Life through the lens-2. Long-focus lenses and their features.
Continuing the review started in the previous issue of the magazine, we propose to consider in more detail the main characteristics of long-focus lenses, as well as touch upon some issues related to the features of their use in video surveillance systems (CCTV).
Long-focus lenses are designed for monitoring distant (remote) objects in cases where close-range monitoring is not possible or for the purpose of early detection of unauthorized actions or violations within the controlled area. Such lenses are also called «telephoto lenses».
First of all, it is necessary to recall the classification of lenses.
The angle of view of a TV camera depends on the physical dimensions of its photoreceiver and the focal length of the lens. According to the angle of view, lenses are short-focus (wide-angle), normal, and long-focus (narrow-angle).
Long-focus lenses are those with a focal length that exceeds the diagonal of the photodetector (matrix). For the most common matrix format of 1/3”, used in TV security cameras, the diagonal is 6 mm. Therefore, for such cameras, all lenses with a focal length greater than 6 mm are formally considered long-focus lenses.
Among long-focus lenses, we should highlight ultra-long-focus lenses with field of view angles less than 9 degrees. For example, for a photodetector format of 1/3”, these include lenses with a focal length greater than 30 mm, and for a format of 1/2”, these include lenses with a focal length greater than 40 mm.
Let's start our review of telephoto lenses with fixed focal length lenses, which are most often used in stationary TV cameras. In this class of lenses, mirror-lens lenses stand out for their unique characteristics. The main advantage of these lenses, in contrast to lens lenses, is the absence of chromatic aberrations due to a fundamentally different optical design. With the help of such lenses, you can obtain a photographic-quality image, since their optical resolution is very high. The main disadvantage is low light intensity.
In case of using a telephoto lens with a fixed focal length of a large value, an additional overview camera with a large angle of view may be required for its orientation and increasing the observation area.
Most often, lenses with a variable focal length are used in video surveillance systems — zoom lenses. They can be divided into several groups. Each group is distinguished primarily by its purpose, as well as functional capabilities, focal length range, weight and dimensions.
1. Varifocals 1/3”
10x — f (5-50) mm
20x — f (5-100) mm
Varifocal lenses with a focal length range of 5 to 50 (100) mm with automatic diaphragm. A group of lenses that is most often used in stationary TV cameras to solve most of the problems facing a video surveillance system. Allows you to select the optimal viewing angle of the TV camera when installing it directly on site. Versatility, relatively low cost, comparable to the cost of monofocal lenses, compactness and ease of setup determine the high applicability of this group. As a rule, these are lenses with a 1/3 ”format.
2. Motor ZOOM
1/4 ”
f from 3 to 130 mm
10x – 36x
Small-sized zoom lenses (both modular and in a housing) with a magnification of 10 to 36 and a focal length range of 3 to 130 mm. Most often used in high-speed dome PTZ cameras. As a rule, they are a finished product integrated with a TV camera, which has a module for telemetry control of modes. They are distinguished by their compactness, small format (1/4″) and high speed of scaling and focusing. There are models with a built-in autofocus system and a day-night mode.
3. Motor ZOOM
1/3” and 1/2”
f from 5 to 300 mm
10x – 20x
Motorized zoom lenses from 10 to 20 times and a focal length range of approximately 5 to 300 mm. They are distinguished by their medium dimensions, relatively low weight, focal length range and, accordingly, cost. They are used in TV cameras with a protective casing and are installed on a separately purchased swivel device. At large distances, TV cameras are controlled from the observation post using a special telemetry control module, at distances of up to 20 m — directly from the remote control. They exert a relatively small load on the support-swivel mechanism.
4. Motor ZOOM
1/3” and 1/2”
f from 10 to 100 0 mm
(with 2x extender)
f from 20 to 2000 mm
20x — 50x
High-power motorized zoom lenses (from 20 to 50x and higher) with a maximum focal length (in the TELE position) from 300 to 1000 mm and more. They are distinguished by their large size, weight and cost. Cameras with such lenses require a large protective casing and a swivel mechanism with a high load capacity.
To increase the focal length of lenses (scale) up to two times, extenders (optical attachments) with a magnification of 1.5 or 2 are used.
The magnification of modern zoom models can reach 30 and even 50 times. However, increasing the magnification leads to an increase in the cost of the lens and a significant loss of light intensity when increasing the scale.
Zoom lenses are used in most cases together with TV cameras and PTZ devices – this allows for surveillance in large areas. At the same time, for autonomous operation of such cameras in automatic modes (patrolling, tracking, work at pre-set positions, etc.), there is a need for position sensors of motorized electric zoom and focus drives. Practice has shown that the efficiency of controlling such cameras only by the operator is zero. A PTZ TV camera with a zoom lens in normal mode should operate autonomously according to the algorithm set by the operator in automatic mode, respond to the triggering of alarm sensors, and notify security in case of unauthorized actions. In the event of an alarm event, if necessary, the operator should be able to quickly take control of this camera.
Remote control of such lenses and rotary mechanisms is in most cases carried out by means of special telemetry equipment by sending commands of the selected protocol via a two-wire line. Such equipment can be integrated into the rotary device itself. The most common interface used for telemetry control is the RS-485 standard with the PELCO protocol.
According to the method of diaphragm control, there are the following lenses:
without diaphragm adjustment;
with manual mechanical diaphragm control;
with automatic diaphragm adjustment (ARD);
with a motorized diaphragm drive.
In some cases, it may be necessary to set a specified (or preset) diaphragm position during remote control by an operator command or during autonomous operation of the TV camera according to a program. The diaphragm operating mode that allows such control is called OVER-RIDE. In this case, the diaphragm unit drive operates according to the position sensor.
Features of long-focus optics and their differences from normal (medium-focus) lenses As is known, the aperture ratio is determined by the ratio of the effective diameter of the front lens to the focal length. In our case, an increase in the focal length and aperture ratio leads to a significant increase in the overall dimensions and weight of the lens with all the ensuing consequences. These include an increase in the size of the protective casing, windage, load on the support and rotation mechanism. With an increase in the focal length of the lens, the requirements for the rigidity and stability of the support, as well as the play of the rotation device, increase significantly, which must be taken into account when designing or selecting support structures for installing a TV camera with such optics.
It should be taken into account that the manufacturer specifies the maximum value of the relative aperture of the lens, which characterizes its light intensity with a fully open aperture at the minimum focal length. As the focal length increases, the light intensity decreases. The magnitude of the change in light intensity depends on the design features of a specific zoom lens model, so when choosing it, preference should be given to a lens in which the change in light intensity with a change in focal length is smaller. However, optics manufacturers do not mention this value, which leads to the need to determine it independently.
The quality of the image formed by the lens is determined by its clarity, which, in turn, is described by the amount of contrast between the brightest and darkest areas, depending on the size of such areas (spatial frequency). The characteristic describing this dependence is called the frequency-contrast characteristic (MCF).
To obtain an image of the highest quality, the resolution of the optical system must match the physical dimensions of the resolution elements of the TV camera photodetector. Optical resolution is measured in lines per millimeter using line test targets. With regard to the discrete structure of photodetectors built on the basis of CCD matrices, to obtain the highest possible image resolution, the optical resolution of the lens, i.e. the number of lines per mm, must be no less than the value obtained under the condition that one line (stroke) and a space between the strokes correspond to 2 resolution elements of such a photodetector. Thus, with an optical resolution of the lens of 100 lines per mm, the minimum size of the resolution element (pixel) of the CCD matrix must be no more than: 1 mm : 100 lines per mm : 2 = 0.005 mm = 5 µm.
For example, the dimensions of the resolution element of high-resolution matrices manufactured by SONY with the number of such elements 752 x 582 and the format 1/3” are 6.5 x 6.25 µm.
The ability of an optical lens to collect light radiation not only within the viewing angle, but also from side sources leads to a decrease in the contrast of the formed image due to the parasitic «illumination» of its dark areas. As the viewing angles decrease (the focal length increases), the influence of the parasitic side «illumination» increases more and more.
The contrast of an image is determined by the difference in illumination of the brightest and darkest areas of the image, and increasing the black level and decreasing the white level lead to a decrease in this difference. Contrast decreases at long focal lengths due to a decrease in the overall brightness of the image, which is a consequence of a decrease in aperture ratio.
To combat parasitic side «illumination» and glare, hoods should be used — flares with a light-absorbing inner surface that are attached to the front of the lens and block the entrance pupil of the lens from light sources located outside the field of view of the TV camera.
For each lens model, it is necessary to select a lens hood of an individual shape due to the difference in the angles of view and design. It should be remembered that the lens hood is not a mandatory accessory, as a rule, it is not included in the lens kit and is ordered separately from the equipment supplier if necessary.
The spectral dependence of the refractive index of the lens material is most noticeable in long-focus optics with small viewing angles and leads to chromatic aberrations (distortions), which appear in the image as colored edging at the boundaries of contrast areas. The clarity of the image is noticeably reduced. To combat this phenomenon, optical elements made of low-dispersion glass (with a constant refractive index in the visible radiation range) are used, which are designated by the ED (Extra Low dispersion) index.
The use of aspherical elements in the optical design of the lens (with the AS index) allows for increased image edge sharpness and aperture ratio, reduced geometric distortion and overall dimensions.
A lens with infrared (IR) correction can operate not only in the visible, but also in the near infrared region of the spectrum without additional focusing. Such lenses are intended for monochrome TV cameras and day-night cameras capable of operating in complete darkness with infrared illumination.
Thus, the quality of the image formed in the process of optoelectronic conversion using a TV camera and a lens depends on their joint operation (compatibility).
Any lens is a projection optical system, the main purpose of which is to project an image of the observed area with minimal distortion onto the photosensitive surface of the photodetector.
The most important characteristics of the «Lens — TV camera» optical system are:
optical format is the size of the formed image or matrix;
focal length determines the angle of view of the TV camera;
aperture determines the sensitivity of the TV camera;
frequency-contrast characteristic (MCF) describes the dependence of the contrast of the formed image on the spatial frequency;
spectral characteristic affects the color rendering and integral sensitivity of the TV camera;
resolution determines the minimum size of image elements distinguishable by contrast;
the introduced geometric distortions disrupt the geometric similarity of the image of the observed object.
The purpose of the TV camera and its operating conditions determine the priority requirements for one or another characteristic of the lens.
When performing television surveillance from long distances using lenses with a focal length of more than 500 mm (1000 and 2000 mm), problems may arise related to the characteristics of the air environment, its movement (heat flows, wind), as well as dust, fumes, fog, precipitation in the form of rain, hail and snow. Therefore, when designing a television video surveillance system, these natural factors should be taken into account, which can significantly reduce its detection ability.
In conclusion, we note that long-focus lenses, and in particular zoom lenses, are expensive, highly complex equipment that requires proper use, as well as additional technical measures aimed at maximum use of the capabilities of such optics.
The following parts of the review will consider the features associated with the control of motorized zoom lenses, as well as modern positioning systems that can significantly expand the functionality of video surveillance systems using such optical systems.
1/3-inch long-focus zoom lens with ARD Smartec STL-5055DC
The STL-5055DC varifocal lens by Smartec has automatic aperture control and stands out for its wide focal length range. It is made in Japan and is designed to work with 1/3-inch CCTV cameras with a CS-type mounting thread. The focal length of the STL-5055DC is manually adjusted from 5 to 55 mm, and the relative aperture is automatically adjusted from F1.4 to F360 under the control of a DC signal. Its horizontal viewing angle can be adjusted from 4.8 ° to 51.3 °. Unlike a number of analogues, this varifocal lens provides exemplary image quality without geometric and chromatic aberrations, since its design uses aspherical lenses and optical elements made of low-dispersion glass. The STL-5055DC weighs no more than 140 g and has small dimensions for long-focus models — 42 x 50 x 64 mm.
Computar TG10Z0513FCS (CBC, Japan)
It is one of the most popular lenses in the security systems market. It has a focal length that changes by 10 times (5–50 mm) and provides a viewing angle change of 57.6°~5.6° for cameras with a matrix size of 1/3″.
This is the most compact 10:1 varifocal lens on the CCTV market today. The maximum aperture of F1.3 combined with aspherical optics provides the lens with excellent performance in low light conditions. Locking screws allow you to fix the focus and focal length adjustment rings, which eliminates their accidental shift.
The TG10Z0513FCS lens is supplied with a 9 or 31 cm cable and a mounted 4-pin auto iris connector.
TVR1618DC (f=16–160 mm F/1.8-360 DC IRIS Vari-focal lens) (Tokina, Japan)
Long-focus varifocal lens TVR1618DC with a focal length of 16–160 mm. The lens is compatible with 1/2-size matrices, which allows it to be installed on many types of video cameras.
TVR1618DC is ideal for monitoring traffic on highways, railways, factories, ports and airports. Thus, at maximum zoom with a 1/3 format video camera, you can place a small car in full screen from a distance of 56.7 m to the object. To eliminate parasitic light effects and increase the light transmission coefficient, multi-coating technologies are used in the optical system.
In light of the trend of popularity of IP video cameras, this lens has advantages over zoom lenses, since some IP cameras do not have a back focus adjustment mechanism, and only varifocal lenses are suitable for such cameras. The lens is equipped with a mounting support to reduce the load on the camera's mounting ring.
YV3.3x15SA-SA2 (Fujinon, Japan)
Varifocal megapixel lens with ARD, 1/3″ – 1/4″, f = 15–50 mm, F = 1.5, viewing angle (1/3″, deg: 18.08 x 13.34; 5.35 x 4.12)( 1/4″, deg: 13.34 x 10.10, CS, magnification factor – 3.3). The long-focus megapixel model YV3.3x15SA-SA2 with a focal length of 15–50 mm has a horizontal viewing angle from 5° to 18° (at 1/3″) and provides the greatest proximity to the observed object.
Megapixel lenses are recommended for installation on IP cameras that perform video surveillance in large areas, outdoors, and at sites with excessive, insufficient, uneven, and variable illumination.
The lens uses lenses made of low-dispersion glass with a high refractive index, which reduces light loss when rays pass through the lens, and more light ultimately reaches the megapixel camera matrix. Such optical elements have a noticeably smaller thickness, so the megapixel lens has compact dimensions and light weight, which allows them to be installed even in the most common models of dome housings on the market.
Motorized 25x Zoom Series LMZ250, LMZ375 and LMZ750 (Kowa Company Ltd., Japan)
The use of low-dispersion glass (ED) reduces chromatic aberrations and increases image clarity. 3-CAM DESIGN technology ensures focusing stability and high reliability of mechanical components at temperature changes.
The lenses of this series are distinguished by high aperture ratio, low light scattering, minimal internal reflections, excellent image quality, Japanese reliability and quality, and competitive cost.
Optical format – ¼ inch. Focal length range: LMZ250 – 10–250 mm; LMZ375 – 15–375 mm; LMZ750 – 30–750 mm.
Relative aperture (F-number): LMZ250 – 1.5; LMZ375 – 2.3; LMZ750 – 4.6.
Lens design options and markings: Direct Drive AM-DC, AMP-DC ARD (with position sensors); Video Drive AM, AMP ARD (with position sensors); motorized diaphragm; Over-ride AM-OR, AMP-OR mode (with position sensors).
1/3” long-focus zoom lenses Spacecom DRACO TV555M IR with RD
Long-focus day-night varifocal lenses with manual diaphragm are produced under the Japanese brand Spacecom and are compatible with 1/3-inch TV cameras of any manufacturer. They allow you to adjust the focal length in the range from 5 to 55 mm, have 11x optical zoom, the maximum possible horizontal viewing angle of 53 degrees, aperture ratio F1.4 and allow the camera to carry out high-quality round-the-clock video surveillance. TV555M IR is equipped with photochromic lenses made of low-dispersion optical glass ED Glass (Extra-low Dispersion). These lenses equalize the refraction angles of waves of different lengths and change their light transmission depending on the illumination in the video surveillance zone without the effect of image defocusing.
Vari-focal lenses 13VM20100AS/13VG20100AS (Tamron, Japan)
Vari-focal lenses with a focal length of up to 100 mm provide an advantage when capturing a precision target, such as a car license plate, at a long distance. When used in video surveillance systems, for example, in cities, the lenses provide high image quality when tracking at long distances in a variety of applications. Due to the reduction of aberrations and the use of spherical elements in the lenses, the lenses have become more compact.
Compatible matrix size — 1/3, mount — GS-type. Focal length: 20-100 mm. Aperture range: — F/1.6-Close F/1.6-360. The lenses use manual (13VM20100AS) and DC (13VG20100AS) apertures.
