Aberration of Lens

Edited By Vishal kumar | Updated on Jul 02, 2025 05:17 PM IST

Lenses are widely used in microscopes, telescopes, spectacles and other optical instruments. It is a transparent substance of plastic or glass which is bounded by two surfaces, one or both of which may be curved. The lens works on the principle of refraction. During the formation of images by lenses, it was assumed that objects are very small in size and objects are placed very close to the principal axis of the lens. But in actual practice, these assumptions are not satisfied. So, certain defects take place. Due to these defects, the images are blurred and distorted instead of being sharp and well-defined. These types of defects are called aberrations.

This Story also Contains

Different Types of Aberrations
Spherical Aberration
Cometic Aberration
Astigmatism Aberration
Distortion and Curvature of Field

Aberration of Lens

Different Types of Aberrations

The deviations from the actual size, position and shape of an image as calculated by simple equations are called aberrations produced by the lens. There are two types of aberrations:

Monochromatic Aberration: The aberration which occurs when monochromatic light is used is called monochromatic aberration.
Chromatic Aberration: The aberration that occurs due to the dispersion of light is called chromatic aberration. Chromatic aberration is generally produced by the variation of refractive index with the colour of light.

Monochromatic aberrations are further classified into three steps:

Spherical Aberration
Cometic Aberration
Astigmatism Aberration
Distortion and Curvature of Field

Spherical Aberration

The rays refracting from different regions of the lens do not come to the same focus but are focused at different points which are known as spherical aberration. As shown in the figure given below, light rays refract when passing through the lens.

The marginal rays come to a focus at a shorter distance along the principal axis, while the paraxial rays come to a focus at a longer distance. This creates different focal points along the same axis. The focal point with the “circle of least confusion” is shown in the above figure. The focal length for paraxial rays is greater than the focal length for marginal rays and the difference in focal length of the lens for paraxial rays and marginal rays are called longitudinal spherical aberration. In spherical aberration, a single-point image is not possible at any point on the screen. The image formed in spherical aberration would always be a circle.

There are three methods by which we can reduce spherical aberration:

By using a plano-convex or plano-concave lens with a curved side facing the incident rays.
By using suitable stops, we can reduce spherical aberration.
By using a parabolic mirror and by using a lens of large focal length.

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Cometic Aberration

When a point object is situated away from the principal axis of the lens, then the image formed has a comet-like appearance which is shown in the figure given below. So, the deviation of an image's shape from that object is known as comatic aberration or coma. The image is comet-shaped and hence the name is given as Coma.

This aberration is similar to spherical aberration because in both cases, the lens failed to bring all the rays from an object to focus at the same point. Cometic aberration arises due to the inability of the lens to focus the central and marginal rays at the same point. Another reason is that the linear magnification produced by different zones of the lens is different for a point object situated away from the principal axis. Each zone forms the image of a point in the form of a circle known as a comatic circle.

To avoid comatic aberration,

We must follow the abbe’s sine condition to reduce comatic aberration.

\mu _{1}h_{1}sin\theta _{1}=\mu _{2}h_{2}sin\theta _{2}

Where \mu _{1} and \mu _{2} are the refractive indices of the object and image respectively. h_{1} and h_{2} are the lengths of the object and image for a particular zone. \theta _{1} and \theta _{2} are the angles made by the rays with the axis.

By using stops of suitable diameters, we can minimise the comatic aberration.

Astigmatism Aberration

This aberration is similar to comatic aberration because, in both aberrations, the object lies off the principal axis. In comatic aberration, the spreading of the image takes place in a plane perpendicular to the lens axis and in astigmatism, the spreading takes place along the lens axis.

Astigmatism aberration occurs due to the large difference in the angle between rays and the principal axis.

By using a suitable combination of a convex and concave lens, astigmatism aberration can be reduced.
With the help of suitable stops, astigmatism aberration can be reduced.
By using the toric lens, we can reduce this aberration.

Distortion and Curvature of Field

Distortion

Distortion is also a type of monochromatic aberration which describes how the magnification in an image changes across the field of view at a fixed distance. The inability of a lens to form a flat image of a flat object is called distortion. It arises due to the fact that the magnification produced by the lens for different parts of the object is different. Because the different parts of the object are having different axial distances from the lens and that is why different parts of the object are magnified differently. By using a suitable combination of thin lenses, the distortion formed by lenses can be minimised. It can also be minimised by using suitable stops.

Curvature of Field

The image of an extended object due to a single lens is not a flat one but will be a curved surface. This effect is known as the curvature of the field. The central part of the image will be well-focused but the outer part will not be well-focused and that is why we will get a blurred image. The curvature of the field is due to the different focal lengths of the lens for paraxial and marginal rays. By using suitable convergent and divergent lenses or by using stops, we can eliminate the curvature of the field.

Frequently Asked Questions (FAQs)

1. Which lens is known as a converging lens? How do we define the principal focus of a convex lens?

A Convex lens is known as a diverging lens. The principal focus of a convex lens is a fixed point on its principal axis. A beam of light incident parallel to its principal axis converges to this point after passing through the convex lens.

2. How can we distinguish a concave lens from a convex lens?

A concave lens is thinner in the middle than at the edges and a convex lens is thicker in the middle than at the edges.

3. How can chromatic aberration be minimised?

The Chromatic aberration can be minimised by using a thin and small aperture lens.

4. Define magnification.

Magnification is the ratio of the size of the image to the size of the object. This ratio is equal to the ratio of image and object distance from the lens.

5. Define the principal axis of a lens in simple words.

An imaginary straight line which is passing through both the centres of curvature of a lens is called the principal axis of a lens.

6. How does astigmatism differ from other lens aberrations?

Astigmatism is an aberration where light from off-axis points forms two line foci at different distances instead of a single point focus. This results in different parts of the image being in focus at different distances, unlike spherical aberration which affects the entire image uniformly.

7. How do refractive index and dispersion of lens materials affect aberrations?

The refractive index determines how much light bends when entering the lens, affecting all aberrations. Dispersion, which is the variation of refractive index with wavelength, specifically affects chromatic aberration. Materials with high dispersion cause more chromatic aberration.

8. How do lens coatings help reduce aberrations?

Lens coatings don't directly reduce aberrations, but they help minimize reflections and light scattering within the lens. This improves contrast and reduces ghosting and flare, which can make the effects of aberrations less noticeable in the final image.

9. What is the difference between monochromatic and polychromatic aberrations?

Monochromatic aberrations (like spherical aberration, coma, and astigmatism) occur even with a single wavelength of light. Polychromatic aberrations (like chromatic aberration) only occur when multiple wavelengths of light are present.

10. How does temperature affect lens aberrations?

Temperature changes can cause slight expansions or contractions in lens elements and their housing, potentially altering the spacing and alignment of elements. This can lead to small changes in aberrations, particularly noticeable in high-precision optical systems.

11. What is coma aberration and how does it affect image quality?

Coma aberration occurs when light rays from off-axis points are not focused to a single point in the image plane. This results in a comet-shaped blur (hence the name "coma") that becomes more pronounced for points further from the optical axis, degrading image sharpness especially at the edges.

12. What is the impact of lens aberrations on the resolution of an optical system?

Lens aberrations reduce the resolution of an optical system by spreading light from a point source over a larger area in the image plane. This limits the system's ability to distinguish fine details and decreases overall image sharpness.

13. How do aberrations affect the depth of field in an image?

Aberrations can make the transition from in-focus to out-of-focus areas less distinct, effectively increasing the perceived depth of field. However, this comes at the cost of overall image sharpness and clarity.

14. Why are some aberrations more noticeable in wide-angle lenses?

Wide-angle lenses often have more pronounced aberrations, especially towards the edges, because they need to bend light rays more severely to achieve a wide field of view. This makes effects like distortion, coma, and chromatic aberration more noticeable.

15. How do zoom lenses manage aberrations across their focal length range?

Zoom lenses use complex designs with multiple moving elements to balance aberrations across their focal length range. Different aberrations are often prioritized at different focal lengths, and compromises are made to maintain acceptable image quality throughout the zoom range.

16. What is aberration of lens?

Aberration of lens refers to the imperfections or defects in the image formed by a lens that cause it to deviate from the ideal, sharp image. These imperfections occur due to various factors related to the lens design, material, and the nature of light itself.

17. How does chromatic aberration differ from spherical aberration?

Chromatic aberration occurs due to different wavelengths of light focusing at different points, causing color fringes in the image. Spherical aberration, on the other hand, is caused by light rays passing through different parts of a spherical lens focusing at different points, resulting in a blurred image regardless of color.

18. Why do different colors of light focus at different points in chromatic aberration?

Different colors of light have different wavelengths, and the refractive index of a lens material varies with wavelength. This causes each color to bend (refract) at a slightly different angle, resulting in different focal points for each color.

19. Can chromatic aberration be completely eliminated in a single lens?

No, chromatic aberration cannot be completely eliminated in a single lens. However, it can be significantly reduced by using combinations of lenses with different refractive indices, such as in achromatic doublets or apochromatic lens systems.

20. What is the difference between longitudinal and lateral chromatic aberration?

Longitudinal chromatic aberration occurs when different colors focus at different distances along the optical axis, causing blurring. Lateral chromatic aberration happens when different colors focus at different positions perpendicular to the optical axis, resulting in color fringing at the edges of the image.

21. What causes field curvature in lenses?

Field curvature occurs because a flat object plane is imaged onto a curved surface rather than a flat image plane. This is due to the inherent properties of spherical lenses and results in the edges of a flat subject being out of focus when the center is in focus, or vice versa.

22. How does distortion aberration affect the shape of the image?

Distortion aberration causes straight lines in the object to appear curved in the image. Positive (or pincushion) distortion makes lines bow outwards, while negative (or barrel) distortion makes lines bow inwards. This aberration doesn't affect image sharpness but alters the geometry of the image.

23. Why do some lenses produce images with "softer" edges?

Softer edges in images are often due to a combination of aberrations, particularly coma and astigmatism. These aberrations become more pronounced towards the edges of the lens, causing light from off-axis points to spread out rather than focusing to a sharp point.

24. How does stopping down a lens (using a smaller aperture) affect aberrations?

Stopping down a lens generally reduces aberrations because it blocks light rays passing through the outer edges of the lens, which are typically most affected by aberrations. This improves image quality but reduces the amount of light reaching the sensor or film.

25. What is the Airy disk and how is it related to lens aberrations?

The Airy disk is the best-focused spot of light that a perfect lens with a circular aperture can make, limited only by diffraction. Real lenses produce larger spots due to aberrations, and the goal of lens design is to make these spots as close to the Airy disk size as possible.

26. How does lens shape affect spherical aberration?

The curvature of a spherical lens causes light rays passing through its edges to focus at a different point than rays passing through its center. Lenses with steeper curves generally produce more spherical aberration, while flatter lenses or aspheric designs can reduce this effect.

27. What is the concept of "circle of least confusion" in relation to spherical aberration?

The circle of least confusion is the smallest blur spot produced by a lens with spherical aberration. It occurs at a point between the focal points of the central and edge rays, representing the best compromise focus position for the entire lens.

28. How do lens aberrations affect bokeh (the quality of out-of-focus areas)?

Lens aberrations can significantly influence bokeh. Spherical aberration, in particular, affects how out-of-focus points of light are rendered, potentially making them appear more or less smooth or introducing rings or other artifacts in the bokeh.

29. How do aspherical lens elements help reduce aberrations?

Aspherical lens elements have surfaces that deviate from a perfect sphere, allowing lens designers to correct for spherical aberration and other aberrations more effectively than with spherical elements alone. This can lead to better image quality with fewer elements.

30. What is the difference between "corrected" and "well-corrected" lenses?

A "corrected" lens has had some effort made to reduce aberrations, while a "well-corrected" lens has had extensive design work to minimize aberrations across its entire usage range. Well-corrected lenses typically offer superior image quality but are often more complex and expensive.

31. What is the relationship between lens aperture and chromatic aberration?

Larger apertures (smaller f-numbers) generally exhibit more noticeable chromatic aberration because they allow light to pass through a larger area of the lens, including the edges where chromatic effects are more pronounced.

32. What is the concept of "balancing aberrations" in lens design?

Balancing aberrations involves designing a lens system where different elements introduce opposing aberrations that partially cancel each other out. This approach allows for better overall performance than trying to eliminate each aberration individually.

33. How does the focal length of a lens affect its aberrations?

Generally, longer focal length lenses tend to have less pronounced aberrations because they require less extreme light bending to form an image. However, they may be more susceptible to certain aberrations like chromatic aberration due to their longer light path.

34. What is the impact of lens aberrations on astrophotography?

In astrophotography, lens aberrations can significantly affect image quality. Coma can cause stars to appear elongated or comet-shaped, especially at the edges of the frame, while chromatic aberration can create color fringes around bright stars.

35. How do lens aberrations affect the contrast in an image?

Lens aberrations can reduce image contrast by spreading light from bright areas into darker areas. This effect is particularly noticeable with spherical aberration and coma, which can create a hazy or washed-out appearance in high-contrast scenes.

36. What is the concept of "diffraction-limited" performance in relation to lens aberrations?

A lens is considered diffraction-limited when its aberrations have been reduced to the point where diffraction, rather than aberrations, is the primary factor limiting image sharpness. This represents the theoretical best performance for a given lens aperture.

37. How do lens aberrations affect the performance of autofocus systems?

Lens aberrations can impact autofocus performance by reducing the contrast and sharpness of the image projected onto the autofocus sensors. This can make it more difficult for the system to accurately determine focus, potentially leading to slower or less accurate autofocus.

38. What is longitudinal chromatic aberration (LoCA) and how does it differ from lateral chromatic aberration?

Longitudinal chromatic aberration (LoCA) occurs when different colors focus at different distances along the optical axis. This can cause colored fringes around objects throughout the image, not just at the edges. Lateral chromatic aberration, in contrast, causes color fringing primarily at the edges of the frame.

39. How do lens aberrations affect the appearance of specular highlights in an image?

Lens aberrations can cause specular highlights (bright reflections) to appear distorted or to take on unusual shapes. For example, coma aberration can cause off-axis highlights to appear comet-shaped, while spherical aberration can create a "donut" effect in out-of-focus highlights.

40. What is the relationship between lens aberrations and vignetting?

While vignetting (darkening of image corners) is not strictly an aberration, it often occurs alongside other aberrations in lenses. Both tend to be more pronounced at wider apertures and can be reduced by stopping down the lens.

41. How do lens aberrations affect the perception of sharpness in different parts of an image?

Lens aberrations often cause sharpness to vary across the image field. Typically, the center of the image appears sharpest, with sharpness decreasing towards the edges and corners due to increasing effects of aberrations like coma and astigmatism.

42. What is field curvature and how does it affect focus across an image?

Field curvature is an aberration where the focal plane of the lens is curved rather than flat. This causes the focus to vary across the image field, typically resulting in either the center or the edges being in focus, but not both simultaneously when shooting a flat subject.

43. How do lens aberrations affect the rendering of fine textures and patterns in an image?

Lens aberrations can reduce the clarity and contrast of fine textures and patterns, especially towards the edges of the image. This can result in a loss of detail and a "smearing" effect on intricate patterns, particularly noticeable in architectural or nature photography.

44. What is the concept of "spherochromatism" in lens aberrations?

Spherochromatism is a complex aberration that combines aspects of both spherical aberration and chromatic aberration. It occurs when different wavelengths of light exhibit different amounts of spherical aberration, leading to color-dependent blurring effects.

45. How do lens aberrations affect the appearance of out-of-focus highlights at different apertures?

Lens aberrations can cause out-of-focus highlights to take on different shapes at different apertures. For example, spherical aberration can cause these highlights to have a bright edge and darker center when the lens is wide open, changing to a more uniform appearance as the lens is stopped down.

46. What is the impact of lens aberrations on macro photography?

In macro photography, lens aberrations can be more noticeable due to the extreme close focusing distances involved. Chromatic aberration and spherical aberration can be particularly problematic, affecting the sharpness and color accuracy of fine details in close-up subjects.

47. How do lens aberrations affect the perception of depth and dimensionality in an image?

While aberrations generally degrade image quality, some photographers argue that a controlled amount of certain aberrations (like spherical aberration) can contribute to a more three-dimensional appearance in images by softening transitions and creating a more gradual falloff in sharpness with depth.

48. What is the relationship between lens aberrations and lens flare?

While lens flare is not strictly an aberration, both phenomena are related to how light interacts with the lens. Aberrations can exacerbate the effects of lens flare by causing light to scatter in unpredictable ways within the lens, potentially leading to more pronounced or complex flare patterns.

49. How do lens aberrations affect the performance of teleconverters?

Teleconverters magnify not only the image but also any aberrations present in the lens. This can lead to a noticeable decrease in image quality, particularly with lenses that aren't well-corrected to begin with or when using higher-magnification teleconverters.

50. What is the concept of "focus shift" and how is it related to spherical aberration?

Focus shift is a phenomenon where the point of best focus changes as the lens aperture is adjusted. It's primarily caused by spherical aberration and can lead to images being slightly out of focus when stopping down after focusing at a wider aperture.

51. How do lens aberrations affect the rendering of point light sources, such as stars in astrophotography?

Lens aberrations can cause point light sources to appear distorted or spread out. In astrophotography, this can make stars appear as streaks (coma), colored dots (chromatic aberration), or with diffraction spikes. Well-corrected lenses aim to render stars as small, round points of light.

52. What is the impact of lens aberrations on the effectiveness of image stabilization systems?

While image stabilization primarily combats motion blur, lens aberrations can affect its performance. Aberrations that reduce image sharpness or contrast can make it more challenging for the system to detect and correct for small movements, potentially reducing the effectiveness of stabilization.

53. How do lens aberrations interact with sensor technologies like anti-aliasing filters?

Lens aberrations and anti-aliasing filters both have a softening effect on images. In some cases, the softening from lens aberrations may reduce the need for strong anti-aliasing filters. Conversely, sharp, well-corrected lenses may benefit more from anti-aliasing to prevent moiré patterns.

54. What is the relationship between lens aberrations and diffraction effects as a lens is stopped down?

As a lens is stopped down, the impact of many aberrations (like spherical aberration and coma) tends to decrease. However, diffraction effects become more pronounced at smaller apertures. There's often a "sweet spot" aperture where the balance between reduced aberrations and increased diffraction yields the sharpest overall image.

55. How do lens aberrations affect the perception of color and tonal transitions in an image?

Lens aberrations can affect color rendition and tonal transitions in subtle ways. Chromatic aberration directly impacts color accuracy, especially in high-contrast areas. Other aberrations can affect how smoothly tones transition across an image, potentially impacting the perceived dynamic range and color depth of the final image.