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LENS GLOSSARY  

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ABERRATION: Factors in an optical system that generate adverse effect on the resultative image. Any design work in making optics entails many different approaches to correct various aberrations, such as spherical and chromatic aberrations, astigmatism, comatic flare, and distortion.

AGC: Abreviation of Automatic Gain Control. A feature built in a camera to automatically control gain level.

ALC CONTROL (Automatic Light Compensation): Photometric control that sets the auto-iris to react to bright objects in a picture that does not affect the overall video level. Turning the control towards Peak will increase sensitivity, towards Average will decrease sensitivity. It is normally set to "Average" under factory-shipped conditions.

ANGLE OF VIEW (Angular Field): The angular range that can be covered within the image field. Small focal lengths give a wide angle of view (wideangle), and large focal lengths give a narrow angle of view (telephoto). (ref. Field of View)

APERTURE RATIO: The ratio of the effective lens opening to its focal length (1/ F#)

ASPHERICAL: An optical element processed with non-spherical surface(s). There are a couple of different ways to create aspherics; e.g. grinding, press molding, injection molding and hybrid methods, any of which requires high-precision technology.

AUTO-IRIS LENS: A lens with an electrically controlled iris. The circuit controlling the iris is set to maintain a constant video level in varying lighting conditions. Depending on the placement of the driving circuitry (i.e. on the lens side or incorporated on the camera side), there are two types of Auto-Iris; with a driving circuitry built in and DC meter (galvanometer) only. Make sure to identify, before ordering an AI lens, whether the camera outputs video signal or DC current to actuate the auto-iris. (ref. DC-Type Lens)

BACK FOCUS (Back Focal Distance): The distance from the rear-most portion of the lens element to the image plane. It is important to adjust the back focus correctly in order to obtain the best image. Certain lenses come with a back focus adjustment mechanism, while others do not. Also, most of the cameras incorporate back focus adjustment, if it is not available on the lens side.

BARREL: The chassis of a lens, usually cylindrical, that contains the lens elements and iris diaphragm.

CCD Iris: Automatic brightness level adjustment feature built in certain types of camera. Number of steps (multiple levels of brightness) available varies dependent on the camera model. While there are tremendous advantages to use a camera with CCD Iris, use of lenses with manual and/or Auto-iris will enhance the efficiency of the camera in certain set-up conditions.

COATING: A thin coating applied to the lens surfaces to reduce reflections, thus increasing the amount of light transmission. Also, for color-corrected lenses, it significantly contributes to rendering optimal color balance of the lens. Sophisticated coating techniques allow applying as many as seven layers.

DC-TYPE LENS (lens with Galvanometer iris): An auto-iris lens without a driving circuit to actuate the iris. Iris control voltage is supplied from a circuit located within the camera.

FIELD OF VIEW: The maximum area in angular field that can be seen through a lens or an optical system. (ref. Angle of View)

FIXED FOCUS: Lenses that are not provided with a means of focusing operation regardless of the distance to the subject. .

FLANGE BACK (Flange Back Focal Distance): The distance from the mechanical flange of the lens (rear edge surface of the lens mount) to the focal plane. C-mount lenses have a flange back distance of 17.526mm while CS-mount lenses have 12.5mm. Because of this, C-mount lenses can be used on CS-mount cameras with an adapter ring of 5mm thickness (however, CS-mount lenses cannot be used on C-mount cameras).

F-NUMBER (F/#): Expression denoting the ratio of the equivalent focal length of a lens to the diameter of its entrance pupil (smaller F/# provides larger aperture of the lens, transmitting greater amount of light).

FOCAL LENGTH: The basic parameter to determine the image position, magnification, and angle of view of a lens.

GAIN CONTROL: A control that allows adjustment of iris response speed of Auto-Iris lenses. When an oscillation or "chattering" (iris opens and closes rapidly in bright light) occurs, reduce the gain level until iris stops oscillating.

IMAGE SIZE: Reference to the size of an image formed by the lens onto the camera pick-up device. The current standards are: 1", 2/3", 1/2", and 1/3", corresponding to 16mm, 11mm, 8mm and 6mm measured diagonally.

LEVEL CONTROL: Main iris control. Used to set the auto-iris circuit to a video level desired by the user. After set-up, the circuit will adjust the iris to maintain this video level in changing lighting conditions. Turning the control towards High will open the iris, towards Low will close the iris.

MANUAL IRIS LENS: A lens with a manual adjustment to set the iris opening (F-stop) to a given position. Generally used for relatively constant lighting applications.

MINIMUM OBJECT DISTANCE (M.O.D.): The closest focusing distance of a lens. A measurement from the vertex (front) of the lens to the object. Wide angle lenses generally have a smaller M.O.D. than large focal length lenses (telephoto).

NEUTRAL DENSITY (ND) FILTER: A type of filter to reduce the amount of light transmission without cutting of any particular frequency range of the light. Some lenses incorporate ND filter as a built-in feature for the purpose of helping the diaphragm function toward the minimum aperture range. Optional filters of different diameters are available for attachment to the front of a lens (ND 2X, 4X, etc.).

OBJECT DISTANCE: Distance to the object from the front element surface of the lens.

PRE-POSITION LENS (Preset Lens): Zoom lenses which incorporate variable-resistors (potentio meter) to index zoom, focus and/or aperture positions to the lens controller. After initial set-up, this allows the operator to view different preset areas quickly without having to readjust the zoom, focus and/or aperture each time.

SPOT FILTER: A supplement to the iris which allows the lens to have a larger aperture opening than is physically possible with the iris only. These usually range from F/88 to F/1600. This allows very sensitive cameras to view bright scenes easily. The iris of a lens without a spot filter would not be able to close down enough in bright light without creating an image degradation caused by refraction.

VARIFOCAL LENS: Optical assembly containing several movable elements to permit changing the effective focal length (EFL). Unlike a zoom lens a varifocal lens requires refocusing with each change.

VIDEO TYPE LENS: An auto-iris lens with an internal circuit that converts the video signal to DC voltages that control the iris meter (galvanomic meter) (ref. Auto-Iris Lens).

VIGNETTING: Fall-off of light illumination observed at the image corners. When gradual, it is likely to be inherent to the optical system. In the case of eclipse, it might be caused by mechanical factors such as housing. (Port hole effect as when a 1/2" lens is viewed on a 1" camera is a result of smaller image circle of the lens as opposed to the size of the imager).

ZOOM LENS: A lens that delivers different focal lengths without creating a shift of focus regardless of the focal length setting.

ZOOM RATIO: The ratio of the starting focal length (wide end) to the ending focal length (tele end) of a zoom lens. A lens with a 10X zoom ratio will magnify the image at the tele-end by 10 times.

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Tekstar Optical, Inc.  |  75 Lackawaca Hill, Napanoch, NY 12458  | 
Phone/Fax: 845-647-5367 (LENS)  |  Cell: 516-582-3458
Email: info@tekstaroptical.com