Any deviation of an optical system from stigmatic imaging.
Angle of incidence
The angle between a ray incident to a mirror and the surface normal at the point of incidence.
Angle of reflection
The angle between a ray reflected off a mirror and the surface normal at the point of incidence.
The disparity in focal length for rays from a single object point that are incident at different meridians of the lens.
The ratio of the axial extent (depth) of the image of an extended source object to the depth of the object—measured along the optic axis.
A variation in the power of a lens system with the wavelength of incident light.
A disparity in focal length for rays from a single off-axis object point that are refracted at different distances from the center of the lens.
Points that share an object–image relationship.
Conoid of Sturm
A geometric figure traced by the refraction of a single point source object by an astigmatic lens.
The angle at which a light ray passing from one medium to another with a lower refractive index undergoes total internal reflection.
Curvature of field
A disparity in focal length for objects at different distances from the optic axis.
An aberration corresponding to an axial refractive error, a disparity between the power and the axial length of the eye.
Depth of field
The range of locations of a source object for which an optical system forms acceptably sharp images.
Depth of focus
The range of image locations over which an optical system forms acceptably sharp images of a fixed source object.
The reciprocal meter, the customary unit for vergence and the power of lenses.
A disparity in transverse magnification for objects at different distances from the optic axis.
Effectivity of lenses
The adjustment in the power of a lens necessary to compensate for changes in the distance between the lens and the desired image location.
Among alternative paths between 2 points, light rays travel along the path with the shortest total travel time.
A telescope with a convex objective lens and a concave ocular lens.
An efficient mathematical treatment for the paraxial optical regime by means of linear algebra.
Optical phenomena that are effectively described in terms of light rays, which travel along straight lines unless deviated by lenses or mirrors.
Jackson cross cylinder
A lens superimposing cylindrical lenses of equal and opposite power, placed with axes perpendicular to each other.
Keplerian (or astronomical) telescope
A telescope with convex objective and ocular lenses.
Law of reflection
The statement that the angle of incidence equals the angle of reflection.
A mathematical formula for the power of a curved refracting surface.
The conjugate points on the optic axis through which incident and exiting light rays form equal angles with the optic axis.
Paraxial (or paraxial ray)
The optical regime in which light rays travel close to the optic axis and form only small angles with the optic axis. Rays conforming to this description are termed paraxial rays.
Point spread function
The distribution of light from a point source in the image plane, which characterizes the aberration of an optical system.
A graphical depiction of the action of a toric lens, showing the orientation of the principal axes and the power along each axis.
A graph showing the power of an astigmatic lens at each meridian.
An orientation for an ophthalmic prism such that incoming or exiting light rays strike one of the prism surfaces perpendicularly.
One of 2 planes at which the refraction of a general lens system appears to take place.
The intersection of either principal plane with the optic axis.
A unit describing the deflection of light by an ophthalmic prism equal to 100 times the tangent of the angle of deflection.
The graphical localization of images by drawing rays of light from the source object through the focal points and nodal points of an optical system.
An image formed by the actual convergence of light rays.
An object that serves as the source of light rays in an optical system.
Reduced optical system
A simplified optical system equivalent to a general multi-element optical system characterized by the power and the locations of the principal planes and the nodal points.
The product of object or image vergence and the refractive index of the medium through which the light travels.
The hypothetical spherical wavefront surface that corresponds to a perfect point image.
The ratio of the speed of light in a vacuum to the speed of light in a material medium.
A series of common aberrations of simple optical systems.
A mathematical description of the deflection of light as it passes between media with different refractive indices.
A disparity in focal length for rays from a single axial object point that are refracted at different distances from the center of the lens.
The power of a spherical lens approximately equivalent to the average power of an astigmatic lens.
Imaging by a lens system that focuses the light from a point source at a single image point.
A line perpendicular to a surface through a central point of interest.
An approximate treatment appropriate for thin lenses that ignores the spacing between the front and back lens surfaces.
An aberration equivalent to the direction of light rays at an angle from the optic axis, as if deflected by a prism.
A curved (spherocylindrical) surface resembling a portion of the outer curvature of a torus, such as an automobile tire inner tube.
Total internal reflection
The reflection of a light beam directed from one medium to another with a lower refractive index when the angle of incidence exceeds the critical angle, and no emerging ray can satisfy Snell’s law.
The ratio of the height of an image to the height of the source object, measured perpendicular to the optic axis.
A mathematical relationship between object vergence, the power of a lens or mirror, and the resultant image vergence.
The reciprocal of the distance between a source object or image and a lens or mirror.
An apparent image inferred as the source of a divergent bundle of light rays.
An intermediate image formed by an optical system that serves as the apparent source of a bundle of light rays, which in turn is imaged by a subsequent optical element.
A surface connecting the points of equal travel time for rays of light emerging from a single point source.
A standard mathematical system of polynomial functions used to describe the deviation of a wavefront from the reference sphere as a description of the aberration of an optical system.