Glossary of Camera Lens Terms

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Glossary of Camera Lens Terms

 

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

AGC (AUTOMATIC GAIN CONTROL) – A feature built in a camera to automatically control gain level.

ALC- Iris sensitivity adjustment. Sets the iris reaction time fast or slow for changing light levels. Adjustment towards peak will increase sensitivity and reaction time, towards average will decrease both. Set to mid-range for most applications. You will not see a major change in the picture when adjusting the ALC. 2.) 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.

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 -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. See formula to determine angle of view.   2.) 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 (wide angle), while the longer it becomes, the narrower the angle of view (telephoto). (ref. Field of View) “Horizontal” angle of view is that represented by the 4:3 aspect ratio image of a video sensor. The height of the video image is always 75% of horizontal. Horizontal field of view is less than (diagonal) angular field of view. 2.) 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. See “Field of View”.

Basscam primarily uses Field of View measured in horizontal degrees as measured from the lens outward towards the scene. Ranges are from 120° to 0.5°. Vertical Field of View is always, in all cases, 75% of horizontal.

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 catadioptic 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 aspheric lenses; 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. There are two types to auto-iris lenses currently available. “Video” type lenses contain an internal amplifier to compare the incoming video signal from the camera to adjust the iris to a level set by the user. “DC” type lenses contain no amplifier and are controlled by a circuit located within the camera to adjust the iris. 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, Video type auto iris and electronic iris.)

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.

C-MOUNT/CS-MOUNT – CCTV lenses are available in two different lens mounts. “C-mount” lenses have a flange back distance of (0.69”) 17.53mm vs. (0.5”) 12.5mm for “CS-mount” lenses. Many of today’s cameras can accept either type of lens, but it is important to make sure that camera and lens are compatible and set up properly. C-mount lenses can be used on CS-mount cameras by utilizing a 5mm adapter or adjusting the camera 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.

COATED LENS – 1.) 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. 2.) 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 AUTO IRIS 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. See Video type auto iris lens definition. 2.) An auto-iris lens without an internal circuit to control the iris. All iris control voltages come from a circuit located within the camera. Conversely, see Video Type auto iris lens.

 

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 (closed) 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.

Example: A lens has a “depth of field of 2 feet (plus or minus 1 foot)”. The optimal focus distance is 10 feet. The portions of the image from 9 feet out to 11 feet will be “in focus”. Portions of the scene less than 9 feet and more than 11 feet will be out of focus.

DEPTH OF FIELD, DEFINITION #2 – 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 wide angle 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. cont.

(Depth of Field is challenging to understand, hence the longer than normal definition and example.)

 

ELECTRONIC IRIS -A misnomer. Not a genuine “iris”. A term used to describe an electronic circuit which mimics a genuine mechanical iris. The Electronic Iris (EI) circuit will darken and lighten the video image according to the relative brightness of the scene being viewed by the camera. EI circuits do not have as broad a range of light level operability as do mechanical iris mechanisms. EI circuits are satisfactory for many scenes where there is no direct sun illumination. One can “trick” the EI into being useful in sun illuminated scenes by closing a manual iris on the lens. This, however, results in a lowering of the camera’s ability to see in its lowest light capability since the mechanical iris is partly closed. (This is similar to the effects of wearing sunglasses at night.) (ref. DC-Type Lens, Video type auto iris and electronic iris.)

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).

f-NUMBER – Indicates the brightness of an image formed by the lens. A smaller F-number means a brighter image. All lenses are rated in “f” number as their greatest light transmitting ability. A “1.0” is exceptionally high. Most lenses are rated in the range from f1.2 to f1.8. Zoom lenses have one f rating at full wide angle and a higher f rating number as the lens changes towards telephoto (“zoomed-in”).

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.

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.

G Gain – Some lenses are equipped with a gain adjustment to reduce iris oscillation (iris opens and closes rapidly in bright light) with certain cameras. If oscillation occurs, adjust until the iris settles down.

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).

MANUAL IRIS LENS -A lens with a manual adjustment to change the iris opening (f stop) to a new, fixed position. Generally used for fixed lighting applications. See “Electronic Iris” and Automatic Iris above. A manual iris generally is adjusted by means of a rotating ring.

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. Difference between Optical and Mechanical Back Focal Distance, see above.

MECHANICAL IRIS – A mechanical concentric device which opens and closes allowing more or less light to pass through the iris. Often called an “aperture”. The human eye has an iris, called the “pupil” and opens and closes automatically with light variations.

ND FILTER – A neutral density 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.

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.

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.

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.

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.

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.

TRACKING -A zoom lens’ ability to remain in focus during the entire zoom range from wide angle to telephoto position. A zoom lens not having its “tracking” set correctly will change from in-focus to out-of-focus as the lens field of view is changed.

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.

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. A vari-focal lens is not a true zoom lens but does allow a change in the focal length of the lens just as a zoom lens does.

VIDEO TYPE AUTO IRIS 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). See DC type auto iris lens definition.

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.

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