What is the selection standard of camera lens?

Lead: What is the selection standard of camera lens? On the whole, the market needs an integrated digital multimedia monitoring system. According to users' security needs and their own civil air defense situation, a security system that meets the actual needs is formed to realize video and audio monitoring, control and record storage. DVR products are fixed as a universal digital monitoring core by some manufacturers in the marketing process.

What is the selection standard of camera lens? 1 1. Technical basis for lens selection.

1, focal length

The focal length of the lens is in millimeters, which is mutually causal with the observation angle obtained by the lens. The small focal length lens has a large observation angle, while the long focal length lens has a telescopic effect and a narrow observation angle. Usually, lenses with similar visual angle to human eyes are called ordinary standard lenses. COMPUTAR's slide rule is a convenient tool to estimate the focal length, the size of the observed object and the viewing angle. In addition, the VM300 viewfinder is also an optical device for selecting focal length.

2. Camera specifications

The size of the camera image sensor (CCD) will also affect the viewing angle. In the case of using the same lens, the smaller the CCD, the smaller the viewing angle. The requirement of lens specification is that the image it forms can cover all CCD, such as:

Use a lens with the same specification as the camera or a lens with a larger specification than the camera. This also means that 1/3 camera can use the lens in the whole range of 1/3- 1 inch, and the camera with 1/3 8mm lens can get the same viewing angle as that with 2/3 8mm lens. Only when the latter lens is used can it provide better image quality and higher resolution, because it makes more use of the central optical path of the lens with more accurate molding.

3. Aperture value

Lens is usually measured by lens aperture or aperture value, which is defined as the maximum aperture (minimum aperture value) when the lens is fully opened and the minimum aperture (maximum aperture value) when the effective aperture of the lens is adjusted to the minimum (incomplete closure). Aperture value has many effects on the final image. A small aperture value means that the lens can pass more light in a dark environment, so that the camera can present a better image. However, in the high brightness or reflective environment, the lens with high aperture value will avoid the camera imaging "whitening" and maintain a stable video level. All automatic apertures are equipped with neutral opaque filters to increase the aperture value. Aperture value also has a direct impact on depth of field.

4. Depth of field

The area that can be clearly imaged in the whole observation area is defined as the depth of field. Large depth of field means that more areas can be clearly imaged from the observation target near the lens to the target at infinity, and the whole area is usually from the observation target near the lens to infinity. A small depth of field means that only a few areas can be clearly imaged. The scenery is deeply influenced by many factors. Generally, the depth of field of a wide-angle lens is larger than that of a telephoto lens, and the larger the aperture value, the larger the depth of field range. For automatic aperture lens, the constant adjustment of aperture also means the corresponding change of depth of field. At night, the automatic aperture is fully opened, and the depth of field reaches the minimum, which also means that the target clearly observed during the day may become blurred at this time.

5, automatic and manual aperture

Usually, we tend to use an automatic aperture lens outdoors where the light changes greatly, and a manual aperture lens indoors where the light is basically constant. With the electronic shutter of the camera, electronic compensation can be carried out, and now it can also be used in places where the light changes. However, some factors need to be considered in this form of use, and the setting of aperture value becomes more strict. If the aperture is opened to the maximum in order to make the camera work at night, the depth of field will be very small, and it is difficult to get a clear image even during the day. Although the camera can keep the output level stable, it cannot keep the depth of field accordingly. If the aperture is lowered to increase the depth of field, the performance of the camera in dark environment will be compromised.

6, video drive and direct drive

Automatic aperture lens must control the operation of aperture to achieve the best image effect.

Video-driven automatic aperture lens contains magnification, which can convert the video signal of the camera to control the aperture.

Direct-drive automatic aperture lens is cheaper than video-driven lens because it only contains galvanometer aperture motor, but the camera it uses must have built-in amplification circuit. The basis of choosing two kinds of automatic aperture lenses is what kind of automatic aperture driving form the camera used has. Usually in today's situation, the camera has two driving forms.

7. Type C and CS interfaces

Nowadays, cameras and lenses are usually CS-type interfaces, and CS-type cameras can connect C-type and CS-type lenses. Once the C-lens is connected, a 5mm connecting ring needs to be added between the camera and the lens to get a clear image. The camera with C interface cannot be connected with the lens with CS interface, because it is practically impossible to get a clear image by getting the image of the lens close to CCD.

Second, the basic concepts

1. Imaging plane: The imaging plane is the plane where incident light is imaged through the lens. This plane is a circle.

2.CCD chip: a device used in a camera to convert optical signals into electrical signals. There are often 1/2 ",1/3" and 2/3 "in the camera manual, which refer to the diagonal length of CCD chip in inches.

3. Focal length: it is the distance from the lens to the imaging surface, in millimeters. ..

4. Angle of view: it is the angle of view, that is, how wide the lens can "see".

5. Minimum working distance: refers to the shortest distance from the lens to the object that can be seen clearly.

6. Field of view: refers to the effective working area that the lens can cover.

7. Depth of field: refers to the depth range that the lens can image.

Third, the classification of shots.

1, according to shape function, according to size, according to aperture, according to zoom type, according to focal length and moment.

Spherical lens 1" 25mm automatic aperture electric zoom long focal length lens

Aspheric lens 1/2" 3mm manual aperture manual zoom standard lens

Pinhole lens1/3 "8.5mm fixed aperture fixed focal length wide-angle lens.

Fisheye lens 2/3 inch17mm

2. According to the lens installation classification

There are two kinds of lens installation methods: C-type installation and CS-type installation.

The lens commonly used in TV monitoring system is C-type mounting lens (in32 threaded seat), which is an internationally recognized standard. The aperture of the lens installation part is 25. The distance from the lens mounting datum plane to the focus is 17. 526 mm The lens interfaces of most cameras are CS-type, so when the C-type lens is installed on the camera with CS interface, a 5 mm thick collar is needed, while when the CS lens is installed on the camera with CS interface, there is no collar. In practical application, if the CS lens is installed on the CS interface camera by mistake, because the imaging surface of the lens cannot fall on the CCD target surface of the camera, a clear image will not be obtained. However, if the C lens is directly connected to the CS interface camera without a ring, the rear mirror of the lens may touch the protective glass of the CCD target surface, causing damage to the CCD camera. We should pay special attention to this point in practice.

3. Classification by camera lens specifications

The camera lens specification depends on the CCD size of the camera, and the two should correspond. That is, when the CCD target size of the camera is 1/2 inches, the lens should be 1/2 inches. When the CCD target size of the camera is 1/3 inches, the lens should be 1/3 inches. When the CCD target size of the camera is 1/4 inches, the lens should be 1/4 inches. If the lens size is inconsistent with the CCD target size of the camera, the observation angle will not meet the design requirements, or the picture will be out of focus.

4. According to the lens aperture, the lens can be divided into manual iris and automatic iris.

When used with a camera, the manual aperture lens is suitable for the application with constant brightness, and the aperture of the automatic aperture lens is automatically adjusted when the brightness changes, so it is suitable for the application with changing brightness.

There are two types of automatic aperture lenses:

One is to control the aperture on the lens by transmitting video signals and power from the camera to the lens, which is called video input type, and the other is to directly control the aperture by using the DC voltage on the camera, which is called DC input type.

ALC (Automatic Lens Control) adjustment on the automatic aperture lens is used to set the metering system, and the reference signal intensity can be set for the average brightness of the whole picture or the brightest part (peak) in the picture for automatic aperture adjustment. Generally speaking, ALC has been set in the factory, so it can be adjusted. However, when the shooting scene contains extremely bright targets, the image of bright targets may cause "white clipping" and make the whole screen white. At this time, ALC can adjust and change the picture. In addition, the automatic aperture lens is equipped with an aperture ring. When the aperture ring rotates, the luminous flux passing through the lens will change. The luminous flux is the aperture, which is generally expressed by f, and its value is the ratio of the focal length of the lens to the aperture of the lens, that is, F=f (focal length) /D (actual effective aperture of the lens). The smaller the f value, the larger the aperture. The use of automatic aperture lens is an ideal choice for the following applications: in very bright occasions such as direct sunlight, using automatic aperture lens can have a wide dynamic range. When the whole field of vision is well focused, the depth of field of the automatic aperture lens is greater than that of the fixed aperture lens. Under strong light, when the blur caused by optical signal is minimal, it is required to use automatic aperture lens.

5, according to the size of the lens field of view classification.

Standard lens: the viewing angle is about 30 degrees. In 1/2-inch CCD camera, the focal length of the standard lens is 12 mm, and in 1/3-inch CCD camera, the focal length of the standard lens is 8 mm. Wide-angle lens: the viewing angle is above 90 degrees, and the focal length can be less than a few millimeters, which can provide a wider field of vision. Telefocal lens: the viewing angle is less than 20 degrees, and the focal length can reach several meters or even dozens of meters. This kind of lens can enlarge the influence of the photographed object at a long distance, but make the observation range smaller.

Zoom lens: also known as telescope head, there are two kinds of zoom lenses: manual zoom lens and electric zoom lens.

Zoom lens: it is between the standard lens and the wide-angle lens, and its focal length is continuously variable, which can enlarge distant objects and provide a broad field of vision, thus increasing the monitoring range.

The zoom lens can be set to the minimum focal length and the maximum focal length by auto-focusing, but the focus from the minimum focal length to the maximum focal length needs to be achieved by manual focusing.

Pinhole lens: The lens is several millimeters in diameter and can be concealed.

6, from the lens focal length.

Short focal length lens: Because of the large incident angle, it can provide a wider field of vision.

Medium focal length lens: standard lens, the focal length depends on the size of CCD.

Long focal length lens: Due to the narrow incident angle, it can only provide a narrow field of vision, which is suitable for remote monitoring.

Zoom lens: usually electric, it can be used as a wide angle, standard or telephoto lens.

Four, zoom lens (zoom lens)

1. Zoom lenses can be divided into two categories: manual telescopic lens and automatic telescopic lens.

Telescope head is often called zoom lens because it can change the focal length of the lens within a certain range, so it can enlarge or reduce the monitored object.

Typical optical magnification specifications are 6 times (6.0~36mm, F 1.2), 8 times (4.5~36mm, F 1.6), 10 times (8.0~80mm, f1. In order to increase the magnification, electronic digital amplification can be applied in addition to optical amplification.

In the electric telescope head, there are three kinds of aperture adjustment, namely: automatic aperture, DC-driven automatic aperture and electric aperture adjustment.

There are only two ways to adjust focus and zoom: electric adjustment and preset. The electric adjustment is driven by the motor in the lens, and the preset is to set the adjustment stop position in advance through the potentiometer in the lens, which can avoid the process that the imaging must be adjusted one by one, and the positioning is accurate and fast. In the integrated camera system with spherical cover, the telescope head with preset position is mostly used.

Another thing that interests users is the fast focus function, which consists of a focus measurement system and an electric zoom feedback control system.

2. Relationship between camera lens and CCD size

1/2 "lens can be used for either 1/2" camera or 1/3 "camera, but the viewing angle will be reduced by about 25%. 1/3 "lens cannot be used for 1/2" camera, but only for 1/3 "camera.

Verb (abbreviation of verb) the scope of application of different kinds of lenses

1, application range of manual and automatic aperture lenses

Manual aperture lens is the simplest lens and is suitable for relatively stable lighting conditions. Manual aperture is composed of several pieces of metal. The luminous flux is regulated by a ring on the outer diameter of the lens. Turn this circle to make the aperture smaller or larger.

Automatic aperture lens should be used in the environment where lighting conditions change greatly or are not used to monitor fixed targets. For example, in outdoor or places where artificial lighting is often switched on and off, the aperture of the automatic aperture lens is driven by a motor, which is controlled by the video signal of the camera.

Manual aperture lens and automatic aperture lens are divided into fixed focal length (aperture) lens, automatic aperture lens and electric zoom lens.

2, fixed focal length (aperture) lens

Generally matched with electronic shutter camera, it is suitable for indoor monitoring of fixed targets.

Fixed focal length lenses are generally divided into long focal length lenses, medium focal length lenses and short focal length lenses.

The medium focal length lens is a lens with a focal length similar to the imaging size;

A lens with a focal length smaller than the imaging size is called a close-up lens, and a close-up lens is also called a wide-angle lens. The focal length of this lens is usually less than 28 mm, and the short-distance lens is mainly used in situations with poor ambient lighting conditions and large monitoring range.

A lens with a focal length greater than the imaging size is called a long focal length lens, also known as a telescope head. The focal length of this kind of lens is generally above 150mm, which is mainly used to monitor distant scenes.

3. Manual aperture lens can be matched with electronic shutter camera, which can be used in all kinds of light.

4, automatic aperture lens, (EF) can be matched with any CCD camera, can be used in all kinds of light, especially suitable for places where the brightness of the monitored surface changes greatly and the range is large. In order to avoid halo phenomenon and burn the target surface, an automatic aperture lens is usually equipped.

5, electric zoom lens, can be matched with any CCD camera, can be used for all kinds of light, zoom lens is through the remote control device to focus light, aperture open, change the focal length.

Six, focal length calculation:

1 formula calculation method: the calculation of the field of view and focal length refers to the size of the subject, which is determined by the distance from the lens to the subject, the focal length of the lens and the required imaging size.

The focal length of the lens, the size of the field of view and the distance from the lens to the subject are calculated as follows:

f=wL/W 2、f=hL/h

f； focal distance

W: the width of the image (the imaging width of the object on the ccd target surface)

W: the width of the theme.

L: the distance from the subject to the lens

H: image height (imaging height of the object on the ccd target surface) and field of view height (shooting scene)

H: the height of the main body

Ccd target size: mm specification

Specification 1/3 inch 1/2 inch 2/3 inch 1 inch

W 4.8 6.4 8.8 12.7

H 3.6 4.8 6.6 9.6

Because the width and height of the camera picture are the same as those of the TV receiver, the ratio is 4:3. When l is constant and h or w increases, f decreases, and when h or w is constant and l increases, f increases.

Extension: camera lens

classify

Divide by structure

Fixed aperture fixed focus lens: simple. The lens has only one focus adjustment ring that can be manually adjusted, and rotating the adjustment ring left and right can make the image imaged on the CCD target surface clearest. Without the aperture adjusting ring, the aperture cannot be adjusted, and the luminous flux entering the lens cannot be changed by changing the lens factor, only by changing the illumination of the field of view. Simple structure and low price.

Manual aperture fixed-focus lens: Compared with the fixed-focus lens, the manual aperture fixed-focus lens has an aperture adjustment ring, and the aperture range is generally from F 1.2 or F 1.4 to full closure, which can easily adapt to the illumination of the shooting scene, and the aperture adjustment is done manually'. The illumination is relatively uniform and the price is relatively cheap.

Automatic aperture fixed-focus lens: Add a gear-driven micromotor to the aperture adjusting ring of manual aperture fixed-focus lens, and lead out 3 or 4 core shielded wires from the driving circuit and connect them to the camera automatic aperture interface seat. When the luminous flux entering the lens changes, the charge generated by the CCD target surface of the camera also changes, so that the video signal level changes, and a control signal is generated and transmitted to the automatic aperture lens, so that the motor in the lens rotates forward or backward accordingly, thus completing the task of resizing.

Manual aperture fixed focus lens: the focal length is variable, and there is a focal length adjustment ring, which can adjust the focal length of the lens within a certain range. The variable magnification is generally 2 ~ 3 times and the focal length is generally 3.6 ~ 8 mm In practical application, the market angle of the monitored market can be easily selected by manually adjusting the zoom ring of the lens. However, it is inconvenient to manually adjust the zoom frequently when the camera installation position is fixed. Therefore, after the completion of the project, the focal length of the manual zoom lens is rarely adjusted. It only acts as a fixed-focus lens.

Automatic aperture electric zoom lens: Compared with automatic aperture fixed focus lens, two micro motors are added, one of which is connected with the zoom ring of the lens to control the focal length when the lens rotates; The other motor is connected to the focus ring of the lens, and when it is controlled to rotate, the focus of the lens can be completed. However, due to the addition of two motors and the increase in the number of lens groups, the volume of the lens has also increased accordingly. Compared with the automatic aperture electric zoom lens, the electric three-variable lens only changes the control of the aperture adjusting motor from automatic control to manual control by the controller.

Sometimes, division

According to the size of the field of view, it can be divided into: small field of view lens, ordinary lens (about 50 degrees), wide-angle lens and ultra-wide-angle lens (100- 120 degrees).

Standard lens: the viewing angle is about 50 degrees, which is also the viewing angle that a person can see with one eye without turning his head and eyes, so it is also called standard lens. The standard lens focal length of a 5mm camera is mostly 40mm, 50mm or 555 mm.. The standard lens focal length of 120 camera is usually 80mm or 75mm. The larger the CCD chip, the longer the focal length of the standard lens.

Wide-angle lens: the viewing angle is above 90 degrees, which is suitable for shooting scenes with close distance and large range. It can deliberately exaggerate the foreground to show a strong sense of distance, that is, perspective. A typical wide-angle lens of a 35mm camera has a focal length of 28mm and a viewing angle of 72 degrees. The 50,28mm lens of 120 camera is equivalent to the 35,28mm lens of 40mm camera.

Telephoto lens: Suitable for shooting distant scenes. Small depth of field is easy to make the subject with blurred background stand out, but it is huge and difficult to focus on the dynamic subject. The long focal length lens of a 35mm camera is usually divided into three levels. The lens below 135mm is called the medium focal length, and 135-500mm is called the long focal length, 500mm.

It's called ultra-long focal length. 150mm lens of 120 camera is equivalent to 105mm lens of 35mm camera. Because the lens with long focal length is too heavy, there is a design of telescope head, which is to add a negative lens behind the lens and move the main plane of the lens forward to get the lens body with short focal length.

Reflective telescope head: it is another design of super telescope head, which uses a mirror to image, but due to design reasons, it is impossible to install an aperture, so the exposure can only be adjusted through the shutter.

Marco lens: In addition to shooting extremely close macro photography, you can also shoot from a distance.

Interface type to be divided

C-lens: the focal length of the flange is the distance from the mounting flange to the convergence point of parallel light incident on the lens. The flange focal length is 17.526mm or 0.690in. Mounting rib: diameter 1in, 32 teeth. The lens can be used for a linear sensor with a length of 0.5 12in (13mm). However, due to geometric deformation and market angle characteristics, it is necessary to distinguish whether the short-focus lens is suitable or not. For example, a lens with a focal length of 12.6mm should not use a linear array with a length greater than 6.5 mm If the distance between the lens and the array is determined by the focal length of the flange, then when the magnification of the object is less than 20 times, a lens connecting ring should be added. A connecting ring is added behind the lens to increase the distance from the lens to the image, which means that the focus range of most lenses is 5- 10%. The lens contact distance is the focal length/object magnification.

Cs-type lens: With a 5mm adapter ring, the C-type lens can be used on CS-type cameras.

U-shaped lens: a lens with variable focal length. The flange focal length is 47.526mm or 1.79 13in, and the mounting rib is M42× 1. It is mainly designed for 35mm camera applications (such as domestic and imported 135 camera lenses) and can be used for any array with a length less than 1.25in(38. 1mm). Short focal length lenses are not recommended. 4 42mm lens 3 L fixed focal length large field of view lens was originally designed for photographic amplification (such as the lenses of various domestic magnifying glasses), and it has good characteristics at a field of view angle of 2.25in(63.5mm). The focal length of the flange is a function of the specific lens. The mounting thread is M39× 1.0. It can be used for arrays with a length less than 1.25 inches (35. 1), and there is no restriction.

Special lens: such as micro-magnifying system. Pay special attention to the difference between CS and C, and customize the adapter ring when connecting different types of cameras and different types of lenses. It's expensive abroad, about 50 dollars each, so it's better to process it yourself.

main feature

Lens magnification (~x zoom)

The combination of optical zoom lens and digital zoom lens produces the maximum magnification.

Resolution (~ TV line)

This value represents the camera's ability to handle details. The resolution test is completed by a test chart composed of more and more dense line clusters. The value of resolution is the maximum value of the area where the dense line can distinguish the lines in the camera image. The resolution is often expressed by TV scanning lines. The higher the value, the better the camera quality.

focal distance

Refers to the distance between the focus of parallel light passing through the lens and the lens.

hum

Used to zoom in and out of the theme.

Focus (focus)

Used to fine-tune objects to achieve the best visual effect.

Main parameter

Clarity/clarity

Sharpness refers to the sharpness of every detail shadow and its boundary on the image. Usually, the resolution above 720P is called HD camera.

intensity of illumination

Illuminance refers to the degree to which an object is vertically irradiated, expressed by the luminous flux received per unit area and expressed by lux (LX), that is, lm/m2.

Electronic shutter

Electronic shutter is to use electronic technology to control the generation and transfer of charge on CCD chip in time, so as to get the "shutter" effect. Can cooperate with the lens to expand the photographic range of the camera.

white balance

White balance is an index to describe the white accuracy of the mixed red, green and blue primary colors in the display.

Backlight compensation

The essence of backlight compensation is to adjust the gain of the camera and the charge accumulation time of CCD according to the brightness of the image.

Wide dynamic

Wide dynamic technology is a technology used to let the camera see the image features in a very strong contrast.

Strong light suppression

Strong light suppression is the use of DSP technology, simply speaking, it is to weaken the strong light part and brighten the dark light part to achieve light balance.

Clean the lens

The lens is particularly easy to get dirty, but the dust on the lens often cannot be wiped off. There is a protective film on the surface of the lens, and frequent wiping of the lens will destroy these tissues. If the lens is stained with dust, do not wipe it with towels, paper towels and other items. You can use a "leather tiger" (that is, blowing balloons) to blow the dust off the lens surface with air. Sometimes with air balls, the dust can't be blown off. If there is not a lot of dust on the lens, the dust will not affect the image quality.