Optical aberrations are imperfections in the way lenses converge rays of light to a point. Lens aberrations can be divided into two types: imperfect converging of light to a focused point (which affects sharpness) and flawed geometric projection of the scene (which manifest as distortions or warping). When designing a lens, an optical engineer is faced with making a series of complex compromises to limit aberrations. Since there’s no such thing as a perfect lens, engineers must make the best possible compromises given the restrictions of the intended use, features, production costs, and marketable price. While there’s little you can do about aberrations in your lenses (except for stopping down the aperture), it’s helpful to know what they are when setting out to make a new purchase. More importantly, as a photographer, you should be aware of the limitations of your lenses; this allows you to exploit their strengths and avoid emphasizing their weaknesses.
Types of lens aberrations
Aberrations are the primary cause of reduced acuity in lenses that are correctly focused. There are five types you should know about, chromatic aberration, spherical aberration, curvature of field, coma, and astigmatism.
Chromatic aberrations manifest themselves as fringes of colour on high-contrast edges. They are caused by different wavelengths of light (i.e., different colours) undergoing varying degrees of refraction and being focused at different positions as they pass through the lens. Two types of chromatic aberration exist: longitudinal and lateral. Longitudinal aberrations are caused by different wavelengths of light focusing to varying distances from the lens. Blue-violet light focuses closer to the lens than red, with green coming into focus between them. Longitudinal aberrations can be minimized by stopping down your aperture, which brings the wavelengths into acceptable focus. Lateral aberrations occur when different wavelengths of light focus on different positions of the focal plane (i.e., on the image sensor). These typically happen with short-focus, and especially, ultra-wide angle lenses. Lateral aberrations cannot be diminished by stopping down the aperture, but are effectively minimized by some photo-editing software.
Spherical aberrations cause soft-focused images that lack fine contrast. They occur when light passing through the edges of a lens focuses closer to the lens than light passing through its centre. In most lenses, the presence of spherical aberration is an undesirable technical flaw; however, in the past, deliberate and controlled spherical aberration was implemented for use in “soft-focus” portrait lenses. Spherical aberration can be reduced by stopping down the aperture.
Curvature of field occurs when a lens cannot focus a flat subject normal to its optical axis onto a flat image plane. To a certain degree, this aberration isn’t problematic for portrait, landscape, and street photography; however, it’s highly undesirable in fields apt to feature prominent flat planes, such as architectural, technical, and macro photography. The effects of curvature of field can be reduced by stopping down the aperture.
Coma describes the reduced ability of a lens to render a sharp point image that originates away from the lens axis. As the name implies, the rendition of such a point source is characterized by a shape similar to a comet’s tail. Coma may be minimized by stopping down the aperture.
In photography, astigmatism causes a subject point originating away from the lens axis to render as a highly stretched oval at one focus distance, as a highly stretched oval perpendicular to the first at another focus distance, and as a blurry disc in between. There are two types, tangential and sagittal astigmatism. In tangential astigmatism, the elongation of the subject points occurs along an imaginary line radiating from the optical axis, whereas in sagittal astigmatism, the elongation is normal to this, appearing to follow imaginary rings circling the optical axis. Much like most of the other types of aberration, astigmatism may be reduced by stopping down the aperture.
Linear distortions are deviations from an ideal rectilinear projection. Recall that rectilinear lenses are designed to render the straight elements found in a scene as straight lines in the image. There are three types of distortions: barrel (convex), pincushion (concave), and complex.
Unlike the aberrations above, distortions do not generally affect image sharpness; furthermore, distortions cannot be minimized by stopping down a lens. Despite this, barrel and pincushion distortions are efficiently corrected using software, and some cameras automatically apply the appropriate corrections when taking a picture. Correcting complex distortions is also possible, but requires a correction profile that knows the exact structure of the warped geometry.
Barrel and pincushion distortion are often associated with specific focal lengths. For instance, short-focus lenses tend to exhibit barrel distortion, while long-focus lenses are more susceptible to pincushion distortion. Zoom lenses are commonly afflicted by both, showing barrel distortion on their wide end and transitioning to pincushion distortion on the long end of the zoom range. This behaviour is present regardless of the absolute focal length of the zoom lens. For example, both 16–35 mm and 70–200 mm lenses will show barrel distortion at 16 mm and 70 mm, respectively, and pincushion distortion at 35 mm and 200 mm, respectively.
Despite the presence of optical distortions in many lenses, the effects are hard to notice in most photographs. Images where optical distortions are most evident often feature straight lines running parallel and close to the edges of the frame.