The imagery formed by a toric lens, with power that varies according to meridian
A small light-tight room or box in which a pinhole aperture forms an inverted image.
The distance between a lens and the image it forms of an object at great distance (optical infinity).
A refractive error in which distant objects are imaged behind the retina.
The distance from a lens to the image it forms of an object. Distances to the left of a lens are considered as negative numbers; distances to the right of a lens are considered as positive numbers.
The orientation of a plane passing through the optic axis of a lens, or of the intersection curve of such a plane with a lens surface. This orientation is usually specified in degrees, increasing counterclockwise from the horizontal as viewed from in front of the lens. The horizontal meridian is by convention designated as 180° (not 0°); the vertical meridian is at 90°.
A refractive error in which distant objects are imaged in front of the retina.
The distance from a source object to the lens, in meters. The sign conventions are the same as for image distance.
Power (of a lens)
The reciprocal of the focal length. Measured in m–1, referred to as diopters (D).
A diagrammatic representation of the action of a toric lens, showing the power and orientation of the 2 (perpendicular) principal meridians.
The flattest or steepest meridian of a toric lens. The principal meridians are in general perpendicular to each other.
The speed of light in air (or a vacuum) divided by the speed of light in a different medium. As light always travels more slowly through a material medium (sometimes referred to as a denser medium) than in a vacuum, the refractive index is always greater than 1.00.
A lens with a surface resembling the outer rim of a torus, such as an automobile tire or the side of a rugby ball or an American football.
Vergence (in air)
The reciprocal of the object distance (object vergence) or image distance (image vergence).
Vergence (in media other than air or a vacuum)
The refractive index of the medium in which light travels divided by the object distance or image distance. Sometimes referred to as reduced vergence, even though this is numerically greater than the vergence in air, as refractive indices of denser media are always greater than 1.00.
The formula relating object vergence, lens power, and image vergence.