Glossary of Lens Terms
Abberation - Factors in an optical system that generate adverse effect in 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 (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 area size captured by a photographic lens is expressed as a diagonal angular field called Angle of View. The shorter the focal length of a lens, the wider the angle of view (wideangle), while the longer it becomes, the narrower the angle of view (telephoto). (ref. Field of View)
Aperture - Lens opening. The opening in a camera lens through which light passes to expose the film. The size of aperture is either fixed, as in a reflex or catadioptric lens, or adjustable. The aperture size controls depth of field. The smaller the aperture used (e.g.: f/22, f/16 or f/11) the greater the depth of field.
Aperture Ratio - The ratio of the effective lens opening to its focal length (ref. Relative Aperture, F-Number)
Aspect Ratio - The ratio of width to height in photographic prints. A 24mm x 36mm 35mm negative produces a 2:3 aspect ratio. This aspect ratio produces the most common 3.5 x 5" or 4 x 6" photographs.
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.
Coated Lens - A lens covered with a very thin layer of transparent material that reduces the amount of light reflected by the surface of the lens. A coated lens is faster (transmits more light) than an un-coated lens.
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.
Depth of Field - When focused on a subject, there are areas in front of and behind the main subject where details are sharp. This area is referred to as depth of field. When the image area is narrow, it is expressed as shallow depth of field. When it is wide, it is called deep depth of field. A wideangle lens will deliver deeper depth of field compared with a telephoto which delivers a shallow depth of field. Depth of field is also controlled by the aperture. The depth of field becomes shallower as the lens aperture is shifted towards full open aperture. It becomes deeper when the aperture gets closer to the fully stopped down position.
Field of View - The maximum area in angular field that can be seen through a lens or an optical system. (ref. Angle of View)
Finder - A viewing device on a camera to show the subject area that will be recorded on the film. Also known as viewfinder and projected frame.
Fixed Focal - A lens designed and built to provide a single focal length.
Fixed Focus (Pan 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 - A lens comprises multiple lens elements, it can be regarded as a single convex element. The focal length is defined as the distance from the center of such a convex element (principle point) to the focal point (image plane) and it is one of the most decisive factors that determines the characteristics 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.
Lens - One or more pieces of optical glass or similar material designed to collect and focus rays of light to form a sharp image on film, paper, or projection screen.
Lens Shade - A hood (detachable or permanent) at the front of a lens that keeps unwanted or stray light from striking the lens and causing flare. Lens hoods must be properly selected based on the type of lens to avoid vignetting of the image.
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).
ND Filter - A filter that attenuates light evenly over the visible light spectrum. It reduces the light entering a lens, thus forcing the iris to open to its maximum.
Noise - The word "noise" originated in audio practice and refers to random spurts of electrical energy or interference. In some cases, it will produce a "salt-and-pepper" pattern over the televised picture. Heavy noise is sometimes referred to as "snow".
Non-Composite Video - A video signal containing all information except sync.
(NTSC) National Television Systems Committee - A committee that worked with the FCC in formulating standards for the present day United States color television system.
Object Distance - Distance to the object from the front element surface of the lens.
(PAL) Phase Alternating Line - Color encoding system for analogue television used in broadcast television systems in most countries broadcasting at 576i.
Saturation - An attribute of perceived color, or the percentage of hue in a color. Saturated colors are called vivid, strong, or deep. De-saturated colors are called dull, weak, or washed out.
Telephoto Lens - A lens that makes a subject appear larger on film than does a normal lens at the same camera-to-subject distance. A telephoto lens has a longer focal length and narrower field of view than a normal lens.
Varifocal Lens - Optical assembly containing several movable elements to permit changing the effective focal length. Unlike a zoom lens a varifocal lens requires refocusing with each change of the focal length setting, yet offering a tremendous versatility.
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).
Wide-angle Lens - A lens that has a shorter focal length and a wider field of view (includes more subject area) than a normal lens
Zoom Lens - A zoom lens allows continuous shift of focal length without shifting the focus point and therefore provides the versatility of having several focal length lenses in one convenient lens.