Xinsheng IntelligentIndustry NewsExplanation of some difficult problems in camera lens optics

1. What is the difference between zoom and focus?

Zooming is to change the focal length of the lens (accurately speaking, the image distance) to change the angle of view of shooting, which is commonly referred to as zooming in or out of the subject. For example, 18-55mm and 70-200mm lenses are typical zoom lenses. The longer the focal length, the narrower the angle of view.

Focusing usually refers to adjusting the distance between the lens group and the negative film (sensor plane), so that the image of the subject on the CCD/CMOS is clear.

We usually say "focusing" generally refers to "focusing". Some people believe that the statement that a prime lens cannot focus is false.

2. Why is the image at the maximum aperture of the lens usually poor, or "imaging is more fleshy"?

The pursuit of sharpness in imaging should be the pursuit of all lenses. The aperture value of the lens generally ranges from F1.2 - F32, such as F1.8 - F16. For ordinary SLR lenses, the aperture value for the sharpest image is usually F5.6 or F8. Why? This involves two concepts, one is spherical aberration and the other is diffraction.

Diffraction: The factor that affects sharpness at ultra-small apertures.

Diffraction refers to the physical phenomenon in which the wave deviates from the original straight line when it encounters an obstacle. In classical physics, waves propagate to varying degrees after passing through obstacles such as slits, holes, or disks.

The related concepts include Airi-Ban and Rayleigh criterion. Those who are interested can search by themselves, and will not be expanded here.

Spherical aberration: The factor that affects sharpness at large aperture.

In the simplified diagram of lens imaging that we usually see, the parallel light converges to a point after passing through the lens, which is called the focus. In fact, this is not the case in reality. As shown below:

Since the light cannot be perfectly converged, it is impossible to produce a sharp image. When the aperture is large (the aperture F value is small), the circular surface of the lens to receive light is very large, so the convergence of the light will be very scattered, resulting in an unsharp image. When the aperture is narrowed, the circular surface of the lens to receive light is very small. At this time, after the light passes through the lens, the convergence point is relatively more concentrated, so that the image is sharper.

Of course, generally speaking, the focus will not be very accurate. When the aperture is small, the depth of field will be deeper and it will appear sharper, but this is another story.

Of course, for an actual lens, the lenses inside the lens are very complicated, there may be several or even a dozen lenses, and some lenses are made of expensive materials, such as fluorite, whose main function is to improve the optics of the lens. Quality, correct various spherical aberration, coma, dispersion, distortion and so on.

For example the EF 200mm f/2L IS USM lens:

3. Why does the industrial field never mention ISO as a parameter?

Engineers working in industrial machine vision generally know aperture and shutter (exposure time), but if you ask him what ISO is, he probably doesn't know. Is there an ISO parameter in the field of industrial machine vision? Yes, but the name was changed, called gain (gain), this parameter is generally not adjusted by anyone, and generally does not need to be adjusted.

Aperture, shutter time, and ISO are the troika for getting proper exposure, and the three work together and restrict each other. ISO usually takes a value of 100-3200.

Usually, the higher the ISO, the stronger the camera's ability to sense light, the more noise and the worse the image quality. For every photographer, in the vast majority of cases, he wants to fix the ISO at 100, because this can get the purest picture, but it is almost impossible. Because the ISO is too low in a dark light environment, the aperture can only be adjusted forcibly, but the aperture is limited, so the shutter time can only be increased, but if the shutter time is long, the hand-held shooting will inevitably blur.

However, this problem generally does not exist in the industrial field (or this problem is a secondary problem). First, because the industrial camera generally does not move, the exposure time is very long, and it will not paste the film. In addition, the light source of industrial vision equipment has its own, almost no There is a dark environment.

4. Why is the higher the brightness of the light source, the better?

High-brightness light sources have at least these advantages: high light source brightness, the shutter time can be reduced, it is possible to improve the acquisition speed; secondly, the high brightness of the light source can greatly reduce the impact of ambient stray light; in addition, the high brightness of the light source can reduce the aperture Zooming out, and usually closing down the aperture, results in a sharper picture and greater depth of field, which are extremely important for machine vision systems. Therefore, when choosing a light source, the higher the brightness of the light source, the better.

5. What is the single pixel quality?

A single pixel refers to a pixel on the sensor, and the quality of a single pixel is usually directly related to the size of the pixel particle. For example, the pixel particle size of some cameras is 5um × 5um, while the pixel particle size of many mobile phones is 1.12um × 1.12um, or 1.3um × 1.3um. The larger the value, the better the quality of a single pixel.

The main reason is that there is telecommunication interference between pixels. The smaller the pixel particles, the smaller the spacing between adjacent pixels, the easier it is to interfere with each other, and the result is a decrease in the purity of the picture quality.

For example, there are 1,000 aunties dancing square dancing in a small square, and each person is only allocated 1 square meter of space, so crowded aunts are likely to fight, but if only 10 aunts are dancing square dancing, then it will be much more spacious and comfortable. Maybe after the square dance, three tables can be set up to fight landlords, and of course the remaining one can wait for rotation.