How to choose a camera module
August 5, 2021
Overview of how to choose a camera module
It took a few days to hand over the video capture + image processing board to the factory, and then wait for the return of the board, then write the driver, plug the hardware into the PC, and install and debug the driver. If all goes well If so, the remaining problem is that the host computer and the camera are driven. I have used the camera driver before. The host computer has a headache after thinking about it. I don't plan to do it myself. Then I will find a great god to help me make one. It may be done in minutes.
This news article introduces the knowledge related to the camera.
For the camera, there are three most important parameters: focal length f, maximum relative aperture D/f, and field of view 2w. After these three parameters are determined, the resolution, depth of field, distortion, interface and other factors are considered.
Focal length F
Reflects the ability of an optical system to converge light and determines the size of the image. The focal length refers to the distance from the focal point to the center of the lens. Each lens has a fixed or variable focal length, which is represented by a capital letter F. As shown below.
F20 indicates that the focal length of this lens is 20mm, and F28-70 indicates that the focal length of this lens is 28mm~70mm, a zoom lens.
Maximum relative aperture
The maximum relative aperture of the lens is used to describe the light transmission capability of the lens. The larger the maximum relative aperture of the lens, the greater the amount of light transmission. The maximum relative aperture of the lens is expressed by the ratio of the cylindrical beam diameter D that can pass through the lens to the focal length f of the lens, that is, D/f. For example, F20mm and 1:2 indicate that the maximum relative aperture of the lens is one-half of the focal length of the lens. The maximum relative aperture of the lens is large, the amount of light entering is large, and the light passing ability is strong. In order to meet the needs of photography when it is dark, or for photography of high-speed moving objects, a short exposure time is required to increase the illuminance of the image surface.
The maximum relative aperture size determines the resolution, image surface illuminance and image quality of the lens. In order to control the amount of luminous flux passing through the lens, it is necessary to set an aperture at the back of the lens, F means, F is the reciprocal of the relative aperture. The larger the F, the smaller the aperture, and the smaller the F, the smaller the range of use of the lens, thereby adjusting the amount of light entering and the illuminance on the image surface. The standard value of F is 1.4/2/2.8/4/5.6/8/11/16/22 and other serial values. The smaller the F, the greater the luminous flux reaching the target surface of the camera.
Field of view
The field of view is divided into the field of view on the object side and the field of view on the image side. What is concerned is the angle of view of the object. Optical instruments use the diameter of the imaged object as the angle of view calculation. For sensors, since the photosensitive surface is rectangular, the angle of view is often calculated by the diameter of the imaged object on the diagonal of the rectangular photosensitive surface.
The angle of view of the lens refers to the angle formed by the edge of the effective imaging plane of the camera tube and the node behind the lens. Determines the scope of the scene being photographed. Expressed by 2w. When the focal length of the lens is constant, the larger the field of view, the larger the image. The relationship between the angle of view and the focal length is as follows:
y = f *tanw;
Where y is the diameter of the image area.
If the focal length is short, the field of view is large, and the image is large, if the focal length is long, the field of view is reduced, and the image is small.
For a long focal length lens, it can only have a small angle of view, and it is suitable for long-distance photography to take a larger image of a distant scene. The short focal length lens has a larger field of view, which can capture the nearby scenes in a larger range into the image surface. It is also called a wide-angle lens with a larger field of view.
The difference between linear array and area array sensors
The photosensitive sensors used in industrial cameras mainly include CCD or CMOS sensors, which are divided into two types: area array and linear array. The working area is divided into rectangular or linear. The working area of the sensor must be included in the circular area of the image plane determined by the lens.
The area array sensor is a rectangular array composed of many pixel units, and each pixel unit is a square sensor. The commonly used sensor sizes are shown in the figure below:
The pixel units of the line array sensor are arranged in a row, there are 1K, 2K, 4K, 8K, 12K, and the pixel units are 5μm, 7μm, 10μm, 14μm and so on.
Resolution is an important parameter to evaluate the lens. It is defined as the logarithm of black and white stripes that the lens can distinguish within a millimeter at the image plane, as shown in the figure below:
Resolution is divided into lens resolution and pixel resolution, and the final image quality is determined by the pixel resolution. In actual use, due to the existence of the depth of field, the resolution of the lens is generally higher than that of the pixel.
When the field of view is large, the image will lose its approximation relative to the object. This deformation of the image is called distortion.
Depth of field
Some calculation formulas for optical system
Lens selection parameters
Select some parameters of the lens