The goal of this article
is to provide an overview of some of the most commonly
encountered optical distortions. Distortion is often mentioned — either
as being present to some degree, or absent — when
compact digital cameras are considered, because of the
very short focal lengths of their lenses. We have no intention
of getting into any technical detail here. Optics is a
highly complex field, and not our area of expertise. The
aim here is simply to explain the most important types
of distortion as they relate to compact digital cameras,
and the methods employed to correct them and minimize their
effect on images.
Focusing light, and the distortions that the
process can induce was recognized and understood long before
the development of photography. The need to focus the light
from lighthouses for example, caused Augustin Fresnel to
create an optical surface — one that still bears his
name to this day, the Fresnel lens — to focus
light over a long distance. Just one of many lens types invented,
and part of the history of optical science.
The Fresnel lens of an old lighthouse in the
Halifax Maritime museum.
In the context of photography, references to optical distortion
apply to specific, and undesirable, anomalies that occur
in the process of focusing light onto a surface. Over the
years, complex lens designs and optical formulas have been
devised to eliminate, or minimize the anomalies.
Distortions occur as light passes through the different glass elements
that make up a lens and which focus the image. The function of the lens
is to capture all light rays, at all the visible wavelengths, and to
precisely place them onto a small flat area while maintaining a sharp
image. Since light travels on a straight path, the optical elements of
the lens must bend, and correct all the light reflected from a scene.
For instance, a subject measuring 3 meters across must be sharply focused
on a surface less than 2 centimetres wide in the camera.
As light travels through any
of the elements of a lens, as it enters and exits each element,
the curved surfaces change its angle, bending it as required.
To allow the light to pass through unimpeded, not only must the glass be
completely free of any impurities which might disperse it, it also has
to minimize diffraction which might cause a loss of focus as it reaches
the image plane. 1. Converging lens. 2. transmitted
light. 3. Focal point. 4. Image plane.
With zoom lenses, coatings and the complex geometry of
each element are in large part responsible for the cost
associated with the construction of the lens. Zoom lenses
must control and focus the various wavelengths of light,
limit dispersion through the use of specially formulated
glass (ED glass), while simultaneously correcting most
of the distortions that are induced by the various possible
focal lengths of the zoom.
Certain types of corrections though, are more difficult to obtain than
others, and it happens that lenses that are otherwise excellent, suffer
from slight distortions at the periphery of the image plane, where light
arrives at a more pronounced angle than at the centre of the image plane.
With compact digital cameras, the small size of the sensor requires the
use of much shorter focal lengths than 35mm cameras. This in turn makes
the needed corrections even more complicated.
Barrel distortion
One of the most commonly observed types of distortion with a wide angle
is barrel distortion. As the name implies, the image is distorted
into a barrel shape, curving outwards at the edges.
While barrel distortion is
a desirable feature for one type of lens - called a fisheye,
which allows for creative images - with most wide angle lenses,
the lens should form an image that looks as natural as possible.
To this end, barrel distortion is corrected as much as possible.
To control barrel distortion, aspherical surfaces are used on some of the
elements of the lens. Aspherical surfaces provide a way to apply a gradual
correction that progressively increases towards the periphery of the lens,
and which straightens out the image.
The barrel distortion shown here has been exaggerated to illustrate the
effect.
Pincushion distortion
The pincushion distortion shown here has been exaggerated to illustrate
the effect.
If barrel distortion can
affect the wide angle end of a lens, pincushion distortion
is its equivalent at the telephoto end.
Pincushion distortion can show up in zoom lenses that have a broad range
of focal lengths. For example a 10X zoom that has a focal length the
equivalent of a 40mm to a 400 mm.
Pincushion distortion causes the outer parts of the image to curve inwards,
towards the centre.
Astigmatism
Astigmatism is a not a distortion
per se, but a phenomenon that causes the outer corners of an
image to lose sharpness.
Generally, it is caused by
an irregularity, or an inappropriate curvature in the outer
part of the lens, causing a loss of focus when the light
reaches the peripheral parts of the image plane.
This anomaly can be sometimes seen when a zoom is at its widest angle,
a point at which the greatest surface area of the lens is used, and when
the proximity of the combined optical elements can engender a loss of sharpness
on the peripheral part of the image. It is also a phenomenon that can be
seen with some simple fixed-focus lenses.
Chromatic aberrations
In the area of phenomena that can cause a loss of image quality, chromatic
aberrations are variations that are not caused by the geometric qualities
of the lens elements, but by the way they let light pass through the
glass. The optical surfaces of optical glass are usually treated with
special coatings that control the way light is transmitted.
As each colour component of
light has a specific wavelength, the coatings serve to control
them so they arrive at the same point on the focal plane.
In some cases however, this control isn't perfect. Then, all the colour
components of light are not precisely focused where they should be. The
result is a purple, or violet fringe on the edges of image elements.
In these cases, the aberration is most visible in the periphery of the
frame where the incidence of the light rays is the most pronounced, and
at light/dark boundaries within the image.
Vignetting
Vignetting is a loss of brightness in the corners of the image. In this
case the source of the problem is most commonly an external element to
the lens, due to something blocking some of the light from entering the
lens.
The problem can be caused
by a poorly designed lens barrel or lens trim, but is most
commonly the result of an inappropriate lens hood, or the
use of a filter not large enough to cover the focal range
of the lens.
Many manufacturers avoid this problem by engineering hoods specific to
the zoom (perfect lens hoods) which have a "flower" shape
that offers protection for the lens from sunlight, but which is adapted
to all the lens' focal lengths.
We have covered here those phenomena that are the most frequently mentioned
in our reviews. Quite obviously, correcting any of these demands high
quality lenses, which adds to the price of a camera. Aspherical surfaces
or ED glass for instance, entail a greater cost when lenses are designed
and built. Yet, it is a necessary cost as it directly impacts the quality
of the image, and needs to be valued as much as the sensor itself.
