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Older Revision of Photographer s Glossary

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Photographer's Glossary

Angle of Incidence - The angle at which the light rays strikes the surface.

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Angle of Reflection - The angle at which the light rays reflects from the surface. The angle of reflection equals the angle of incidence. Only applies to a specular reflection where the surface is uniform. In a diffused reflection, light is reflected in all different angles.

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Aspherical Lens - A lens that is not spherical in shape. The technology to create non-sperical lenses wsa invented to reduce image distortion that is a common characteristic of a spherical lens. Aspherical lenses are generally molded out of plastic or the plastic coated onto the surface of a glass lens.

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Aperture - The lens opening that lets in light. The diameter of this opening, in addition to the focal length, determines the f-stop number. The f-stop number is sometimes referred to as the aperture number, or aperture for short. F-number is focal length divided by aperture diameter. Both units are in millimeters and the resulting f-number is unit-less.

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Chromatic Aberration - Occurs when light rays travels through glass (or lens). Each light wave frequency bends (refracts) at a different angle while traveling through the lens. Short wavelengths (toward the blue) are refracted more than long wavelengths (toward the red). Thus, different color wavelengths are exposed on different portions of the photosensitive material (film or sensor). Achromatic lens solves this problem by combining lenses with different refractive indexes.

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Color Fringing - One of the effects of lateral chromatic aberration. It appears as magenta and green bands at contrast boundaries. Color fringing is always more apparent on the edge of the image than the center due to the lens curvature. It becomes the worst at the image corners.

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Depth-of-Field (DOF) - The distance range at which the subject acceptably sharp is defined as DOF. Depth-of-Field can be changed by changing the size of the aperture. A larger aperture (smaller f-number) causes narrow DOF range. A smaller aperture (larger f-number) creates a wide DOF range.

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Dispersion - The variation in a lens-element's ability to bend light.

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Exposure Latitude - The range of exposure at which the photosensitive material (film or sensor) will produce an acceptable image. Most slide films have small exposure latitude (in the 1-stop range). Most print films have big exposure latitude (in the 5-stop range), because exposure can be compensated in the printing stage.

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ISO - In photograhy, ISO refers to the light gathering capability of the photosensitivity material (film or sensor) used to capture the image. Because the International Organization of Standardization defined this standard, the name ISO speed has became synonymous to film speed. The sensitivity of the material increases as the ISO number increase. Every doubling of the ISO number is one stop increase in exposure. Every halving of the ISO speed is one stop decrease in exposure.

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Focal Length - With a pinhole camera, the focal length is the distance between the pinhole (serving as the aperture) and the film/sensor plane. An equivalent lens, with the same focal length as the pinhole, capturing an object will produce an object image that is the same size as the object image produced by the pinhole.

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Reflection - When light ray strikes a surface, it is bounced away from the surface. The act of being redirected from its original path is called reflection. Two types of reflection: specular and diffused. A specular reflection occurs when light is reflected from a smooth uniform surface, such as a mirror. In a specular reflection, the entire light wave continues to travel in the same direction after being reflected. A diffused reflection occurs when light is reflected from a rough, non-uniform surface. In a diffused reflection, the light wave is scattered in different directions due to the irregular surface.

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Refraction - The property that light exhibits when traveling from one transparent medium into another transparent medium, such as from air into water. When light travels from one medium into another, part of the light ray is reflected away from the second medium. Another portion of the light ray enters the second medium and is bent at the boundary. This bent ray in the second medium is called the refracted ray. The angle of the bent depends on the difference between the two mediums, the wavelength of the light ray, and the angle of incidence.

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