If you spend much time perusing the Lenses section of the B&H website, or follow along with the latest announcements of new glass, you're likely to run into a range of phrases that are not inherently known to those with less than a keen, honed understanding of photographic and optical geekery. Scientific-sounding words like aspherical elements, chromatic aberration, coma, low dispersion, and high refractive index to the layman often lead to imprecise thoughts regarding how a lens performs or what it does to better image quality. But what exactly does an anomalous partial dispersion element do? And why do you not want spherical aberration? This glossary of terms and explanations should help to sort out some of the linguistic and conceptual hurdles faced when learning about a new lens.
In its most basic definition, an aberration is something that deviates from the norm, usually in an unwanted way. In regard to optics, this describes the failure of rays of light passing through a lens to converge at a single point. Aberrations are divided into two categories: chromatic and monochromatic—which are then further subdivided into specific types of each aberration.
One of the most frequently called-out optical anomalies, and one of the main reasons new lens development continues to occur, chromatic aberration describes the way a lens is unable to focus various wavelengths of color at the same point. Think back to your physics classes and recall the color spectrum and how you can use a prism to separate white light into a rainbow; this is essentially what is happening when light reaches your lens and disperses. Well-designed lenses are able to reorganize this light and focus each wavelength at the same point, giving a high degree of color accuracy and registration.
An example of chromatic aberration, where the red, green, and blue wavelengths do not converge at the same point, causing color fringing.
In more practical terms, chromatic aberrations can commonly be recognized as, or called, color fringing. This is most often seen in high–contrast situations where a dark subject is set against a bright background. With a lens unable to correct for chromatic aberration, the subject's edges will acquire a colored haze, often purple, however sometimes a range of other colors that also decreases clarity and apparent sharpness. Rather than your image being of a black subject against a white background, an image infected with chromatic aberration will show that black subject surrounded by a blurred “fringe” of color before the white background. Further refining this concept, chromatic aberrations are typically divided into two sub-genres:
Strong chromatic aberration is present along the edges of the blinds, where red, green, and blue fringing can be seen.
Longitudinal Chromatic Aberration This type of aberration occurs when different colored wavelengths do not converge at the same point, leading to color fringing around subjects throughout the image, from the center to the edges. Longitudinal chromatic aberrations occur most frequently with wider aperture settings, and can be controlled by stopping down your lens.
Lateral Chromatic Aberration This type of aberration occurs when different wavelengths of light (colors) are focused on the same plane, but at different positions, due to the angle of light entering the lens. Lateral chromatic aberrations are only visible at the edges of the frame, rather than in the center, and cannot be corrected by stopping down your lens. Instead, you must rely on post-production or in-camera solutions to alleviate this type of aberration.
Well-corrected, more sophisticated optical designs handle both of these types of aberrations successfully, with lower quality and sometimes more extreme lens designs (fisheyes or ultra fast lenses) being prone to chromatic aberration.
Monochromatic aberration is caused by single wavelengths rather than different (colored) wavelengths. They are dubbed monochromatic because the aberrations occur due to imperfections in a lens’s optical design and are not dependent on color and the focusing of various wavelengths.
Spherical Aberration This type of aberration is caused by light rays entering the lens and not converging at the same point. Spherical lens elements refract rays less when they are entering along the horizontal axis—perpendicular to the film or sensor plane—than rays that enter the lens closer to the periphery. Due to this difference in refraction, light rays that enter the lens parallel do not end up converging at the same point after passing through the optics. In short, the inability to produce this convergence can cause a noticeable decrease in image clarity, sharpness, and resolution.
An example of spherical aberration, where various light rays are not converging at the same point, causing a loss of clarity and sharpness.
Coma (also called a comatic aberration) Coma is an effect that occurs when light rays from point sources pass through a lens at an angle, as opposed to straight on. When a lens design cannot focus these angular light rays at the same point, the point light source will be depicted as a teardrop- or comet-shaped highlight, rather than a circular highlight. Similar to a spherical aberration, coma can be minimized by stopping down your lens.
An example of coma, where the angular light rays result in point light sources being reproduced as a teardrop shape rather than a circular highlight.
The teardrop-shaped highlights in this image are an example of comatic aberration.
Astigmatism One of the more complicated aberrations to describe, astigmatism is similar in concept to coma and is caused by rays entering the lens along the sagittal plane being focused at a different point than rays along the tangential plane. This causes distortion along the edges and in the corners of an image. Astigmatism exists to a degree in all lenses, but is more prominent in instances where the optical design is not completely parallel or symmetric. Like other monochromatic aberrations, the effects of astigmatism can be reduced by stopping down your lens.
Field Curvature Related to astigmatism, field curvature, or curvature of field, is a natural aberration of essentially all lenses, due to their curved structure and how they project light onto the flat sensor or film plane. Since a lens naturally projects light in a curved manner, the edges and corners of an image can appear soft or distorted compared to the sharper central area of an image. To further complicate the issue, some lenses do not project a cleanly shaped field curvature and, rather, have more abstract, wavy field curvature due to the various combinations of different lens elements. Field curvature can be seen using a lens’s MTF chart where dips, curves, or slopes in the lines indicate relative sharpness from the center of a frame to the edges. Stopping down a lens can, again, reduce the effects of field curvature.
An example of strong field curvature, this image shows a sharp central region with strong blurring and distortion in the corners and edges of the frame.
Distortion Finally, distortion is a form of aberration that describes when an image produced by a lens does not retain its rectilinearity. Depending on the type of lens in use, two main forms of distortion occur: barrel distortion and pincushion distortion. The types of distortion they describe are self-referential, where barrel distortion can make straight lines bulge toward the edges of the image (like a wooden barrel) and pincushion distortion renders straight lines with a center-facing bend (like a cushion). Both of these distortions are most commonly seen in zoom lenses, especially near the wider end of their focal length range, but can also show in some prime lenses, especially those with wide or long focal lengths. Wide-angle lenses are most likely to exhibit barrel distortion and telephoto lenses tend to show some form of pincushion distortion. Additionally, a third type of distortion—moustache distortion—is also possible. This is a combination of barrel distortion in the center and pincushion distortion near the edges of an image and is named after the shape of an exaggerated moustache (think Captain Hook).
|Barrel Distortion||Pincushion Distortion||Moustache Distortion|
|This photo shows noticeable barrel distortion with some slight moustache distortion at the edges.||This photo shows pincushion distortion.|
One final optical anomaly, related to spherical aberration, is focus shift. Not technically an aberration, this issue occurs when an image is focused at a lens’s maximum aperture before stopping down to make an image, resulting in a less-than-sharp photo. Common to prime lenses with fast maximum apertures, this problem is the sum of an uncorrected lens being used in its most flawed manner to acquire focus (wide open) and then having its spherical aberration corrected by stopping down despite the fact that the point at which the light rays are converging is no longer the plane of intended focus. In addition to affecting faster lenses, this issue occurs often when working with close-up subjects since missing focus by an inch at a one-foot working distance is more significant than missing focus by an inch on a subject thirty feet away.
Focus shift can happen when using manual or (phase-detection) autofocus since, with both means, focus is being acquired at a lens’s maximum aperture. A handful of ways to correct for focus shift all involve some form of a compromise, which will usually affect how your lens or camera performs in other situations. You can use the AF fine-tuning settings to purposely introduce front or back-focus to compensate for focus shift. You can work with stop-down focusing or contrast-detection focusing, since those methods can operate with lenses at less than maximum aperture. Finally, you can simply rely on depth of field to compensate for minor amounts of focus shift at close range.
Now that we've covered a good array of aberrations you're bound to experience in some way, let's take a look at some of the ways these aberrations are corrected or minimized.
One of the most common types of specialized elements highlighted when describing a lens’s traits is an aspherical element. It is exactly as it sounds: a lens element that is ground, molded, or otherwise formed into to a shape that is not entirely spherical. As mentioned above with spherical aberrations, a simple spherical element is not capable of refracting light rays into one converging point due to its curved shape. An aspherical element, on the other hand, can more efficiently focus rays entering from the edges and corners to lessen the amount of spherical aberration, coma, and astigmatism. Aspherical elements tend to be more beneficial in wider focal lengths, although they are present in some longer, telephoto lenses. Additionally, aspherical elements are added to a lens design in order to replace multiple spherical elements, thereby reducing weight and complexity in the composition of the lens.
|Spherical Lens||Aspherical Lens|
Often presented using varying degrees of intensity, such as extra-low, ultra-low, or super-low dispersion, as well as anomalous dispersion or anomalous partial dispersion, low dispersion glass, in short, is used to reduce or control the effects of chromatic aberration. As aspherical elements are typically used to correct monochromatic aberrations, various types of low dispersion glass are used to combat both longitudinal and lateral chromatic aberrations. This special type of glass ensures that various colored light rays are refracted in the same manner to achieve proper convergence and registration of each, resulting in images void of color fringing. Just as how aspherical elements are more common in wide to normal focal length lenses, low dispersion glass is more frequently used in longer focal lengths and telephoto lens designs.
This apochromatic design shows red, green, and blue wavelengths converging at the same point, resulting in neutral color balance that is void of fringing.
A special type of low–dispersion element, these elements, commonly found in telephoto lenses, are comprised of a naturally-occurring, although now synthetically produced, type of crystal that has notably low dispersion and a low refractive index. Outperforming other types of low dispersion glass, fluorite elements are used to significantly reduce chromatic aberrations and are also lighter in weight than their glass counterparts. The drawback to fluorite is that it is a costlier and lengthier process to produce this material compared to other types of low dispersion glass and, as such, is reserved for more exotic and complex lens designs.
Fluorite, shown in its various forms, is used to reduce chromatic aberrations.
Relating to the above topics, an apochromatic lens, also called an apochromat or signified by the inclusion of Apo in the lens name, is a highly corrected lens design that should produce imagery with fewer chromatic and spherical aberrations than other modern lenses, specifically called achromats (or achromatic lenses). All modern lenses fall into one of these two loosely-defined categories, although historically there are also simple lenses as a third category. Where achromats are designed to bring red and blue wavelengths into focus on the same plane, to reduce most common types of color fringing, apochromats further scrutinize on color accuracy by also bringing green-colored wavelengths into focus at the same point to further minimize chromatic aberrations. Additionally, apochromats are more adept at reducing spherical aberrations than achromats.
Diffractive Optics and Phase Fresnel Elements
Relatively new to the optics world, and only seen in a handful of telephoto lenses from a couple prominent manufacturers, Diffractive Optics and Phase Fresnel elements are other means to reduce chromatic aberrations. Rather than relying on special types of glass to affect how different wavelengths of light refract, these elements physically adjust the path of light prior to being focused. The elements themselves are composed of small concentric circles (à la Fresnel) and are paired with a common refractive element in the lens design. This marriage then essentially defocuses and, more accurately, refocuses different–colored wavelengths onto the same point than other optical combinations. And it does so in a manner that avoids several glass elements, thereby reducing weight from the overall design. The drawback to Diffractive Optics and Phase Fresnel elements is that they can contribute to more intense flare when photographing strong point light sources, which can only be corrected during post production.
A Diffractive Optics/Phase Fresnel design, this type of element resembles a Fresnel lens and helps to control aberrations and replaces the need to use several glass elements.
High Refractive Index Element
Another type of element whose job is to replace several “normal” glass elements to lessen overall weight, high refractive index glass is used to correct for field curvature and other monochromatic aberrations for increased clarity and sharpness.
Specifically designed to improve image quality at closer focusing distances, a floating element, or floating elements system, is a single element or group of optics that adjusts its position during focusing to offer consistent performance throughout the focusing range. Many of the aforementioned corrective elements and techniques are implemented to reduce aberrations when focused at infinity and are no longer as useful when focusing on subjects at close range. By implementing an element or a group of elements that shifts its position during focusing, the effects of the corrective elements are maintained.
This floating elements design shows how only the rear groups are moved to adjust focus to offer consistent performance throughout the focus range.