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What are the types of optical aberrations?

And how do optical aberrations manifest in bokeh?

Optical Aberrations in Photography

In photography, optical aberrations are imperfections in image formation that occur when light rays passing through a lens fail to converge properly at a single point. These aberrations can dramatically affect a photograph’s sharpness, clarity, and color accuracy, and understanding them is crucial for both amateur and professional photographers. This article explores the various optical aberrations, their impact on photography and bokeh, and strategies for correction and mitigation.

What Is an Aberration in Photography?

An optical aberration in photography refers to a deviation from ideal image formation caused by a lens’s inherent limitations. These imperfections result in blurred or distorted images with color fringing, affecting the overall visual quality. Aberrations arise due to the complex nature of light and the geometric and physical constraints of lens design.

Is It Possible to Design a Photographic Lens Without Aberrations?

Designing a photographic lens entirely free from aberrations is theoretically impossible due to the fundamental optical principles and material limitations. However, lens designers strive to minimize these imperfections using advanced materials and sophisticated design techniques. Both prime lenses (with a fixed focal length) and zoom lenses (which cover a range of focal lengths) exhibit specific aberrations. Still, prime lenses often have simpler designs that can be more precisely corrected for specific aberrations. By their nature, Zoom lenses face greater challenges in controlling aberrations across their variable focal lengths, requiring more complex arrangements of optical elements to achieve acceptable correction levels.

Two Types of Lens Aberrations: Chromatic and Monochromatic

In the intricate world of optical imaging, lens aberrations present challenges that can significantly affect the quality of photographs. Understanding the two primary types of lens aberrations, chromatic and monochromatic, is essential for photographers aiming to achieve high-quality images. Each type has distinct causes and manifestations, influencing photographs’ focus accuracy and color fidelity.

Chromatic Aberrations

Chromatic aberrations (Longitudinal and Lateral) occur due to the lens’s inability to focus all light colors at the same convergence point. This type of aberration stems from the physical properties of light itself – different wavelengths (colors) of light refract at slightly different angles when passing through a lens.

Chromatic aberration usually appears as “color fringing” or “halos” around high-contrast edges in an image. These fringes typically manifest in colors of purple, blue, green, or red, depending on the background and lighting conditions.

  • Causes: The root cause of chromatic aberrations is the dispersion of light, where different colors focus at different distances from the lens due to their varying wavelengths. This dispersion is a fundamental characteristic of transparent optical materials used in lens manufacturing.
  • Visualization on the Photograph: In practical terms, photographers often notice chromatic aberration in areas of extreme contrast, such as the edges of buildings against a bright sky or tree branches outlined by backlighting. These aberrations can detract from the overall sharpness and clarity of the image, making high-contrast edges appear less defined.

Monochromatic Aberrations

Unlike chromatic aberrations, monochromatic aberrations are unrelated to color and occur even if the lens is used with monochrome light. These aberrations are associated with the geometry and design of the lens and are Spherical, Comatic, Astigmatism, Field Curvature, and Distortion.

Monochromatic aberrations include several types, such as spherical aberration, coma, astigmatism, field curvature, and distortion. Each affects the image differently but generally leads to a loss of sharpness or distortion.

  • Causes: These aberrations are caused by the imperfect shape and alignment of the optical elements within the lens. For example, spherical aberration occurs because spherical surfaces are not ideal for focusing light on a single point. Similarly, astigmatism arises from the lens surfaces’ inability to simultaneously focus light to an end in both vertical and horizontal planes.
  • Visualization on the Photograph: Monochromatic aberrations manifest as various distortions in the image. Spherical aberration can make images appear soft, especially at wide apertures. Comatic aberration creates tail-like smears radiating from points of light, which is particularly noticeable in astrophotography. Astigmatism causes points to stretch into lines that can distort the shape of objects within the image, while field curvature results in images that are sharp in the center but blur towards the edges.

Two Different Types of Chromatic Aberrations

Chromatic aberrations are optical flaws caused by the lens’s inability to focus all light colors onto the same point. This kind of aberration is particularly problematic because it can be visibly distracting and impact the aesthetic quality of photographs. There are two main types of chromatic aberrations: longitudinal chromatic aberration and lateral chromatic aberration. Each affects images differently and arises from distinct optical behaviors.

Longitudinal Chromatic Aberration (Axial Chromatic Aberration)

Longitudinal chromatic aberration occurs when different light colors are not focused at the same distance along the lens’s optical axis. This results in various colors focusing slightly in front of or behind the plane of focus.

  • Common in Lenses: This type of aberration is typically seen in lenses with large apertures, such as fast primes. Vintage lenses like the Canon 85mm f/1.2 and modern lenses like the Nikon 50mm f/1.4G are known to exhibit this problem when used wide open.
  • Causes: Longitudinal chromatic aberration is caused by the dispersion of light, where different wavelengths (colors) refract differently as they pass through the lens elements. This dispersion leads to the focal plane’s variation for each color, creating a soft focus effect in color-specific areas of the image.

Lateral Chromatic Aberration (Transverse Chromatic Aberration)

Lateral chromatic aberration appears as color fringes along the edges and corners of objects within an image where the contrast is high. These fringes typically manifest in complementary colors such as purple and green.

  • Common in Lenses: Lateral chromatic aberration can occur in a wide range of lenses but is often more controlled in modern lens designs with advanced coatings and specially formulated glass. Older and lower-quality modern lenses, especially at wider focal lengths, such as the wide-angle zooms, may show more pronounced lateral chromatic aberrations.
  • Causes: This aberration is caused by the lens’s inability to focus all light colors at the same point across the image plane. Unlike longitudinal chromatic aberration, which varies along the optical axis, lateral chromatic aberration varies across the field of view and is more noticeable towards the edges of the frame.

Visualizing and Mitigating Chromatic Aberrations

In terms of how these aberrations visually manifest in photographs:

  • Longitudinal Chromatic Aberration: Often seen as a halo of colors around out-of-focus areas, particularly noticeable in bokeh, where it can give a color “glow” that can be aesthetically pleasing or distracting, depending on the context.
  • Lateral Chromatic Aberration: Visible as sharp color edges along high-contrast boundaries, which can be particularly distracting in landscape photography or any images containing straight lines and geometric patterns.

Photographers can mitigate these effects in several ways:

  • Lens Selection: Choose lenses known for minimal chromatic aberration or use apochromatic lenses specifically designed to reduce this flaw.
  • Aperture Adjustment: Stopping down the lens’s aperture can help reduce chromatic aberration by narrowing the path through which different wavelengths of light enter the lens.
  • Post-Processing: Most modern image editing software, like Adobe Photoshop and Lightroom, offers tools to correct chromatic aberrations. These tools can effectively remove unwanted color fringing, especially for lateral chromatic aberration.

Five Different Types of Monochromatic Aberration

Monochromatic aberrations, which affect the image quality regardless of color, are primarily influenced by the lens design and the physical properties of the lens elements. These aberrations can significantly impact image sharpness, clarity, and distortion.

Spherical Aberration

Spherical aberration occurs when light rays passing through different lens parts converge at various points along the optical axis. This leads to a softening of the image, especially noticeable when using wide apertures.

  • Vintage Lens Example: The Helios 44-2 58mm f/2 is notorious for its spherical aberration at wide apertures, which creates a dreamy, soft-focus effect often prized for portrait photography.
  • Modern Lens Example: Modern lenses like the Canon EF 50mm f/1.2L USM also display spherical aberration at wide apertures but to a lesser degree due to advanced lens designs that better correct this aberration.

Comatic Aberration (Coma)

Coma causes off-axis point sources, such as stars, to appear comet-like or to have a tail, particularly noticeable in the corners of an image.

  • Vintage Lens Example: Older wide-angle lenses like the Nikon 28mm f/2.8 AI-S can exhibit noticeable coma when shooting point light sources at night.
  • Modern Lens Example: The Sigma 14mm f/1.8 DG HSM Art is designed to significantly reduce coma, making it ideal for astrophotography where point light source rendering is critical.

Tangential and Sagittal Astigmatism

This aberration causes point images to stretch into line segments in the tangential and sagittal planes, affecting image sharpness across the field.

  • Vintage Lens Example: The Canon FD 85mm f/1.8 manual focus lens shows astigmatism at wide apertures, especially noticeable in the bokeh rendering.
  • Modern Lens Example: The Zeiss Otus 85mm f/1.4 and Otus 55mmf/1.4, known for its exceptional correction of optical aberrations, handles astigmatism very well, maintaining sharpness and consistency across the frame.

Field Curvature

Field curvature refers to the phenomenon where the plane of best focus is curved, rather than flat, causing the edges of the image to appear out of focus when the center is in focus.

  • Vintage Lens Example: The Nikon 50mm f/1.4 AI-S exhibits field curvature, especially in landscape photography, where edge sharpness is crucial.
  • Modern Lens Example: The Nikon AF-S Nikkor 58mm f/1.4G has been designed to minimize field curvature, improving edge-to-edge sharpness in images.

Learn what causes Field Curvature in photographic lenses.

Distortion

Distortion manifests as either barrel distortion, where lines bow out toward the edges of the image, or pincushion distortion, where lines pinch in.

  • Vintage Lens Example: The Minolta MC Rokkor-PF 58mm f/1.4 shows noticeable barrel distortion, typical for older lens designs.
  • Modern Lens Example: The Canon EF 11-24mm f/4L USM controls distortion exceptionally well, even at the wide end of its zoom range, making it a favorite for architectural and interior photography.

Mitigation in Photography

To address these monochromatic aberrations effectively:

  • Lens Selection and Use: Choose lenses known for minimal aberrations or use techniques like stopping down the aperture to reduce effects like spherical aberration and coma.
  • In-Camera Correction: Many modern cameras offer built-in correction profiles for popular lenses, which can automatically adjust for distortion and some other aberrations.
  • Post-Processing: Software like Adobe Lightroom and Photoshop provides powerful tools for correcting lens aberrations, including detailed controls for adjusting individual parameters like distortion, vignetting, and sharpness.

How Do These Aberrations Manifest in Bokeh?

Optical aberrations not only affect the sharpness and distortion in the in-focus areas of an image but also significantly influence the character and quality of bokeh, the aesthetic quality of the out-of-focus areas. Each type of aberration can uniquely alter the appearance of bokeh, impacting the overall aesthetic appeal of a photograph.

Effect of Spherical Aberration on Bokeh

Spherical aberration often results in bokeh that has a soft, halo-like glow around highlight areas. This can create a dreamy, less defined appearance of the out-of-focus lights.

  • Example: In portraits taken with the Helios 44-2 lens at wide apertures, the bokeh exhibits a very soft, glowing quality that many photographers find appealing for its vintage look.
  • Mitigation: Stopping down the aperture or using lenses with better spherical aberration correction can help achieve a crisper bokeh. Post-processing can also slightly enhance the definition by increasing contrast in the bokeh areas.

Impact of Comatic Aberration on Bokeh

Comatic aberration, or coma, causes bokeh shapes to stretch into tail-like forms, especially noticeable around the edges of the frame. This can lead to bokeh that appears asymmetric and distorted.

  • Example: Wide-angle lenses used in astrophotography, like older versions of the Nikon 28mm, can show comet-like tails on stars towards the edges of the image, disrupting the uniform circular bokeh desired in such images.
  • Mitigation: Using coma-corrected lenses or shooting at narrower apertures where the coma effect is less pronounced can help. Corrective software adjustments are generally limited in addressing coma effectively.

Influence of Astigmatism on Bokeh

Astigmatism causes bokeh to elongate into oval or line shapes instead of rounded circles, affecting the smoothness and uniformity of the background blur.

  • Example: Lenses like the Canon FD 85mm f/1.8 may produce elongated bokeh shapes at wide apertures, particularly noticeable in images with vertical and horizontal structures in the background.
  • Mitigation: Minimizing astigmatism involves selecting lenses known for low astigmatism or adjusting the focus slightly to place critical elements well within the depth of field. Post-processing can blur the background further, softening the impact of astigmatic distortion.

Effects of Field Curvature on Bokeh

Field curvature leads to bokeh inconsistency across an image, with the central area possibly in focus and the edges blurred, or vice versa, creating a wrap-around effect.

  • Example: With the Nikon 50mm f/1.4 AI-S, landscapes focused at the center may have soft, blurred edges even when the central area is sharp, affecting the overall depth feel of the bokeh.
  • Mitigation: Field curvature can be reduced by stopping down the lens to increase depth of field. Choosing lenses with minimal field curvature or using software to adjust the focus plane can also help.

Distortion Effects on Bokeh

Distortion, especially in wide-angle lenses, can alter the shape and symmetry of bokeh, making it appear squeezed or stretched along the image’s edges.

  • Example: The Minolta MC Rokkor-PF 58mm f/1.4 may exhibit noticeable stretching of bokeh shapes toward the image periphery, creating an unnaturally distorted background blur.
  • Mitigation: Opting for lenses with lower distortion or correcting distortion in post-processing can help maintain more natural bokeh shapes across the frame.

Each type of optical aberration has a distinct impact on bokeh, affecting how photographers utilize their tools and techniques to capture images. The choice of lens, aperture setting, and post-processing adjustments are all critical in managing these aberrations to achieve the desired visual outcome in photography.

How does Vignetting Differ from Aberrations?

  • Focus vs. Light Falloff: Unlike aberrations, which directly affect how light is focused and thus how sharp or distorted an image appears, vignetting primarily impacts the evenness of exposure across the image. It doesn’t distort the image but rather affects its brightness and saturation on the edges.
  • Corrections and Controls: Vignetting is often easier to correct than many optical aberrations. It can be significantly reduced or compensated for in-camera (through lens corrections) or in post-processing with software that adjusts the brightness and color uniformity across the image.
  • Discover more about vignetting by clicking here.

VADEMECUM'S QUICK CONTENTS

Index (all topics)
Types of Bokeh (scenarios)
Technical Factors Affecting Bokeh
- Spherical Aberration and Bokeh
Image Processing & Bokeh
- Enhancing & Simulating Bokeh
- Software Limitations
Bokeh Appraisal
Creative Use of Bokeh
Practical Applications
Limitations & Challenges
Lenses and Examples
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