Chromatic Aberration

Chromatic aberration, also known as chromatic distortion, achromatism, and spherochromatism is a distortion that occurs in optics due to the failure of the lens in focusing all the colors at the same convergence point. The distortion occurs due to the varying refractive indices of the lenses. In other words, the glass behaves differently for different wavelengths of the light. The refractive indices of the transparent materials change and usually decrease with an increase in the wavelength.

Chromatic aberration establishes itself as fringes along the boundary of the subject that separates the bright and dark parts. The fringes appear because it is difficult for the optical spectrum to create a common point for all the color wavelengths. The focal range of the lens is reliant on the refractive index. Therefore, wavelengths of different colors focus in different positions when the pass through the lens.

Types of Chromatic Aberration

Chromatic aberration occurs in two types – axial and transverse. Axial or longitudinal chromatic aberration occurs due to the different distances of the focusing points of the wavelengths from the lens. In photography, the term that describes this difference is focus shift. The transfers or lateral chromatic aberration occurs when there is a difference in positions of the wavelengths in the focal plane. The magnification of the lens also varies the wavelength, with changes the positions of the wavelengths of different colors.

The characteristics exhibited by the two types of chromatic aberrations are different. There is a possibility that both aberrations occur together. Axial or longitudinal chromatic aberration is present throughout the image. Optometrists, optical engineers, and vision scientists term the unit of focus as diopters. Reducing the longitudinal chromatic aberration is possible by stopping down, which increases the depth of field. An increase in the depth of field narrows the distance between the points on the axis, which together fall in the acceptable focus range. Transverse chromatic aberration increases towards the edges but does not occur at the center. Increasing the depth of field has no effect on the transverse chromatic aberration.

In digital chromatic aberration, the axial aberration results in defocused planes of the blue and red wavelengths, which becomes difficult to correct it in the post processing stage. On the other hand, the transverse aberration results in different magnifications of the red, blue, and green planes, which is possible to correct using scaling the planes appropriately to minimize the error.

Minimization

Minimizing the chromatic aberration was possible by increasing the focal length of the lens. For instance, the long telescopes such as the aerial telescopes found in the 17th century. Newton’s discovery of the white light consisting of a spectrum of colors helped him in concluding that uneven refraction of the light was responsible for the development of chromatic aberration. The theory helped him to develop reflecting telescope, the Newtonian telescope in the year 1668.

Minimizing chromatic aberration is possible due to the presence of the circle of least confusion. The aberration is further reducible by using an achromatic lens. An achromatic lens is a combination of different dispersion lenses, which together form a compound lens. The standard construction of the compound lens is the achromatic doublet, where the development of the glass elements uses flint and crown glass.

The use of the materials assists in reducing the development of chromatic aberration for specified wavelengths. A combination of the two or more lens elements increases the degree of correction, as seen in an achromatic lens. Note that the term “achromat” refers to the correction of two or three wavelengths, where they focus correctly.

Achromatic lens manufactured with low dispersion glass results in better correction than the lens made of conventional glass. Additionally, the benefit of using achromatic glass is not constrained to focusing two or three wavelengths correctly, but the fact that the occurrence of an error for other wavelengths is small.

Over the years, considerable research led to the development of different glass types that were helpful in reducing the chromatic aberration. The most notable material of all is the low dispersion glass, with a high content of fluorite. The hybrid material of the glass offers a greater degree of correction – only two elements of glasses is necessary to correct chromatic aberration.

An alternative to the achromatic doublets is the use of diffractive elements. Diffractive elements are capable of generating arbitrary complex waves from a sample of optical material that is usually a flat surface. Diffractive elements use complementary dispersion characteristics of plastics and optical glasses. In a visible spectrum, the diffractive elements carry an Abbe Number of -3.5.

Chromatic Aberration in Photography

Chromatic aberration in photography is a common characteristic due to the use of lenses. However, the certainty depends on the construction, the design, and the use of the glass elements in a lens. Poor or low-quality lenses have a higher degree of chromatic aberration in comparison to expensive lenses. The reason is the use of diffraction or low dispersion glass elements.

The sensor in the DSLR receives the light through the lens. The construction of the glass elements ensures that the wavelengths of the light fall within the circle of least confusion to keep the value of CA to the minimum. Apart from the design of the lens, the aperture value also defines the amount of the chromatic aberration in an image.

The occurrence of the chromatic aberration results in color fringing across the borders of the subject or in the regions where there is a difference between the dark and light portions. With the help of post processing software, such as Photoshop, removing the chromatic aberration is possible. Nonetheless, the reduction is acceptable only for a particular limit. Additionally, post processing requires spending a lot of time, which can be a tedious stage for professionals.

As the occurrence of the chromatic aberration is due to the different wavelengths and their behavior to the glass element, considering the nature of the reaction is important to generate a high-quality image. Different wavelengths of the light strike the glass element at the same time and behave uniquely before they reach the sensor. The lens design or the construction of the glass elements ensures that the behavior of the wavelengths is minimal.

The arrangement and alignment of the glass elements in the lens ensure that the different wavelengths of the light fall ahead of the sensor with each point on the axis placed at a minimal distance. There is a high probability that a lens uses more than 16 elements to correct different wavelengths that disperse when the ray of light strikes the first glass element at the beginning of the lens.

Unfortunately, chromatic aberration persists even with such a robust construction of the lens due to defects in the glass material or the overall design of the lens itself. An important factor to remember here is that every lens (even the expensive or the professional lenses) display visible chromatic aberration. The appearance of the aberration is dependent on the focal length of the lens, the distance from the subject, and lighting conditions.

Avoiding Chromatic Aberration

Although it is impossible to avoid chromatic aberration, reducing it to the minimal state is possible. Another way to remove the aberration is by using post-processing software, such as Photoshop. Nonetheless, it is crucial to remember that the support offered by software has limitation. It is a healthy practice to decrease the occurrence by addressing the issues in the camera and using an appropriate lens.

The following points help in reducing the chromatic aberration in the camera.

1. Avoiding High Contrast

Chromatic aberration is its highest in high contrast scenes, particularly in scenes with white backgrounds and landscape images with bright sunlight. Although it is impossible to control the settings in such a condition, reframing the shot and adjusting the subject helps in reducing the amount of chromatic aberration.

It is also useful to switch the background that matches the subject’s primary color. Another option is by waiting for favorable lighting conditions. If the shot requires capturing the scene as-is, then storing it in RAW format will be helpful. During post processing action, the RAW file will provide complete details of the image, making it easy to adjust the CA.

2. Focal Lengths

It is natural for photographers to possess a wide range of focal length optics. However, it is a fact that zoom lenses exhibit chromatic aberrations at their shortest and longest focal value. Therefore, choosing a different focal length will be helpful in reducing chromatic aberration. Additionally, using a zoom lens also introduces other defects at its widest opening. Under such circumstances, using a wide-angle prime lens that practically eliminates other flaws is a better option.

3. Stopping the Aperture

Stopping the aperture will increase the depth of field, which reduces the appearance of the chromatic aberration. However, stopping down the aperture depends on the lens used for a particular session. It is also crucial to remember that stopping down the aperture requires compensation in the form of increased ISO or dropping the shutter speed. Access to lighting instruments will be helpful in compensation and provides easy setup without worrying about the presence of the chromatic aberration or other defects.

4. Reframing the Subject

Chromatic aberration frequently occurs when the subject moves away from the center of the frame. As a practice, it is preferable to position the subject at the center of the frame. The appearance of the CA, in this case, is due to the curvature of the lens within the barrel. By reframing the subject to the center, eliminating the presence of the aberration is possible.

It becomes difficult to reframe the subject at the center in certain cases. Under such instances, cropping the image is a suitable answer to achieving the desired frame. The difference is not noticeable for small prints and digital distributions.

Image processing is helpful in correcting chromatic aberration along with other defects. However, there is a limitation to the processing software and may not be a perfect solution to eliminate CA. Therefore, as a practice, choosing an appropriate point from the factors mentioned above will prove beneficial. It further depends on the kind of lens used, the lighting conditions, the background, and the experience of the photographer.

Chromatic aberration is complex and has a lasting relationship with photography. Few manufacturers of the lens offer lens-specific chromatic aberration reduction techniques. These techniques assist in engineering the lenses in reducing the aberration. However, a careful approach to selecting a proper lens, settings, and using third-party post-processing software will be helpful in removing chromatic aberration from the image.

Conclusion

Chromatic aberration is present in optics. It is impossible to eliminate it, but there is a possibility to reduce it to a negligible quantity. Understanding the construction of the lens elements in a lens, the material of the glass elements, aperture value, and depth of field provide a chance to bring down the presence of chromatic aberrations in a picture. Exposure to third-party post-processing software, such as Photoshop, is also helpful in improving the quality of the images.

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