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What are Lens Aberrations?
- Introduction to Aberration Theory
- Spherical Aberration
- Comatic Aberration, aka Coma
- Chromatic Aberration
- Distortion of the Image
- Curvature of the Field of the Image
1. Introduction to Aberration Theory
Whenever the topic of lens characteristics is brought up, the word “aberrations” is never far behind. “This is an amazing lens with practically all aberrations removed.” You’re quite likely to encounter a statement like that in a variety of lens reviews and discussions. Somewhat rarer you might also run into someone saying, “That’s a wonderful lens; its residual aberrations are well defined and result in an image with incredible plasticity and beauty.”
So why the difference in opinion? I’ll try to answer that question while touching on why this particular phenomenon is good or bad for lenses and various genres of photography in general. To start off though, let’s figure out what exactly photographic lens aberrations are. We’ll begin with the theory behind them and a number of basic definitions.
In the general sense, the term aberration (from Latin “aberrare” meaning “to wonder”) is defined as a deviation from the norm, an error, a certain divergence from normal performance.
A lens aberration is an optical design image error. It’s caused by the fact that in practice the lens medium can cause substantial deviation of light rays from the direction they are intended to travel in the lens’ theoretical, ideal optical design.
The end result is that aberrations harm the generally accepted quality of photos with afflictions such as lack of sharpness in the center, loss of contrast in general, strong lack of sharpness along the sides, warped geometry and displayed space, color aureoles, etc.
The primary photographic lens aberration types are as follows:
1. Spherical Aberration
2. Comatic Aberration
3. Chromatic Aberration
4. Distortion of the Image
6. Curvature of the Field of the Image
Before taking a closer look at each, let’s think back to the Spherical Lens Types article and recall how light is supposed to pass through a lens in an ideal optical design:
Pic. 1. Passage of light in an ideal optical design.
As we can see from the illustration, all the light rays gather in a singular point F, the primary focus. However, in practice, everything plays out in a far more complicated manner. The essence of optical aberrations lies in the fact that while light rays fall onto a lens from a single light source point, they don’t all gather at one point afterwards. Let’s take a look at the kind of deviations that occur in an optical design given various aberrations.
It’s also worth noting that both simple and complex lenses suffer from all the further depicted aberrations at the same time.
2. Spherical Aberration
Spherical aberration occurs when light rays that fall onto the edges of a lens gather closer to the lens than the rays that fall onto its center. This results in the surface point appearing as a blurry circle or disk.
Pic. 2. Spherical Aberration.
Spherical aberration manifests on photos in the form of a softened image. This effect is especially noticeable on open apertures. Lenses with high lens speeds are particularly prone to suffering from this type of aberration. However, assuming that outline contrast remains under these conditions, this sort of soft effect can be quite useful for certain kinds of photo shoots, i.e. portrait photos.
Pic. 3. Soft effect achieved via an open aperture with spherical aberration.
It’s practically impossible to remove this type of aberrations from a lens consisting entirely of spherical lenses. When it comes to super speed lenses, the only effective way to substantially compensate this type of aberration is by using aspherical elements in the optical design.
3. Comatic Aberration, aka Coma
The coma is a type of spherical aberration that affects off-axis rays. It causes rays that hit the lens at an angle to the optical axis to not gather in a single point. This results in a point of light at the edge of the image that looks like the tail of a flying comet rather than a simple spot. The coma effect can also lead to overexposure for parts of the image that lack sharpness.
Pic. 4. Coma.
Pic. 5. Coma as seen on an actual photo.
4. Chromatic Aberration
Chromatic aberration is a direct consequence of dispersed light. In essence, it occurs when a ray of white light passes through a lens and disperses into its color component rays. Shortwave rays (blue, violet) suffer stronger refraction and unite closer to the lens itself as compared to longwave rays (orange, red).
Pic. 6. Chromatic aberration. F1 being the focus of the violet rays, F2 being the focus of the red rays.
In this case, just as in the case of spherical aberration, the glowing surface spot on the resulting image appears in the form of a blurred circle/disk.
Chromatic aberration manifests on photos in the form of foreign shades and color outlines around various objects in the shot. The effects of aberration are especially noticeable in photos with contrasted subjects. Today it’s simple enough to fix up XA in RAW converters so long as the original shooting was done in RAW format.
Pic. 7. Example of chromatic aberration.
5. Distortion of the Image
Distortion manifests in the warping and distortion of photo geometry. In other words, the image’s scale changes moving from the center of the field to the edges, leading to straight lines curving either towards the center or towards the edges.
The two primary types of distortion are barrel distortion (mostly characteristic of wide angles) and pincushion distortion (most often manifesting on a long focus).
Pic. 8. Pincushion and barrel distortion types.
Distortion is usually substantially stronger for lenses with dynamic focal range (zoom lenses) as opposed to fixed focal length lenses. Some lenses, fish eye lenses for example, intentionally retain distortion effects or even accent them.
Pic. 9. Strong barrel distortion achieved via a Zenitar 16mm FishEye lens.
Modern lenses, especially dynamic focal distance lenses, allow for relatively efficient distortion correction by means of implementing an aspherical lens (or several) into the lens’ optical design.
Astigmatism (from Greek “Stigma”) manifests in the absence of a glowing spot at the edges of the field, either in the form of a spot or even in the form of a disk. Furthermore, a glowing spot located on the primary optical axis is actually transferred as a spot, yet if the spot is off-axis, it ends up transferred as a darkened area, a group of crossed lines, etc.
This effect is most often present along the edges of an image.
Pic. 10. Manifestation of astigmatism.
7. Curvature of the Field of the Image
Curvature of field is an aberration that results in an image of a flat object positioned perpendicular to the lens’ optical axis lies upon a surface that is either concave or convex relative to the lens. This aberration causes uneven image field sharpness. When the central part of the image is sharply focused, its edges will lie out of focus and will appear to lack sharpness. If the sharpness is set along the edges of the image, then its center will end up lacking sharpness.
Pic. 11. Image field curvature.
Curvature of field manifests in the form of lowered resolution, lack of sharpness, and image curvature or radial swirling.
Should the center of the image be in focus, then its edges are out of focus, and vice-versa.
Field curvature cannot be removed by aperturing the lens. This type of aberration can only be corrected by altering the forms of the separate elements within the lens, i.e. altering their thickness and the distance between them, changing aperture position, and implementing aspherical elements. Correcting this aberration generally requires the lens to contain no less than two dispersal lenses.
Pic. 12. Example of strong field curvature achieved via an SLR Magic 35/1.7 lens.
That’s about it. We’ve gone over all the primary optical aberrations and their manifestation on photos. In my next article I’ll try to answer a rather complicated by vital question: Lens Aberrations in Photography: Good or Bad?
(с) 2011 Sergey Borodin. Pictures by author.