33.15 - Lens aberrations
Lens aberration:
Imperfection in the images
formed by a lens.
Spherical aberration: Image
blurring due to the spherical
contour of the lens.
Chromatic aberration: Image
blurring due to differing
refraction of various
wavelengths (colors) of light.
You may have noticed that even when an image created by a lens is focused as sharply
as possible, it is still somewhat blurry. The blurriness results from lens aberrations.
Blurring means that the light rays from a single point on an object do not precisely
converge at one point in the image, as the examples to the right illustrate, using
horizontal rays from a point on a very distant object. A well-crafted lens causes little
blur; lower quality lenses can suffer from substantial blurriness.
There are several types of aberration. Some aberrations may be caused by
imperfections in the manufacturing of the lens, but one type arises unavoidably from the
nature of a spherical lens. The lack of focus arising from spherical aberration most
affects rays that strike a lens near its periphery. Spherical aberration means the light
rays refract at an undesired angle; rather than converging to a single point, they
converge to a larger region. The spherical nature of the lens causes this problem.
Instead of a single point of focus for incoming parallel rays, the lens creates a circle of
least confusion, a region in which the rays approximately converge and an image is
most satisfactorily viewed. This is diagrammed in Concept 1.
A typical image created by a spherical lens is shown above. The paraxial rays focus
sharply in the center of the real image. Nonparaxial rays, having already passed
through their closer focal points, are beginning to spread out again. Their contribution to
the image shows up as a set of concentric rings. The outermost ring corresponds to
rays passing through the outermost portions of the lens which have the shortest focal
lengths.
Spherical aberration can be counteracted in a variety of ways. For instance,
photographers are aware of this phenomenon, and when precise focus is important,
they “stop down” the aperture of the iris that lets light into the camera lens, restricting
the remaining light to the center portion of the lens. Mirrors are also subject to spherical
aberration: In large astronomical telescopes, the problem is avoided by using parabolic
or other non-spherical reflectors.
Chromatic aberration occurs because the refractive index of a material is a function of the wavelength of light. For instance, since blue light
refracts more than red light when it passes from air into glass, a ray of white light disperses into its component colors when refracted by glass.
This notably occurs in prisms, although there it is often the desired effect. You see a diagram illustrating chromatic aberration in Concept 2.
The photo below provides an example of chromatic aberration. There is a noticeable purple halo around the circular windows on the left side of
the image. The right side of the image is actually the center of a larger cropped image. The chromatic aberration is less noticeable in this area,
which corresponds to the center of the lens. Chromatic aberration worsens toward the edge of the lens.
Spherical aberration.Spherical aberration
Due to curvature of lens
Chromatic aberration
Different wavelengths refract differently