Computar Catalog: Glossary
ABERRATION - Any inherent deficiency of a lens or optical system. Aberrations are responsible for imperfections in shape or sharpness of the image.
AGC - Automatic Gain Control, an electronic circuit that amplifies the video signal when the strength of the signal falls below a given value.
ALC - Photometric control, measures light intensity. Determines the iris reaction sensitivity. Sensitivity is increased when the potentiometer is turned towards PEAK, and decreased when turned towards AVERAGE.
ANGLE OF VIEW - May be expressed in Diagonal, Horizontal, or Vertical. Smaller focal lengths give a wider angle of view.
APERTURE - The opening of the lens that controls the amount of light reaching the surface of the pickup device. The size of the aperture is controlled by the iris adjustment.
APERTURE SCALE - The aperture scale is referred to as an F-number. The international aperture scale is: F1, F1.4, F2, F2.8, F4, F4.6, F8, F11, F16, etc.
ASPHERICAL LENS - A lens one or more of whose elements has a non-spherical surface. Aspherical surfaces are shaped to reduce the spherical and other aberrations.
AUTO-IRIS LENS - A lens with an electronically controlled iris. This allows the lens to maintain one light level throughout varying light conditions.
BACK FOCUS - A term used to describe the relationship of the distance of the lens to the image device. This distance is critical to maintaining the proper depth of field through changing focal lengths and varying light conditions. The correct back focus is normally achieved by adjusting the image pick-up device on the camera itself.
BLC - Back light compensation. A function of the camera that compensates for excessive light directed at the camera, which causes the video to bloom or the images in front of the light to be unusable.
CAMERA FORMAT - The approximate size of a camera image pickup device. This measurement is derived from the diagonal line of a chip or the diameter of the tube. Currently there are five format sizes in the CCTV industry: 1/4", 1/3", 1/2", 2/3", 1"
C-MOUNT - An industry standard for mounting a lens to a camera where a 1" x 32 thread is employed and the distance from the image plane is 17.52mm from the shoulder of the lens. A C-mount lens may be used CS-mount camera with the use of a 5mm-adapter ring.
CS-MOUNT - A relatively new industry standard for mounting a lens to a camera where a 1" X 32 thread is employed and the distance from the image plane from the shoulder of the lens is 12.52mm. A CS-mount lens may NOT be used on a C-mount camera.
DC TYPE AUTO-IRIS - Auto-iris lenses where the iris is controlled by the circuitry of the camera.
DEPTH OF FIELD - The regions in front of and behind the focused distance where the image remains in focus. With a greater the depth of field, more of the scene near to far is in focus. Lens aperture and scene lighting will greatly influence the D.O.F.
EXTENSION TUBES - Various size spacers used between the camera and lens to reduce the Minimum Object Distance. Not recommended for use with zoom lenses due to the loss of tracking ability.
F-NUMBER - Indicates the brightness of the image formed by the lens, controlled by the iris. The smaller the F-number the brighter the image.
F-STOP - A term used to indicate the speed of a lens. The smaller the F-number the greater amount of light passes through the lens.
FIELD OF VIEW - The horizontal or vertical scene size at a given length from the camera to the subject.
FOCAL LENGTH - The distance from the center of the lens to a plane at which point a sharp image of an object viewed at an infinite position. The focal length determines the size of the image and angle of FOV seen by the camera through the lens. This is the center of the lens to the image pickup device.
HUNTING - An industry term used to describe a auto-iris lenses inability to stabilize under certain light conditions.
IRIS - A mechanical diaphragm which can be controlled manually or automatically to adjust the lens aperture.
LENS FORMAT - The approximate size of a lens projected image. In most cases the lens will project a image slightly greater than the designated image size to insure the pickup device is completely covered. It is recommended that camera and lenses are the same format size. A lens a larger format size can be used on a smaller format camera, however a smaller format lens should never be used with a larger format camera.
LENS SPEED - Refers to the lens aperture or its ability to transmit light. This is measured in F-stops.
LEVEL CONTROL - Used to set the auto-iris circuit to a video level desired by the user. Turning the level potentiometer towards the HIGH position will open the iris allowing more light to pass through the lens, towards the LOW will close the iris allowing less light to pass through the lens.
MANUAL IRIS LENS - A lens with a manual adjustment to set the iris opening (aperture) to a fixed position. This type lens is generally used in fixed lighting conditions.
MINIMUM OBJECT DISTANCE (M.O.D.) - The closest distance a given lens will be able to focus upon a object. Generally the smaller the focal length the shorter the M.O.D. This distance can be altered with use of extension tubes.
PINHOLE LENS - Lenses used primarily in covert applications where the camera/lens must remain out of sight.
PRE-POSITION LENSES - Pre-position lenses are specially designed lenses with extra mechanical/electrical components to allow for computer interfacing. This function allows the lens (when used with the appropriate controller) to feedback to the controller information relevant to zoom and focus propositioning allowing the controller to quickly scan to a pre-selected scene, arriving in focus at the proper zoom point without operator intervention.
SPOT FILTER - A neutral density filter paced at the center of one of the elements (or on an iris blade) to increase the high end of the F-stop range of the lens.
TELEPHOTO - Telephoto is a term used to describe lenses that have a high focal number causing the reproduced image to appear larger than human eye reproduction.
TRACKING - A zoom lenses ability to remain in focus throughout the entire zoom range.
VARI-FOCAL - A low cost version of a zoom lens designed to meet installers needs for versatility. This lens does not have the ability to track from wide to telephoto.
VIDEO TYPE LENS - An auto-iris lens with internal circuitry for processing of the video signal which controls the iris movements.
ZOOM LENS - A lens with the ability to change its focal length manually or through the use of a controller to cover a variety of needs.
ZOOM RATIO - The ratio of the starting focal length (wide) to the ending focal length (telephoto) of a zoom lens. A 10X zoom will magnify the image at the wide end by 10 times. Examples of a 10X zoom lenses; 8mm~80mm, 12mm~120mm.