lenstems.htm

LENS TERMINOLOGY

ALC CONTROL: Photometric control, measures light intensity. Sets the iris to react to bright objects in a picture that do not affect the overall video level. Turning the control towards Peak will increase sensitivity, towards Average will decrease sensitivity.
ANGLE OF VIEW: The angular range that can be focused within the image size. Small focal lengths give a wide angle of view, and large focal lengths give a narrow angle of view. Sometimes referred to as Field of View.; Relates to the F-number. The effective aperture of a lens is not its actual diameter but the diameter of the image of the iris seen from the front of the lens. Larger apertures equal smaller F-number.
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.
CCTV: Acronym for Closed Circuit Television.
CCVE: Acronym for Closed Circuit Video Equipment.
C-MOUNT: "C-mount" lenses have a flange back distance of 17.526mm vs. 12.5mm for "CS-mount" lenses. C-mount lenses can be used on CS-mount cameras by utilizing a 5mm adapter or adjusting the camera for C-mount lenses.
CS-MOUNT: "CS-mount" lenses have a flange back distance of 12.5mm vs. 17.526mm for "C-mount" lenses. Because of the shorter back focal distance, CS-mount lenses can only be used on CS-mount cameras. Your picture will be out of focus if you use a CS-mount lens on a C-mount camera.
DC TYPE LENS: An auto-iris lens without an internal circuit to control the iris. All iris control voltages come from a circuit located within the camera.
DEPTH OF FIELD (FOCUS): The zone in the front and back of the area focused upon that will remain in focus. Anything within this depth of field will appear sharp. Depth of field has the following features 1. Larger F-numbers give greater depth of field. The more the iris is stopped down the greater the depth of field. 2. Shorter focal lengths give greater depth of field. 3. Greater subject distance gives greater depth of field. 4. Depth of field is greater behind the subject than in front.
EXTENSION TUBE: Kit consisting of various size spacers that are used between the lens and the camera to reduce the lens M.O.D. Generally used for very close-up applications. Not recommended for zoom lenses due to loss of tracking.
F-DROP: The drop of the F-number of a while zooming at full aperture. The entrance pupil of a zoom lens changes in diameter as the focal length is changed. As you zoom towards the telephoto end, the entrance pupil gradually enlarges. When the entrance pupil diameter is equal to the diameter of the focusing lens group, it cannot become any larger. This causes the F-drop.
F-NUMBER: Indicates the brightness of an image formed by the lens. A smaller F-number means a brighter image.
FIELD OF VIEW: See Angle of view.
FOCAL LENGTH: The basic parameter to determine the image position, magnification, and angle of view of a lens.
GAIN CONTROL: Reduces iris oscillation (iris opens and closes rapidly in bright light). If oscillation occurs, adjust CCW 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", 1/3", and 1/4" 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) in a fixed position. Generally used for fixed lighting applications.
MECHANICAL BACK FOCAL DISTANCE (FLANGE BACK): The distance from the flange of the lens (beginning of the lens mount) to the focal plane. C-mount lenses have a flange back distance of 17.526mm vs. 12.5mm for CS-mount.
MINIMUM OBJECT DISTANCE (M.O.D.): The closest distance a given lens will be able to focus upon an object. This is measured 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.
OPTICAL BACK FOCAL DISTANCE: The distance from the rear most portion of the lens glass to the image plane.
PINHOLE LENS: Lens used for applications where the camera/lens must be hidden. Front of lens has a small opening to allow the lens to view an entire room through a small hole in a wall.
PRE-POSITION LENSES (PZF): Zoom lenses which utilize a variable-resistor (potentiometer) to indicate zoom/focus position to the lens controller. After initial set-up, this allows the operator to view different pre-set areas quickly without having to readjust the zoom and focus each time.
SLIP CLUTCH: A part of the lens mounts which allows the lens position to be adjusted to a desired position after the lens is mounted. The lens will turn (slip) up to 350O until reaching a physical limit.
SPOT FILTER: A supplement to the iris which allows the lens to have a larger F-number than is physically possible with the iris only. These usually range from F88 to F1600. 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 to achieve a usable picture.
T-NUMBER: An F-number expresses the speed of the lens on the assumption that the lens transmits 100% of the incident light. In reality, different lenses have different transmittance, so lenses with the same F-number may actually have different speeds. The T-number solves this by taking both the iris diameter and transmittance into account. Two lenses with the same T-number will always give the same image brightness.
TRACKING: A zoom lens' ability to remain in focus during the entire zoom range from wide angle to telephoto position.
VIDEO TYPE LENS: An auto-iris lens with internal circuit which receives voltage and a video signal from the camera to adjust the iris.
ZOOM LENS: A lens with a variable focal length to obtain a wide angle picture to a narrow angle picture to cover a variety of needs.
ZOOM RATIO: The ratio of the starting focal length (wide position) to the ending focal length (telephoto position) of a zoom lens. A lens with a 10X zoom ratio will magnify the image at the wide angle end by 10 times when at the telephoto position.

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