Everything about Astigmatism totally explained
In
optics,
astigmatism is when an optical system has different
foci for
rays that propagate in two perpendicular
planes. If an optical system with astigmatism is used to form an image of a
cross, the vertical and horizontal lines will be in sharp focus at two different distances. The term comes from the
Greek α- (
a-) meaning "without" and στίγμα (
stigma), "a mark, spot, puncture".
Forms of astigmatism
There are two distinct forms of astigmatism. The first is a third-order
aberration, which occurs for objects (or parts of objects) away from the
optical axis. This form of aberration occurs even when the optical system is perfectly symmetrical. This is often referred to as a "monochromatic aberration", because it occurs even for light of a single
wavelength. This terminology may be misleading, however, as the
amount of aberration can vary strongly with wavelength in an optical system.
The second form of astigmatism occurs when the optical system isn't symmetric about the optical axis. This may be by design (as in the case of a
cylindrical lens), or due to manufacturing error in the surfaces of the components or misalignment of the components. In this case, astigmatism is observed even for rays from on-axis object points. This form of astigmatism is extremely important in
ophthalmology, since the human
eye often exhibits this aberration due to imperfections in the shape of the
cornea or the
lens.
Third-order astigmatism
In the analysis of this form of astigmatism, it's most common to consider rays from a given point on the object, which propagate in two special planes. The first plane is the
tangential plane. This is the plane which includes both the object point being considered and the axis of symmetry. Rays that propagate in this plane are called
tangential rays. Planes that include the optical axis are
meridional planes. It is common to simplify problems in radially-symmetric optical systems by choosing object points in the vertical ("
y") plane only. This plane is then sometimes referred to as
the meridional plane.
The second special plane is the
sagittal plane. This is defined as the plane,
orthogonal to the tangential plane, which contains the object point being considered and intersects the optical axis at the
entrance pupil of the optical system. This plane contains the
chief ray, but doesn't contain the optic axis. It is therefore a
skew plane, in other words not a meridional plane. Rays propagating in this plane are called
sagittal rays.
In third-order astigmatism, the sagittal and transverse rays form
foci at different distances along the optic axis. These foci are called the
sagittal focus and the
transverse focus, respectively. In the presence of astigmatism, an off-axis point on the object isn't sharply imaged by the optical system. Instead, sharp
lines are formed at the sagittal and transverse foci. The image at the transverse focus is a short line, oriented in the direction of the
sagittal plane; images of circles centered on the optic axis, or lines tangential to such circles, will be sharp in this plane. The image at the sagittal focus is a short line, oriented in the
tangential direction; images of spokes radiating from the center are sharp at this focus. In between these two foci, a round but "blurry" image is formed. This is called the
medial focus or
circle of least confusion. This plane often represents the best compromise image location in a system with astigmatism.
The amount of aberration due to astigmatism is proportional to the
square of the angle between the rays from the object and the optical axis of the system. With care, an optical system can be designed to reduce or eliminate astigmatism. Such systems are called
anastigmats.
Astigmatism in systems that are not rotationally symmetric
If an optical system isn't axisymmetric, either due to an error in the shape of the optical surfaces or due to misalignment of the components, astigmatism can occur even for on-axis object points. This effect is often used deliberately in complex optical systems, especially certain types of
telescope.
In the analysis of these systems, it's common to consider tangential rays (as defined above), and rays in a meridional plane (a plane containing the optic axis) perpendicular to the tangential plane. This plane is called either the
sagittal meridional plane or, confusingly, just the
sagittal plane.
Ophthalmic astigmatism
In
ophthalmology, the vertical and horizontal planes are identified as
tangential and
sagittal meridians, respectively. Ophthalmic astigmatism is a
refraction error of the
eye in which there's a difference in degree of
refraction in different meridians. It is typically characterized by an aspherical, non-figure of revolution
cornea in which the corneal profile
slope and refractive power in one meridian is greater than that of the perpendicular axis.
Astigmatism causes difficulties in seeing fine detail. In some cases vertical lines and objects such as walls may appear to the patient to be leaning over like the
Tower of Pisa. Astigmatism can be often corrected by
glasses with a
lens that has different
radii of curvature in different planes (a
cylindrical lens),
contact lenses, or
refractive surgery.
Astigmatism is quite common. Studies have shown that about one in three people suffers from it.
(External Link
)(External Link)(External Link) The prevalence of astigmatism increases with age.
(External Link) Although a person may not notice mild astigmatism, higher amounts of astigmatism may cause blurry vision,,
asthenopia,
fatigue, or
headaches.
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There are a number of tests used by
ophthalmologists and
optometrists during
eye examinations to determine the presence of astigmatism and to quantify the amount and axis of the astigmatism.
(External Link) A
Snellen chart or other
eye chart may initially reveal reduced
visual acuity. A
keratometer may be used to measure the curvature of the steepest and flattest meridians in the cornea's front surface.
(External Link) A
corneal topographer may also be used to obtain a more accurate representation of the cornea's shape.
(External Link) An
autorefractor or
retinoscopy may provide an objective estimate of the eye's refractive error and the use of
Jackson cross cylinders in a
phoropter may be used to subjectively refine those measurements.
(External Link)(External Link)(External Link) An alternative technique with the phoropter requires the use of a "clock dial" or "sunburst" chart to determine the astigmatic axis and power.
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Astigmatism due to misaligned or malformed lenses and mirrors
Grinding and polishing of precision optical parts, either by hand or machine, typically employs significant downward pressure, which in turn creates significant frictional side pressures during polishing strokes that can combine to locally flex and distort the parts. These distortions generally don't possess figure-of-revolution symmetry and are thus astigmatic, and slowly become permanently polished into the surface if the problems causing the distortion are not corrected. Astigmatic, distorted surfaces potentially introduce serious degradations in optical system performance.
Surface distortion due to grinding or polishing increases with the
aspect ratio of the part (diameter to thickness ratio). To a first order, glass strength increases as the
cube of the thickness. Thick lenses at 4:1 to 6:1 aspect ratios will flex much less than high aspect ratio parts, such as optical windows, which can have aspect ratios of 15:1 or higher. The combination of surface or wavefront error precision requirements and part aspect ratio drives the degree of back support uniformity required, especially during the higher down pressures and side forces during polishing. Optical working typically involves a degree of randomness that helps greatly in preserving figure-of-revolution surfaces, provided the part isn't flexing during the grind/polish process.
Deliberate astigmatism in optical systems
Compact disc players use an astigmatic lens for focusing. When one axis is more in focus than the other, dot-like features on the disc project to oval shapes. The orientation of the oval indicates which axis is more in focus, and thus which direction the lens needs to move. A square arrangement of only four sensors can observe this bias and use it to bring the read lens to best focus, without being fooled by oblong pits or other features on the disc surface.
Some
telescopes use deliberately astigmatic optics.
Further Information
Get more info on 'Astigmatism'.
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