Optics 101: Understanding Photographic Lens Aberrations
No optical system is perfect. Every lens—whether a modern high-tech giant or a cherished vintage classic—is subject to the laws of physics. As light waves pass through glass elements, they are refracted. This causes deviations between reality and the projected image on the camera sensor: these are known as lens aberrations (optical aberrations).
A fundamental distinction is made between monochromatic aberrations (which occur even with single-color light) and chromatic aberrations (color defects caused by the varying refraction of different wavelengths of light).
Here is a detailed overview of the most important phenomena:
1. Spherical Aberration
Spherical aberration is caused by the geometry of the lenses. Classic lens elements possess a spherical surface because it is the easiest shape to grind during manufacturing.
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The Cause: Light rays passing through the outer marginal zones of a spherical lens are refracted more strongly than rays arriving close to the optical axis (through the center). Consequently, they intersect the optical axis at different focal points.
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Visual Impact on the Image: The image loses contrast and peak sharpness. A characteristic soft veil drapes over the subject—often referred to as a "glow" or a "soft-focus effect." This effect is particularly visible wide open on older, fast lenses.
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The Remedy: Stopping down! Since closing the aperture blocks the aberrant marginal rays, sharpness increases rapidly. In modern lens design, this error is corrected by using aspherical lenses (aspheres), whose surfaces flatten out toward the edges.
2a. Axial / Longitudinal Chromatic Aberration (LoCA)
Axial or longitudinal chromatic aberration (often referred to as LoCA or "bokeh fringing") occurs along the optical axis—meaning in the depth of the image space.
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The Physical Cause: When white light passes through the lens parallel to the optical axis, the different color wavelengths are refracted at different angles. As a result, each color comes into focus at a different position one behind the other along the optical axis. Blue light is refracted the most and focuses closer to the lens, while red light is refracted less and focuses further back.
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Visual Impact on the Image: Because the colors focus at different depths, the camera sensor can logically only align with a single wavelength (color) at the perfect focal plane. The other colors are already out of focus at this point, creating blur circles. This artifact is visible across the entire image field (including the very center of the frame), primarily appearing in highly contrasting out-of-focus areas (bokeh):
Objects located in front of the plane of focus (foreground) typically display unsightly magenta or purple fringes.
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Objects located behind the plane of focus (background) exhibit green or cyan fringes (e.g., specular highlights on water or tree branches against a bright sky).
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Correction in Practice: Stopping down helps immensely here! Closing the aperture narrows the light cone and increases the depth of field. This effectively masks the misfocus of the individual colors, causing the color fringes to almost completely disappear. Optically, this aberration can only be corrected by using expensive specialized glass (such as fluorite or UD/Ultra-low Dispersion elements) in apochromatic (APO) lenses.
2b. Lateral / Transverse Chromatic Aberration (LaCA)
Lateral or transverse chromatic aberration (commonly called lateral CA or lateral color) occurs perpendicular to the optical axis—meaning across the width and height of the final image.
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The Physical Cause: This aberration exclusively affects light rays that enter the lens at an angle relative to the optical axis. Due to the varying refraction of colors, there is no shift in depth here; instead, it causes a difference in magnification scale. This means the lens projects a slightly different-sized image for each color onto the camera sensor. For instance, the blue image component might be fractionally smaller or larger than the red one.
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Visual Impact on the Image: Since all light rays enter perpendicularly and parallel at the exact center of the frame, the center of the image is completely free of this defect. The color misalignment only drifts apart toward the edges and corners of the image. Along high-contrast edges in the corners (such as the silhouette of a building against the sky), you will see very sharp, well-defined color fringes. These usually appear in pairs, typically as red/cyan or blue/yellow. The tricky part: because this is a pure magnification error, these color fringes are tack-sharp even within the perfect plane of focus. It is not a blur issue.
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Correction in Practice: Stopping down does absolutely nothing here! Because closing the aperture does not alter the magnification scale of the different colors, lateral CA remains just as pronounced at f/8 or f/11 as it is wide open. The good news for modern photographers: since the error is purely geometric and mathematically predictable, it can be corrected almost 100% perfectly and seamlessly in digital post-processing (e.g., in Lightroom with a single click on "Remove Chromatic Aberration") or directly in-camera.
3. Coma (often called comatic aberration)
Coma is an aberration that exclusively affects light rays entering the lens obliquely (at an angle) relative to the optical axis. Therefore, it appears primarily in the corners of the image.
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The Cause: When a bundle of light hits a lens obliquely, the marginal rays are refracted differently than the central rays. The various zones of the lens generate images of differing sizes.
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Visual Impact on the Image: Point light sources (such as stars in astrophotography or streetlights at night) in the corners of the frame are not rendered as sharp points. They distort asymmetrically and drag a tail behind them—looking like tiny comets (hence the name "coma") or small swallows.
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The Remedy: Stopping down effectively reduces coma. For astrophotographers, excellent coma correction when shooting wide open is the most critical quality criterion for a wide-angle lens.
4. Astigmatism
Astigmatism is closely related to coma but describes a different geometric problem involving oblique light rays entering the marginal areas of the lens.
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The Cause: An oblique bundle of incoming light strikes the lens in two different planes: the meridional (tangential) plane and the sagittal plane. Due to the angle, the lens optically possesses a different curvature in these two directions. This causes the light to focus not into a single point, but into two focal lines perpendicular to each other.
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Visual Impact on the Image: Points in the corners of the image are distorted into lines or ovals, either in a radial direction (pointing toward the center of the image) or in a tangential direction (concentric around the center). The image loses a massive amount of resolution in the corners.
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The Remedy: Stopping down reduces the length of the focal lines, which improves sharpness. Astigmatism can only be fully corrected through the purposeful use of lens groups with opposing defects (known as anastigmats).
For photographers, the theoretical representation using focal lines in space is often difficult to grasp. In practice, astigmatism is most noticeable when photographing test charts (e.g., Siemens stars or concentric circles) or fine textures like tree branches and brick walls.
Astigmatism causes the lens to render radial structures (structures pointing toward the center of the image) on a different focal plane than tangential structures (structures running perpendicular to them).
5. Field Curvature
In theory, a lens is supposed to project a flat reality onto a flat camera sensor (or film stock). Field curvature prevents exactly that.
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The Cause: The inherent nature of a simple lens dictates that the image plane is not a flat surface, but rather a curved shell (often bowl-shaped).
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Visual Impact on the Image: If you focus on the center of the frame and it is tack-sharp, the sharpness visibly drops off toward the edges and corners because the focal plane there lies slightly in front of or behind the sensor. Conversely, if you focus on the corners, the center becomes blurry. This is particularly troublesome in copy-stand photography (documents) or landscape photography.
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The Remedy: Heavy stopping down increases the depth of field enough so that the blur caused by the curvature disappears within the sharp area of rendition.
6. Distortion
Unlike the other aberrations, distortion does not affect image sharpness, but purely the geometric shape of the image.
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The Cause: Distortion occurs when the magnification scale is not constant across the image field. If the magnification scale decreases or increases toward the edges, the image geometry becomes distorted.
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Visual Impact on the Image: Straight lines (such as building walls, horizons, or architecture) are rendered curved. Three types are distinguished:
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Barrel Distortion: Lines bend outward (typical for wide-angle lenses).
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Pincushion Distortion: Lines bend inward (typical for telephoto lenses).
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Wave Distortion ("Mustache Distortion"): A mixture of both—barrel distortion in the center, turning into pincushion distortion toward the edges (extremely complex to correct).
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The Remedy: Stopping down does absolutely nothing here! Because it is a pure geometric error, distortion remains identical at every aperture setting. However, it can be excellently corrected digitally via lens profiles in post-processing (Lightroom, etc.) or directly in-camera.