Aberration of Lens

Aberration of the lens refers to the failure of a lens to produce a perfect image. It occurs due to the lens’s geometry and light’s physical properties. There are several lens aberrations, each affecting the image in different ways.

In this article, we will discuss lens aberration, its types, corrections methods, and many more in detail.

Lens aberration refers to the imperfections that occur when light passes through a lens, distorting the image. These occur due to the geometry and materials of the lens. They cause light rays to spread out or focus inaccurately. Aberrations affect the clarity, brightness, and colour of images.

Aberrations can degrade the quality of an image by causing blurriness, light halos, and color fringing. The impact of these aberrations becomes noticeable in high-resolution photography where even minor flaws in image clarity are visible.

Here are the key factors that contribute to lens aberration:

• Lens Shape: Spherical lenses, which are common due to ease of manufacturing, do not focus all light rays to a single focal point, leading to spherical aberration.

• Lens Composition: The material used in a lens affects its refractive index and dispersion, influencing aberrations like chromatic aberration.

• Aperture Size: A larger aperture allows more light but increases susceptibility to aberrations like coma and spherical aberration. A smaller aperture reduces these but can increase diffraction.

• Lens Elements Configuration: The arrangement and types of lens elements (e.g., aspherical vs. spherical) affect how well aberrations are corrected within the lens assembly.

• Wavelength of Light: Different wavelengths of light refract at slightly different angles when passing through a lens, leading to chromatic aberration.

• Quality of Lens Manufacturing: Imperfections in lens manufacturing, such as surface irregularities and alignment errors, can introduce various aberrations.

• Lens Coatings: The presence or absence of anti-reflective coatings impacts how much light is reflected internally within the lens, affecting aberrations like ghosting and flare.

There are Various types of aberrations exist, each affecting image quality in specific ways. Here are the types of lens aberrations:

1. Chromatic Aberration
2. Spherical Aberration
3. Coma
4. Astigmatism
5. Curvature of Field and Distortion

Where:

• K is Constant related to the lens shape
• r is Aperture radius
• F is Focal length of the lens
• Δn is Difference in refractive index for two colors
• λ is Wavelength of light
• h is Distance from the optical axis
• L is Distance from the lens to the image plane
• α is Angle of incidence of the chief ray
• β is Angle between the chief ray and the optical axis
• k1​, k2​, k3​ is Distortion coefficients

Also, Check Power of a Lens

Chromatic aberration appears as color fringes around objects in images. It happens because different colors of light refract at slightly different angles. Blue and red light often focus at different distances when passing through a lens. This type of aberration reduces image sharpness and color accuracy.

Correcting chromatic aberration can involve using special lens coatings or additional lens elements that compensate for color dispersion.

There are two types of chromatic aberrations. They are:

• Lateral Chromatic Aberration
• Longitudinal Chromatic Aberration

Lateral Chromatic Aberration

Lateral chromatic aberration occurs when different wavelengths of light are focused at different positions on the same focal plane. It is also known as transverse chromatic aberration. This type of aberration results in color fringes appearing around the edges of objects in the image.

It is more noticeable towards the edges of the optical field and varies with the aperture size. Lateral chromatic aberration is often seen as color outlines that can degrade image quality in high-contrast scenes.

Longitudinal Chromatic Aberration

Longitudinal chromatic aberration happens when different wavelengths of light are focused at different distances along the optical axis. This causes a shift in focus between colors, with some colors focusing slightly in front of or behind the sensor or film plane.

It is also called axial chromatic aberration. This effect is most apparent in images with shallow depth of field, where it shows as halos of color around out-of-focus areas. It is noticeable in the background blur of images.

Spherical aberration occurs when light rays passing through a lens do not converge at the same point. This results from the spherical shape of standard lenses. Since the light rays hitting the lens at different places focus at different distances, this causes blur and reduces image quality, especially in strong lighting.

In lenses with spherical surfaces, rays from the edge focus nearer than those from the center. This variance in focus points across the lens surface degrades the image. It can cause softness in images that cannot be eliminated by simply adjusting the focus. Modern lens designs often use aspherical elements to reduce this effect.

Coma is an aberration that causes off-axis points, like stars, to appear comet-shaped. It gets worse as the aperture of the lens increases. This aberration is especially noticeable in astrophotography and nighttime landscapes.

Coma becomes more prominent in images taken with a wide aperture setting. It distorts points of light away from the center. This can be very distracting in images with pinpoint lights against a dark background. Lens manufacturers combat coma by designing complex aspherical elements and using sophisticated optical formulas to align light more accurately.

Astigmatism in lenses causes images to blur along one axis more than the other. It results from the lens having different focal lengths in different meridians. This makes it hard to get a uniformly sharp focus across an image.

Optical astigmatism can greatly affect the quality of an image, depending on the orientation of the lens elements. It is most noticeable when photographing straight lines, which can appear soft or wavy. Advanced lens designs use multiple aspherical surfaces or specially shaped elements to correct for astigmatism.

Curvature of Field is an optical aberration where the image formed by a lens is not perfectly flat. Instead of a flat plane, the image surface takes the shape of a curved surface, much like a shallow bowl. This causes the edges of the image to appear out of focus when the center is in focus, or vice versa.

This is very noticeable in flat-field applications like microscopy or photographing a flat surface. The result is an image that cannot be entirely in focus at once without stopping down the lens, which increases the depth of field.

Distortion is an aberration that causes straight lines near the edges of an image to bend inward or outward, giving a warped appearance to the image. There are two main types of distortion: barrel and pincushion. Barrel distortion makes images appear bowed outwards at the edges. It is common in wide-angle lenses. Pincushion distortion causes images to pinch in at the edges, which is seen in telephoto lenses.

Both types of distortion are a matter of concern in architectural photography where straight lines are prominent. Modern lenses use complex lens elements and aspherical designs reduce distortion. Many cameras feature built-in digital correction capabilities.

To eliminate aberrations in a lens, several methods can be employed:

1. Lens Design
   • Aspheric Surfaces: Using non-spherical lens surfaces to reduce spherical aberration.
   • Multiple Lens Elements: Combining multiple lens elements with different characteristics to compensate for various aberrations.
   • Apochromatic Lenses: Employing specialized lenses that focus different wavelengths of light to the same point, reducing chromatic aberration.
2. Lens Materials
   • Low Dispersion Materials: Using materials with low dispersion characteristics to minimize chromatic aberration.
   • Fluorite and Extra-low Dispersion (ED) Glass: Incorporating lens elements made from these materials to reduce chromatic aberration and color fringing.
3. Aperture Design
   • Stopping Down: Reducing the aperture size can help mitigate spherical aberration and coma aberration.
4. Advanced Optical Design
   • Computer-Aided Design (CAD): Utilizing advanced software to optimize lens designs and minimize aberrations.
   • Complex Lens Systems: Designing complex lens systems with multiple elements to correct for various aberrations across the entire optical system.

Learn More,

• Convex Lens: Definition, Types, Properties, Uses
• Concave Lens: Definition, Formula, Image Formation
• Lens Sign Convention – Convex and Concave

What is lens aberration in photography?

Lens aberration refers to the imperfections in images produced by lenses, such as blurring or color fringes, due to the lens’s optical limitations.

What causes chromatic aberration in lenses?

Chromatic aberration occurs because lenses refract different wavelengths of light at slightly different angles, leading to color fringing around objects.

Can astigmatism be corrected in camera lenses?

Yes, astigmatism can be corrected by using lens elements with specially designed curvatures to ensure that light focuses evenly at all points.

How can chromatic aberration be reduced using lenses?

Chromatic aberration can be reduced by using an achromatic doublet, which combines two lenses with different dispersion properties to better focus colors at the same point.

How to eliminate aberration in a lens?

Aberrations in lenses can be eliminated through various methods such as using specialized lens materials, incorporating multiple lens elements, utilizing aspheric lens elements, and employing advanced optical designs.

What are the defects or aberrations of the lens?

Common lens aberrations include chromatic aberration, spherical aberration, coma aberration, astigmatism, and field curvature. These aberrations result in various distortions in the image formed by the lens.

What does the term aberration refer to in a lens?

In a lens, aberration refers to the failure of the lens to produce a perfect image. Aberrations cause deviations from the ideal image formation and result in image distortions such as blurring, color fringing, and distortion.