# Ghid sisteme de supraveghere - Obiective şi lentile

BASIC PARAMETERS AND FEATURES OF LENSES
Focal length
The distance from the optical center of a lens (measured along its principal axis) to the image of an object placed in infinity - measured and given in millimeters.
For a given sensor size and focal length, the viewing 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.
Iris
The mechanism of aperture regulation that changes the amount of light going through a lens and falling on the image sensor of a CCTV camera. Lenses with fixed aperture are marked as "NO IRIS".
Lens aperture or F-number
It is the indicator 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 (ratio of root 2). With increase in F-number by one step, the quantity of light passing through the lens decreases by half. In the table below we can find the proportion 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%
The relationship between F-number and the proportion of transmitted light
If the iris of a lens is completely closed, the F-number is equal infinity (it can be also marked as closed iris).
Very often we find two values of F-number - the first one defines the maximum opening of iris, the second - the minimum.
Depth of focus/field
The range of distances from the lens to objects within which the images of the objects are properly focused. Generally, depth of focus increases with closing the iris and decreasing the focal length. This rule is commonly used by photographers, who use good illumination and decrease iris (increase F-number) to achieve good depth of field. If we use auto iris lens and the illumination of a scene drops, in connection with iris opening the depth of focus decreases. Generally, we aim at such depth of focus to reach "infinity" - so that all the objects which are situated farther than some minimal distance can be seen clearly. With a high depth of field, a focus adjustment practically does not influence the sharpness of the image.
Depth of focus increases along with closing the iris. Grey area - the range of depth of field
Transmission number of lens (T)
This is the indicator of the actual ability of light transmission. T scale 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, chromatic aberration.
Camera lenses can be classified according to:
Angle of view:
• narrow-angled (tele-lens) - focal length bigger than the diagonal of the sensor,
• standard - focal length close to the diagonal of the sensor,
• wide-angled - focal length smaller than the diagonal of the sensor.
Type of iris:
• no iris: cheap lenses designed for the use with simple cameras,
• with manual iris,
• with automatic iris.
• fixed - usually the lenses have typical focal length values i.e. 2.5; 3.6; 4.0; 6.0; 12.0; 16.0; 25.0 mm,
• variable (varifocal, zoom) - suggested when the sizes of observed objects change or when an optimum focal length should be chosen experimentally - there are some typical ranges: 3.5-8 mm (1/3" sensors); 6-12 mm (1/2" sensors),
• remotely controlled (so called motor-zoom lens).
Types of lens mounts
Two types are common: C and CS. The distance from the sensor's surface to the mounting surface is 12.5 mm for CS lens, and 17.526 mm for C lens. Board cameras usually use threads with smaller diameter. More about mounts you can find here
The size of a lens versus the size of the image sensor. Manufacturers give the biggest size of the sensor that a specific lens can work with. So the lens must be adjusted to the sensor's size, but it is always possible to use a lens designed for a bigger image sensor, which can be even useful in some cases, as it helps to reduce distortions of images (distortions increase with growing distance from the axis of the lens). With the same lens, use of a smaller sensor will result in displaying smaller part of the observed area. There are lenses suitable for 1", 2/3", 1/2", 1/3" and 1/4" sensors (we may also expect lenses for 1/5" sensors).
Choice of focal length
An appropriate focal length of a lens can be calculated by the simple "Focal length calculator"
The scheme of the optical system of a camera - object, lens, image sensor - with the related parameters
If we know the image sensor format, the size of the object, and its distance from the camera, we can calculate the suitable focal length/viewing angle of the lens. On account of rectangular shape of image sensors, the calculation should be made both for horizontal and vertical plane.
Telescopes, calculators, graphs, lens selection patterns. The selection of lens can be made using patterns, calculators, graphs and special telescope called "view finder".
View finder is placed in the place where a camera is to be installed. After adjusting the rings to achieve the desired viewing angle we can read the value of focal length.
Fixed or varifocal (zoom) lens?
Usually, only after some time of the system's operation the user can precisely decide which objects should be monitored first of all. Therefore, despite a higher price, it is worth to use varifocal lens. It will allow to widen or narrow easily the field of view during operation of the system.
Iris type selection
It should be remembered that sensitivity of cameras is given for a specified F-number of a lens, e.g. 0.05 lx / F=1.2. When we use a F1.8 lens, the sensitivity will be lower, because a smaller amount of light reaches the image sensor. There are available "bright" lenses with F=1.0. Such a lens, in connection with a highly sensitive camera makes it possible to monitor areas in very bad lighting conditions. Lenses for typical conditions feature F=1.2-64. However, using a very sensitive camera, e.g. 0.01 lx, strong sunlight may overexposure the image (the 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 a less sensitive device. Other solution is using gray filters, but they reduce sensitivity at low light. In strong sunlight we may come across situations in which even the use of a lens with auto iris will not guarantee good results. The image on a monitor's screen will not have the same brightness in the whole field of view. Depending on the 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.
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 in moderately changeable light environments, with lens equipped with manual iris. If we want to use such a camera in more variable lighting conditions, i.e. from almost complete darkness to daylight, we have to carefully adjust the manual iris towards opening. We will improve sensitivity, however possibly having problems with focus depth.
There should be no problem in indoor applications. In the case of stable lighting, we we should rather close the iris, which will improve depth of field. However, it has to be made carefully, not to spoil the brightness of the image. The basic advantage of electronic iris is possibility of using simple lens with fixed aperture or with manual aperture control.
Automatic iris ("AI" - Auto Iris)
It keeps the amount of light falling on image sensor constant, no matter the illumination conditions are. Electronic shutter is to be set at 1/50 s, and AI lens is closed or opened adequately to the light intensity. The camera and AI lens are able to work properly even in very dynamic light environments. As a rule, outdoor CCTV cameras, working during the day and night, require this kind of lens. Cameras with AI are equipped with special sockets used for AI lens control. Depending on signal type in this socket, the lens closes or opens its iris keeping the amount of light falling on the image sensor at a constant level.
Auto IRIS control is described in the chapter related to cameras.
Adjustment of automatic iris (AI) parameters
The main purpose is to achieve optimum brightness during the day as well as during the night. A new AI lens is usually adjusted by the manufacturer, so additional regulation is generally not necessary. However, it sometimes happens that we have to perform this adjustment. It consists in setting Level and AGC regulators to a middle position, followed by the adjustment of a desired image brightness with the Level potentiometer. The next step is the adjustment of the ALC potentiometer, depending on the required type of reaction to light changes, and further - putting on a gray filter and setting sharpness of the image.
AI-DC lenses do not use any potentiometers
Problems with image sharpness
A good image sharpness provides the ability to distinguish small details. It is limited by the quality of the lens, resolution of the image sensor, video parameters of the camera and transmission path, and the monitor. The basic adjustment of image sharpness consists in changing the location of the optical center of the lens (along its principal axis) with reference to the surface of the image sensor. Insufficient 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 fully opened iris, using e.g. gray filter. It should be noticed that this adjustment should be made only when necessary - in practice the ring has to be adjusted very rarely.
Elimination of reflections in visible light range
Even choice of suitable focal length and iris does not guarantee good image. Very often 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 lenses. Polarization filter lets in only these light waves which have the same polarization; turning the filter around its axis we change also the polarization of the waves that go through. Turning it round, we can find a position in which the dominant polarization of reflected light is maximally suppressed, so the reflections are minimized.
Camera lenses and infrared spectrum
Monochromatic cameras are sensitive not only in visible light but also in infrared range. It means a possibility of deceiving camera automatics by infrared radiation. If the intensity of infrared light is higher than that of the visible spectrum, the automatics will adjust optimal image parameters for this kind of light. It will result in false brightness of the image in visible light, as well as in problems with good sharpness. Elimination of this phenomenon is possible when using special filters which pass only visible light (IR-cut filters). Other aspect of this phenomenon is the reverse application - CCTV monitoring in IR. Then, we have to use filters which let in only infrared radiation (IR-pass filters). These filters are used to work with infrared illuminators that illuminate the monitored area in darkness.
Special lenses
Pinhole lens
This kind of lens has a very small diameter of the outer (entrance) optical lens. The lenses 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 its optical system are made of special glass that minimizes dispersion, so there is no need to correct sharpness to IR lighting. Using very sensitive cameras, even when they are not dedicated for IR operation, it is recommended to employ such kind of lens to avoid problems with IR spectrum (compare the "Lens and infrared spectrum" section).
Megapixel lens
This kind of lenses provides higher quality with smaller geometric distortions, but first of all minimizes deformation of light waves passing through the lens. The deformation is described by MTF parameter - Modulation Transfer Function. This parameter is essential for sharpness of the images, deciding on the identification capability of megapixel CCTV systems.
Choosing a lens one has to take into consideration its exact specification for a specific group of megapixel cameras (e.g. up to 2 Mpx etc). Very high resolution of a megapixel camera cannot be spoiled by using inadequate lens.
A measure of lens resolution is the number of line pairs (lp) that can be distinguished in one millimeter. This parameter is closely related to the ability of the lens to provide image detail. The lenses for VGA cameras feature about 30 lp/mm, whereas megapixel lenses should guarantee at least 60 lp/mm.
So, megapixel cameras require to employ megapixel lenses. The newest line of megapixel lenses from Japanese Tokina company is dedicated for cooperation with megapixel cameras equipped with 1/2'' sensors of 3 Mpx resolution, but due to outstanding performance the lenses are suitable even for 5 Mpx cameras. They cover practically the whole range of focal lengths suitable for megapixel cameras: 1.4 mm (fish-eye lens - M2236 ), 4.5-13 mm M2235 (the most demanded one), and 16-48 mm M2237.
Telecentric lens - it has been created to use in automatic vision systems and electronic measurement systems which require high accuracy. Traditional lens are characterized by changeable enlargement and angle distortions of different parts of the image that may cause serious problems during measurements and computer analysis.
Aspherical lens
This lens is characterized by a very low F-number, due to larger effective entrance diameter. It ensures bigger aperture and better light transmission. This lens has special profile which compensates aberrations on its edges. The light that is usually lost in a standard lens, is received by the aspherical lens and transmitted to the 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 a B/W camera we also achieve higher sensitivity of the optical system than in the case of traditional lenses.
An interesting group are lenses with built-in IR-illuminator. This solution allows for observation in normal light as well as in total darkness. Prices of such sets are much lower than in the case of additional illuminators and lenses bought separately. However, we should remember that the range of operation is shorter than that of IR halogen or IR LED array illuminators. The basic application field for this kind of lenses is indoor monitoring (e.g. halls) and observation of entrances/exits.
Choosing a lens
The selection of a proper lens is the same importance as the selection of a camera. Inadequate quality of lenses may significantly decrease the potential and quality 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 tables) - it may be decreased by a too big image raster (overscan) in the monitor;
• The light falling on the image sensor in a camera is determined by the aperture number and transmission number of the lens - these values depend on the construction of the lens;
• Internal light reflections inside the lens or moire effect may significantly decrease image quality;
• Some varifocal lenses are vulnerable to ramping phenomenon, which consists in the increase of effective aperture number when increasing focal length.
CAUTION After selecting the camera-lens set, it is suggested - especially in difficult environments - to test the chosen set in similar conditions as those in the final installation.
The compromise between number of cameras and observation accuracy.
Widening the field of view of individual cameras, one may decrease the number of them in the system. However, we should remember that widening the filed of view, so reducing focal length, we also reduce their capability for depicting 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 the recorded video material.
The problem can be easily solved by using megapixel cameras. They allow to observe large areas, record the complete images, and, if necessary - make a digital zoom from the live or recorded video stream.