Glossary |
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| Parallel pencil of rays | A group of light rays traveling parallel to the optical axis from an infinitely far point. When these rays pass through a lens, they converge in the shape of a cone to form a point image within the film plane. |
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| Paraxial ray | A light ray which passes close to the optical axis and is inclined at a very small angle with respect to the optical axis. The point at which paraxial rays converge is called the paraxial focal point. Since the image formed by a monochromatic paraxial ray is in principle free of aberrations, the paraxial ray is an important factor in understanding the basic operation of lens systems. |
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| Peripheral illumination | The brightness of a lens is determined by the F number, but this value only indicates the brightness at the optical axis position, i.e., at the center of the image. The brightness (image surface illuminance) at the edge of the image is called peripheral illumination and is expressed as a percent (%) of the amount of illumination at the image center. Peripheral illumination is affected by lens vignetting and the cos4 (cosine 4) law and is inevitably lower than the center of the image. |
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| Polarized light | Since light is a type of electromagnetic wave, it can be thought of as uniformly vibrating in all directions in a plane perpendicular to the direction of propagation. This type of light is called natural light (or natural polarized light). If the direction of vibration of natural light becomes polarized for some reason, that light is called polarized light. When natural light is reflected from the surface of glass or water, for example, the reflected light vibrates in one direction only and is completely polarized. Also, on a sunny day the light from the area of the sky at a 90° angle from the sun becomes polarized due to the effect of air molecules and particles in the atmosphere. The half-mirrors used in autofocus SLR cameras also cause light polarization. |
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| Principal point (Nodal point) | The focal length of a thin, double-convex, single-element lens is the distance along the optical axis from the center of the lens to its focal point. This center point of the lens is called the principal point. However, since actual photographic lenses consist of combinations of several convex and concave lens elements, it is not visually apparent where the center of the lens might be. The principal point of a multi-element lens is therefore defined as the point on the optical axis at a distance equal to the focal length measured back toward the lens from the focal point. The principal point measured from the front focal point is called the front principal point, and the principal point measured from the rear focal point is called the rear principal point. The distance between these two principal points is called the principal point interval. |
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| Principal ray | A light ray which enters the lens at an angle at a point other than the optical axis point and passes through the center of the diaphragm opening. Principal light rays are the fundamental light rays used for image exposure at all diaphragm openings from maximum aperture to minimum aperture. |
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