to a point or diverge from a point. It is decided by the shape of the lens. A transmissive optical device that is used to focus or disperse the light beam which falls on it is called a lens. It is a thin and transparent material in which the divergence or convergence of the light happens due to the property of refraction of light through it. Generally, lenses are bounded by two surfaces which are spherical or one of the two surfaces as a spherical surface. It is made of transparent materials such as glass or plastic. There are two lenses, a simple lens and compound lens. A simple lens contains a single piece of transparent material but compound lenses are formed by combining many transparent materials.

What is Chromatic Aberration?

In optics sometimes the lens fails to focus the colours in the same point. This is called chromatic aberration. This is because of the dispersion of the light through the lens. The lens has different refractive index spending on the wavelength of the light. In most of the transparent material, the refractive index of the material decreases if the wavelength increases. The change in refractive index changes the focal length of the lens. Lens dispersion causes chromatic aberration in which the travelling speed of different colours are different. This results in a blurred image or the image that is surrounded by colours.

Causes of Chromatic Aberration

When a lens functions as a prism it causes chromatic aberration. The light that passes through the lens gets bent and split into different angles. The main causes of chromatic aberration are

• Poor quality lens

• Uneven aperture blade

• Lens mounted incorrectly

• Incorrect adjustment of objective lens

• Poor colour reproduction camera sensors

• Usage of lens which is designed for another camera

• Light is reflected off the sensor

• Lens which is in out of focus

• Angle distortion

• Wrong settings

Impact of Chromatic Aberration

The quality of the images is affected by the chromatic aberration. The dispersion through lens causes longitudinal chromatic aberration and lateral chromatic aberration. The photographs become blurry or there is a background of different colours such as green, red, yellow, blue, purple and magenta. In the chromatic aberration the colour fringes occurs both at the center and at the edge.

Ways to avoid Chromatic Aberration

Some of the ways to avoid chromatic aberration are given below.

Avoiding high Contrast Scenes

While shooting in a high contrast area, there will be a chance of chromatic aberration. It may be corrected by taking pictures in low contrast surroundings.

Changing Focal Length

At the extreme focal length there may be a chance of chromatic aberration in zooming lenses. To avoid this chromatic aberration, the focal length of the lenses should be fixed at the middle of the lens range.

Aperture Adjustment

Most of the lens faults including chromatic aberration are reduced by adjusting the aperture. By using f/8 or f/11 reduce the chromatic aberration.

Make the Subject to the Center

If the object is adjusted to nearly the center, then the chromatic aberration will be reduced.

Optimizing the Focal Plane

Using lenses with medium focal length will reduce the chromatic aberration.

Using Achromatic Lenses

Achromatic lenses concentrate two wavelengths on the same plane. In ordinary achromatic lenses, there are two separate glasses which have different degrees of dispersion. Chromatic aberration is strongly corrected by apochromatic lenses in which correction is made for three wavelengths of light.

Using cameras within Built Solutions

Transverse aberration is reduced by using cameras with in-built solutions.

Using high quality Lenses and Lenses with low Dispersion Glass

High quality lenses reduce the aberration rather than the cheap lenses. The aberration is also corrected by using lenses with low dispersion glass.

Using Photo editing Software

Post processing software such as photoshop and GIMP reduce the aberration. There is an aberration correction filter in GIMP. It is an open-source software.

Types of Chromatic Aberration

The chromatic aberration is divided into two types. They are

• Axial chromatic aberration

• Lateral chromatic aberration

Axial Chromatic Aberration

Axial chromatic aberration is also called longitudinal chromatic aberration. The lens focuses different wavelengths at different distances from the lens. As the focal point of the colors are different, chromatic aberration occurs around the object. In this type the same color is blurred around the whole object.

Cmglee, Comparison axial lateral chromatic aberration, CC BY-SA 4.0

Lateral Chromatic Aberration

It is also called transverse chromatic aberration. Color fringing is caused by lateral chromatic aberration. It is noticeable at the margin of the image. In this type the aberration is not visible at the center rather it appears at the corners of the images. Multiple lenses with different refractive indexes reduce the lateral chromatic aberration.

Conclusion

The inability of the lens to focus different wavelengths of the light at the same focal plane is called chromatic aberration. Many facts that cause the aberration and they were discussed above. The two types of chromatic aberration such as axial chromatic aberration and lateral chromatic aberration were discussed.

FAQs

Q1. What is the power of the lens?

Ans. When the light falls on the lens it bends through it. This ability of the lens which makes the light bend is called the power of the lens. It is denoted as the reciprocal of the focal length. More light bends through the lens if the focal length is reduced. The power of the lens is given by,

$$\mathrm{P=\frac{1}{f}}$$

Here f denotes the focal length.

Q2. How images are formed in concave lenses?

Ans. If the object is placed at infinity from the concave lens, then the image of the object is formed at the focal point of the lens. The image obtained is virtual and diminishing. If the position of the object is in between infinity and the optical center then the image is formed in between the focal length of the lens and the optical center.

Q3. Does chromatic aberration affect black and white photography?

Ans. Yes. Chromatic aberration affects black and white photography even there is no colors.

Q4. What is spherical aberration?

Ans. The light rays that pass through the spherical surface end up to focus at different points. Due to this spherical aberration, it is hard to obtain sharp images.

Q5. Is chromatic aberration normal?

Ans. Chromatic aberration in an optical system is common and normal while taking photos. There is no such lens to prevent the problem of aberration.