Lens selection affects video quality

No matter what kind of monitoring project is applied, the quality of video quality is the core factor of the evaluation lens. If you use a good camera with a good lens, you cannot get a high-quality monitor. Similarly, if a good camera is equipped with a bad camera, then there will not be a good video display. High-quality lenses not only have excellent performance in image sharpness, contrast, brightness, and color reproduction degree, but also have low distortion of picture distortion. The main parameters of the monitoring lens include focal length and aperture coefficient. At the same time, the lens selection should pay attention to the influencing factors of video quality.

Monitor the main parameters of the lens

1, focal length

We know that the parallel fiber will converge to a focal point through the convex lens. The length of this focal point from the center of the lens is the focal length. Existing lenses can generally be regarded as a convex lens group. For a lens with a fixed focal length, ie, a fixed-focus lens, a lens whose focal length can be adjusted to change is a zoom lens.

The size of the focal length determines the size of the viewing angle, the focal length is small, the angle of view is large, and the observed range is also large, but the distant objects are not clearly distinguished; the focal length is large, the viewing angle is small, and the observation range is small. As long as the focal length is chosen properly, even distant objects can be seen clearly. Since the focal length and the angle of view are one-to-one correspondence, a certain focal length means a certain angle of view. Therefore, when selecting the lens focal length, it is necessary to fully consider whether the observation details are important, or whether there is a large scope of observation. If you want to see the details, choose a long-focus lens; if you look at a close-up scene, choose a small-focus wide-angle lens.

The size of the focal length not only determines the size of the field of view, but also affects the depth of field of the image to some extent. Understanding the depth of field We can talk about “focusing”. When adjusting a high-definition surveillance camera, the process of clearly imaging a scene at a distance from a high-definition surveillance camera by adjusting the camera lens is called focusing. The point where the scene is located is called the focus point.

Depth of field content is the sum of the areas that guarantee the clear image of the scene before and after the focus. It is called the depth of field. It means that any scene within this range can be clearly photographed. The depth of field, first of all, is related to the focal length of the lens. A lens with a long focal length, a small depth of field, and a short focal length have a large depth of field. Second, the depth of field is related to the aperture. The smaller the aperture, the greater the depth of field. The greater the aperture, the greater the depth of field. In general, the foreground depth is less than the depth of field, that is, after precise focusing, the scene in front of the focus point can be clearly imaged only for a short distance, while the scene behind the focus point is clear for a long distance. .

Standard lens: The angle of view is about 30 degrees. In the 1/2 inch CCD camera, the standard lens focal length is set to 12mm. In the 1/3 inch CCD high-definition surveillance camera, the standard lens focal length is set to 8mm, and it is generally used in aisles and Community perimeter and other places. Wide-angle lens: The angle of view is more than 90 degrees, and the focal length can be less than a few millimeters, providing a wider view. Telephoto Lens: Within 20 degrees of viewing angle, the focal length can reach several meters or even tens of meters. This lens can enlarge the effect of shooting objects at a long distance, but make the observation range smaller.

2. Optical coefficient

The aperture coefficient, or aperture index, is represented by F and is measured by the ratio of the lens focal length f and the clear aperture D. Each lens is marked with a maximum F value, for example 6mm/P1.4 represents a maximum aperture of 4.29 mm. The luminous flux is inversely proportional to the square of the F value. The smaller the F value, the greater the luminous flux. The index of the aperture index sequence on the lens is 1.4,2,2.8,4,5.6,8,11,16,22, etc. The rule is that the exposure amount of the previous standard value is exactly the value of the exposure value of the next standard value. Times. In other words, the clear aperture of the lens is 1/1.4, 1/2, 1/2.8, 1/4, 1/5.6, 1/8, 1/11, 1/16, 1/22 respectively. The previous value is The root value of the latter value is 2 times. Therefore, the smaller the aperture index is, the larger the clear aperture is, and the larger the illumination on the imaging target surface is.

The high-definition surveillance camera lens aperture of the video surveillance system has manual (MANUALIRIS) and automatic iris (AUTOIRIS) points. The manual iris is suitable for occasions where the brightness does not change much. The amount of light entering it is adjusted by the aperture ring on the lens, and it can be adjusted once and for all. The automatic iris lens automatically adjusts the clear aperture as the light changes, ensuring that the brightness entering the high-definition surveillance camera is relatively stable, and is used in occasions where lighting, such as outdoor and population changes, is frequent and frequent.

Automatic aperture lenses currently fall into two categories: one is called video (VIDEO)-driven, and the lens itself contains an amplifier circuit that converts the video amplitude signal from the camera into control of the iris motor. The other type is called a direct current (DC) drive type and uses the DC voltage on the camera to directly control the aperture. This lens contains only a galvanometer iris motor and requires an amplifier circuit inside the camera. For various types of auto iris lenses, there are usually two adjustable knobs, one is ALC adjustment (light metering adjustment), there are two options for the peak light metering and the average light metering according to the target lighting conditions, generally take the average light metering file; The other is LEVEL adjustment (sensitivity), which can brighten or darken the output image.

The ALC (Automatic Lens Control) adjustment on the auto iris lens is used to set the metering system. The average brightness of the entire screen or the brightest part (peak) of the screen can be used to set the reference signal intensity for automatic iris adjustment. In general, ALC has been set at the factory and can be adjusted without any adjustment. However, when a scene contains a very bright target, the image of the bright target may cause “white clipping”. Makes all screens white, and ALC can be adjusted to change the screen.

Lens selection affects video quality

In connection with the camera, the lens selection should include the following items.

The smaller the F value, the higher the luminous flux. If the illuminometer provided by the camera itself has a certain requirement on the F value of the light flux, the engineering personnel need to realize that the light flux of the lens must be larger than the light flux of the camera. If the camera is F1.4, at least F1.4 should be used. Or F1.2 lens can match it;

Lens mount matching. That is, C and CSMount must match each other. Of course, most of the current use of 5mm adapter ring under this problem is not big, but in the installation process an endless stream of faults (this part will be explained to the reader in the installation section);

The range of the aperture is large, that is, the larger the range of f values ​​from small to large, the better. This enables the camera equipped to adapt to ambient light conditions to cope with any changes in lighting conditions; the lens coating should be uniform. At present, the coating of higher quality lenses is a multilayer composite coating. The composite coating can effectively suppress the back light and light scattering so that the camera can avoid the adverse effects caused by light;

With aspherical lens matching applications, the overall average degree of camera screen light is greatly improved with aspherical mirrors regardless of millions or non-million camera applications. Because the non-spherical lens, in the sharpness, contrast, the brightness of the screen than the spherical lens have a good effect; the best choice for a focal length, depth of field or aperture has a fixed locking adjustment screw. Because the lens is used in a variety of different environments and under fixed conditions, if there is such a device, image quality changes due to camera shake can be avoided.

Selecting a camera that is easy to install and has a size and structure suitable for the lens. A lens that is too large or too small will cause inconvenience to the installation or adjustment of the camera. In particular, lenses with more complex structures need to be used with care; the selection of infrared lenses is not absolute. , Be cautious when using (Day&Night) day/night cameras and (Low-Light) low-light cameras. The ICR camera does not have a preference for IR lenses. On the contrary, under black and white conditions, the IR lens may be an excessive functional waste.

The (LD low-scattering optical lens) lens is selected by the camera's sensing range of the infrared wave field. Infrared sensing wavelength of general IR is about 300-750nm, but if there is a need for fog, the camera and lens must reach the range of 700-950nm at the infrared sensing wavelength, because only the wavelength reaches this range can it be played. Fog effect;

Lens on the camera must also pay attention to the matching of the caliber, in the past a simple 1/2, 1/3 and 1/4 inch diameter is not a law, with the development of CMOS sensor chip, more and more the size of the image The principle of caliber appears, and the matching principle of the lens is larger than the caliber of the camera. The principle remains unchanged. However, this principle has caused erroneous selection applications in engineering.

For engineers, the above-mentioned lens is crucial for several important reference factors when selecting a camera with a camera. In the same way, the problems that often occur in the installation of lens engineering and the mistakes that can be made are not to be underestimated.