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What Is an Aberration in Photography?
An aberration is an imperfection in the way a lens focuses the light it captures. Aberrations occur because the rays of light passing through a lens fail to converge at a single point.
There are multiple types of aberrations and they can affect sharpness, focus, magnification, distortion, and color in images. As a result, they prevent the accurate reproduction of images.
Aberrations are more prevalent in astrophotography than in other fields of photography due to the tiny sources of light contrasted against the dark background that is space. Aberrations often distort the shape of stars, especially towards the edges of the frame.
Is It Possible to Design a Photographic Lens With No Aberrations?
It is impossible to design a lens with no aberrations. Correcting one type of aberration can create or worsen another, requiring the addition of more lens elements that only complicate matters.
- The movement of zoom lens elements compounds aberration issues, meaning most zooms have clearly detectable aberrations.
- Even the best prime lenses have problems. The Zeiss 21mm f/2.8, an example of wide angle lenses with extremely sharp focus, has significant mustache distortion. The classic Nikon 28 f/1.4 has significant coma. The Canon 50 f/1.2 has field curvature. (More on these aberration types below.)
- Good photographers know the weaknesses of their lenses and use them accordingly. For example, the Sigma 50 f/1.4 is best when shot wide open or at f/2.8 or higher, so an educated photographer will won’t shoot lower.
2 Different Types of Chromatic Aberrations
Chromatic aberrations (also known as “color fringing”) are the resultant imperfection when a lens cannot focus various wavelengths of color at the same point. The dispersion of different colors of light is similar to how a prism separates white light into a rainbow.
Designers have corrected for chromatic aberrations in many lenses, so they focus each wavelength at the same point, giving a high degree of color accuracy and registration.
Chromatic aberrations still occur in faster lenses and those capturing high-contrast areas, such as a dark subject set against a bright background. A colored haze—often purple, but sometimes other colors—appears on a subject’s edges, decreasing clarity and sharpness. As a result, shooting a black subject against a white background creates an image where the subject has a blurred color fringe that separates it from the background.
Chromatic aberrations fall into two subtypes:
- Longitudinal chromatic aberration. “LoCA” or “bokeh fringing” occurs when different wavelengths of color do not converge at the same point after passing through a lens, leading to color fringing around subjects throughout the entire image, from the center to the edges. Longitudinal chromatic aberrations are most common with fast lenses with wider aperture settings. Slower lenses are less prone to LoCA. Photographers correct for this by stopping down the lens.
- Lateral chromatic aberration. “Transverse chromatic aberration” occurs when, due to the angle of light entering the lens, different wavelengths of color focus on the same plane, but at different positions. Lateral chromatic aberrations are visible only at the edges of the frame. Stopping down a lens does not correct this, so photographers rely on post-production or in-camera solutions to alleviate this type of aberration.
5 Different Types of Monochromatic Aberration
Monochromatic aberrations are the resultant imperfection when lenses can’t focus a single color of light. The aberration is due to imperfections in a lens’s optical system.
German mathematician Philipp Ludwig von Seidel identified and described the five monochromatic aberrations—now known as the five Seidel Aberrations—by 1857. In the subsequent 150-plus years, designers have not been able to completely correct for these aberrations.
The subtypes of monochromatic aberration are:
- Spherical aberration: Spherical glass elements in the lens cause light to converge at different places on the sensor. The lens will refract light that enters near the edge more than light that enters near the center. In addition to lens design, the quality of a lens’s glass material and placement of elements within the lens housing result in the aberration, reducing focal strength, image resolution, and clarity. Stopping down the aperture reduces this aberration. Alternatively, aspherical elements in wider focal lengths and telephoto lenses can more efficiently focus rays entering from the edges and corners to lessen the amount of spherical aberration (as well as coma and astigmatism).
- Comatic aberration: This aberration occurs when a single point of light enters a lens at an angle at its edge, rather than straight on at the center of the lens. Unable to focus angular light rays at the same point, point light sources flare out from the point, creating a comet-shaped highlight, rather than a circular one. External coma aberrations occur when the comet “tails” point away from the center of the image—the opposite are internal coma aberrations. Comatic aberrations are more obvious at the edges of frames in images captured with wide apertures. To help combat this problem, photographers raise their f/stop, as they do with spherical aberrations.
- Tangential and sagittal astigmatism: Astigmatism results from rays entering the lens along the sagittal plane and being focused at different points than rays along the tangential plane. This causes distortion along the edges and in the corners of an image, with light sources at the edges of the frame stretching in a line shape, either running from top-right to bottom-left, or vice versa (tangential), or from top-left to bottom-right (sagittal). All lenses have some degree of astigmatism, but it’s more prominent when the optical design is not completely parallel or symmetric. These aberrations are some of the most common in astrophotography. As with most off-axis aberrations, raising the f/stop helps to minimize these imperfections.
- Field curvature: Also known as “Petzval field curvature,” for mathematician Joseph Petzval’s formula, this imperfection occurs when the lens focuses light onto an imaginary curved surface rather than a flat plane. This results in focus issues across an entire image: The center appears to be in focus, while the edges aren’t. All lenses have a curved structure and thus do not project their image onto a flat sensor, but rather form the image on a curved surface, making field curvature a natural aberration. However, this aberration is more common in older lenses than it is in modern lenses. Stopping down a lens can reduce the effects of field curvature. (Some photographers use novelty lenses to intentionally amplify this effect. Excessive vignetting and field curvature create a cool “swirly bokeh” effect.)
- Distortion: Occurring when the lens projects a wider scene across a sensor or film plane, the image fails to retain its rectilinearity. Distortion, which can happen vertically or horizontally, is most noticeable when trying to capture straight lines—bending and curving lines make the scene look unrealistic. “Barrel distortion” is when the captured scene looks smaller at the edges of the image than in the center. “Pincushion distortion” is when the scene at the edges of the frame look bigger than the center. Both distortions are commonly seen in zoom lenses, especially near the wider end of their focal length range, but prime lenses with wide or long focal lengths can also show them. A “mustache distortion” is when a lens shows both types and lines appear wavy due to central and edge distortions. Changing the position of lens elements when zooming or focusing in relation to the stop may increase or decrease the amount of distortion in that lens.
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