The distance from optical center of the lens in which there is created image of an object placed in infinity - measured in millimeters.
For given sensor size and focal length, the view angle of the camera can be calculated. Angle of view is inversely proportional to focal length, so - long focal length means small angle of view, and vice versa. In turn, the bigger image sensor the wider angle of view.
The mechanism of aperture regulation that changes the amount of light going through lens falling on image sensor of CCTV camera. Lenses with fixed iris are marked as "NO IRIS".
Aperture number of lens - F-number
It is factor of lens' ability of light transmission, measured in F scale. The F series is: 1; 1.4; 2; 2.8; 4; 5.6; 8; 11; 16; 22; 32; 45; 64 etc. With increase in F-number by one step, the quantity of light passing through the lens decreases in half. In the table below we can find amount of light that is going through lens with different F values.

F number

Quantity of light passing through
1,0 100,0%
1,4 50,0%
2,0 25,0%
2,8 12,5%
4,0 6,3%
5,6 3,1%
8,0 1,6%
16,0   0,8%
Relationship between F number and amount of transmitted light
If the iris of lens is closed completely, the F number is equal infinity (it can be also marked as closed iris).
Very often there are showed two values of F number, the first defines the maximum opening of Iris and the second - the minimum.
Depth of focus
The range of distance from lens to objects that are still sharply imagined. Generally, depth of focus increases with iris closing and focal length decreasing. This rule is used e.g. by photographers, who use good illumination and decrease iris (increase F-number) to achieve good depth of focus. If we use auto iris lens and illumination drops, in connection with iris opening the depth of focus decreases. Generally, we aim at such depth of focus to reach infinity, so objects which are situated farther than some minimal distance can be seen clearly. With high depth of focus, adjustment of sharpness in wide range does not influent sharpness of the image.
Depth of focus changes along with iris closing. Grey area - the range of depth of focus
Transmission number of lens (T)
The indicator of real ability of light transmission. This scale allows to evaluate lens quality in absolute way, because it takes into consideration the influence of all factors e.g. glass properties, its ability to filter different frequencies of light etc.
Other important parameters:
maximal resolution, sharpness and contrast of images, color correction connected with chromatic aberration.
DIVISION OF LENSES - is made on account of:
Angle of view:
  • narrow-angled (tele-lens), focal length bigger than diagonal of sensor,
  • standard, focal length close to size of diagonal of sensor,
  • wide-angled, focal length smaller than diagonal of sensor.
Type of iris:
  • no iris: cheap lenses designed for the use with simple cameras,
  • with manual iris,
  • with automatic iris.
Focal length:
  • fixed, manufactured lens have typical values i.e. 2.5; 3.6; 4.0; 6.0; 12.0; 16.0; 25.0 mm,
  • variable (varifocal, zoom), suggested when the size of observed object changes or when freedom of focal length selection is required, we also have some typical values here: 3.5-8 mm (1/3" sensors); 6-12 (cameras with 1/2" sensors),
  • remotely controlled (so called motor-zoom lens).
Types of mounts. Two types are common: C and CS, The distance from sensor's surface to mounting surface is 12.5mm for CS lens, and 17.526mm for C lens. Thread with smaller diameter occurs in board cameras. More about mounts here
The size of lens versus the size of image sensor. The size of image which can be thrown by a lens is measured in inches, similarly to image sensor's size. Manufacturers show the biggest size that specific lens can work with. Lens must be adjusted to sensor's size, naturally it is always possible to use lens bigger than camera's image sensor, what can be even useful in some cases, as it helps to reduce deformations of images (lens deformations increase with growing distance from axis). With the same lens, smaller sensor will result in displaying smaller part of observed object. There are common lens suitable for sensors: 1", 2/3", 1/2", 1/3" and 1/4" (in the future lens for 1/5" sensors will also appear).
Selection of focal length
For relevant image sensor's size and focal length, the angle of camera view can be calculated. Angle of view is inversely proportional to focal length. It should be noticed that the angle of view in horizontal plane equal to 30 degrees is similar to human angle of view. And it appears that this angle can be achieved when focal length is the same as image sensor's size. For example, it can be lens with focal length 1/4" (6.2) mm, with image sensor 1/4".
The scheme of optical system object-lens-image sensor and related parameters
If we know image sensor format, distance from observed object and it's dimensions we can calculate what focal length will be needed and also what will be the angle of view of the camera with the lens. On account of rectangular shape of image sensor, calculations should be made both for horizontal and vertical axis.
Telescopes, calculators, graphs, lens selection patterns. The selection of lens can be made using patterns, calculators, graphs and special telescope called "view finder". We also should make field tests if possible, where we can choose from many kinds of lens the types that are most suitable for the task. Selection of focal length basing on graphs amounts to reading value of focal length depending on the width and distance of the object, and image sensor format. Their disadvantage is low accuracy, caused by impossibility of preparing graphs for all possible cases.
View finder is placed in place where camera is to be installed, after that it comes the time to adjust all telescope regulation rings to achieve desired observation range. Then we have to read the value of focal length.
Fixed or varifocal (zoom) lens
Usually, only after some time of work of the system we can precisely decide which objects we want to observe first of all. Therefore, despite higher price, it is worth to use varifocal lens. It will allow to widen or narrow the field of view after some time of operation of the system.
Iris type selection
It should be remembered that camera sensitivity is showed with specified iris, i.e. 0.05 lx F=1.2, and when we use lens with F1.8, sensitivity will be smaller, because smaller amount of light reaches the image sensor. Presently, "bright" lenses with number F=1.0 are available. Such lens, in connection with high-sensitivity cameras, makes it possible to monitor areas with very bad lighting conditions. In typical conditions lenses with F=1.2-64 are used. However, using camera with high sensitivity e.g. 0.01 lx, strong sunlight may overexposure the image (the minimum iris will no be able to limit the light adequately). In this case we should either use lens with bigger maximum F-number specifying minimum iris, or change the camera to less sensitive device. Other solution is using gray filters, but they reduce sensitivity at low light. In truly strong sunlight, we may come across a situation in which even the use of lens with auto iris will not guarantee good results. The image on monitor's screen will not have the same brightness in the field of view. Depending on adjustment of the lens, it may be, for example, overexposed in the central part and underexposed in the corners.
We can "fight" such problems with the use of lenses that are additionally equipped with special filter, so called "spots" plotted on central part of lens surface.
Manually adjustable iris
It is used in stable illumination conditions, usually inside buildings. The adjustment is made once, with a ring or lever.
Besides manual iris, we have also two types of automatic iris.
Electronic iris ("EAI" - Electronic Auto Iris or "EI" - Electronic Iris)
This kind of automatics is used for moderately changeable light environments, with lens equipped with manual iris. If we want to use such camera in more variable light conditions, i.e. from almost complete darkness to sunlight, we have carefully adjust the manual iris towards opening. We may achieve good sensitivity, however may have problems with depth of the field. There should be no problem in indoor applications. In the case of stable light, we have to close iris, which will improve depth of the field. However, in this case it may happen that brightness in low light won't be satisfactory. Basic advantage of electronic iris is possibility of using simple lens with fixed or manual iris.
Automatic iris ("AI" - Auto Iris)
It keeps amount of light falling on image sensor constant, no matter the illumination conditions are. Electronic shutter is to be set at 1/50s, and AI lens is closed or opened adequately to light intensity. Camera and AI lens are able to work properly even in very dynamic light environments. As a rule, outdoor monitoring cameras, working during the day and night, require this kind of lens. Camera with AI is equipped with special socket used for AI lens control. Depending on signal type in this socket, the lens closes or opens its iris keeping amount of light falling on the image sensor at constant level.
Auto IRIS control is described in chapter related to cameras.
Adjustment of automatic iris parameters
Main purpose is to achieve optimum brightness during the day as well as during the night. New AI iris lens is usually adjusted by the manufacturer, so additional regulation is generally not necessary. However, it happens sometimes, that we have to perform this adjustment. It consists in setting the Level and AGC regulators to middle position, followed by adjustment of desired image brightness with Level potentiometer. Next step is adjustment of ALC potentiometer, depending on the type of demanded reaction to light, and further - putting on gray filter and setting sharpness.
DC AI lenses do not use any potentiometers
Problems with image sharpness
Image sharpness means the ability of distinguishing details, limited by resolution of image sensor or monitor. Adjustment of sharpness is in fact change of location of optical center of lens with reference to image sensor. Lack of sharpness is one of the major problems appearing during installation and adjustment of cameras and lens, and it concerns both fixed and varifocal lens. It sometimes happens that we can't achieve clear image during the adjustment of lens sharpness - it appears mainly in extreme positions of lens sharpness adjustment. In this case we should loosen the screw which is used to mount the ring (with the wrench included in the set with almost every camera). With the ring we should adjust the best possible image sharpness. Adjustment should be made at the shortest focal length and sharpness set at infinity. Sharpness adjustment should be made with maximally opened iris, using e.g. gray filter. It should be noticed that this adjustment should be made only when necessary- in practice rings have to be adjusted in rare cases.
Elimination of reflections in visible light range
Even choice of suitable focal length and iris does not guarantee good image. Usually, the main problems are caused by various kinds of reflections from walls, glasses, furniture etc. or windowpanes through which we observe the scene. Generally, the only possibility to remove reflections is use of polarization filters mounted on lens. Polarization filter lets in only these light waves which have the same polarization; turning filter around its axis we change also polarization of waves that go through it. Turning it round, we can find a position in which dominant polarization of reflected light is maximally suppressed, so the reflections are minimized.
Lens and infrared spectrum
Monochromatic cameras are sensitive not only in visible light but also in infrared range. It means possibility of deceiving camera automatics by infrared radiation. If the intensity of infrared light is higher than that of visible spectrum, the automatics will adjust optimal image parameters for this kind of light. It will result in false brightness of image in visible light, as well as problems with good sharpness. Elimination of this phenomenon is possible when using special filters which pass along only visible light (IR-cut). Other aspect of this phenomenon is the reverse problem - monitoring in infrared range. Then we have to use filter which lets in only infrared radiation (IR-pass). These filters are used to work with infrared lamps that illuminate an area in the case of darkness.
Pinhole lens
This kind of lens has very small diameter of outer (entrance) lens. They are used mainly for discrete observation. Cameras with these lenses can be mounted in many different ways hiding their existence (installation inside walls, suitcases, binders, ceilings, PIR or smoke detectors).
Lens with IR correction
The component lenses of the whole lens system are made of special glass that minimizes dispersion, so there is no need to correct sharpness to actual lighting conditions. Using very sensitive cameras, even when they are not dedicated for IR operation, it is recommended to employ such lenses to avoid problems with IR light spectrum (compare the "Lens and infrared spectrum" section).
Megapixel lens
Super high-resolution cameras with megapixel sensors also require adequate lenses - megapixel lenses. These high-quality lenses are characterized by better geometry, and first of all - they do not deform light waves as much as standard devices. The deformation is described by MTF (modulation transfer function) and determines sharpness of images.
Telecentric lens - it has been created to use in automatic vision systems and electronic measure systems which require high accuracy. Traditional lens are characterized by enlargement and angle distortions of image that may cause serious problems during computer analysis of an image.
Aspherical lens
Such lens characterized by very low F number value has bigger effective entrance diameter, ensuring bigger aperture and better light transmission. This lens has different profile, which compensates aberrations on lens edge. The light that is usually lost in standard lens, is being received by aspherical lens and transmitted to image sensor. This feature is especially noticeable with color cameras, because they are less sensitive than B/W ones. When we use this lens with B/W camera we will also achieve higher sensitivity of optical system than with traditional lenses.
An interesting group are lenses with built-in IR-illuminator. This solution allows for observation in normal light conditions as well as in total darkness. Prices of such sets are much lower than additional illuminators and lenses bought separately. An example and parameters of such lens are here. However, we should remember that the range of operation is shorter than that of halogen illuminator. Basic field of use for this kind of lenses is indoor monitoring (e.g. halls, drunk tanks) and observation of entrances/exits.
Selection of proper lens is the same important as selection of the camera. Inadequate quality of lens may significantly decrease potential of the whole system.
Accordingly to EN 50132-7 norm, the selection criteria should take into consideration the following factors:
  • Field of view (showed by manufacturers in specification table) - it may be decreased by too big image raster (overscan) in the monitor,
  • Light falling on image sensor in camera is determined by aperture number and transmission number of lens - these values depend on construction of the lens,
  • Internal light reflections inside lens or moire effect may significantly decrease image quality,
  • Some varifocal lenses are vulnerable to ramping phenomenon, which consists in increase of effective aperture number when increasing focal length.
CAUTION After selecting camera - lens set, it is suggested - especially in difficult environments - to test the chosen set in similar conditions as those in the final installation.
Compromise between number of cameras and observation accuracy.
Widening field of view of cameras one may decrease their number in the system. However, we should remember that widening filed of view, so reducing focal length, we also reduce capability of distinguishing small details. Therefore, lenses with short focal length should be used wisely, because it can happen that we will not be able to distinguish important details during live monitoring or from recorded video material.
The problem can be easily solved by using megapixel cameras. They allow to observe a large area, record the complete image, and, if necessary - make a digital zoom from the "live" or recorded image.