Lens aperture and bi-telecentric lens principle


Aperture is one of the most important tools used to control exposure variables. The other two are shutter speed and ISO sensitivity. Aperture can also change the depth of field, so it is necessary to understand clearly how the selection of the aperture value will affect the final image.

   The aperture of the lens is specified by numbers and f-numbers. Generally speaking, the range is from f/2 to f32. However, this range depends on the lens itself, some are larger and some are smaller. The standard sequence of aperture values u200bu200bis f/2.8, f/4, f/5.6, f8, f11, f16, f22 and f32.

   The above f-numbers are all the aperture values u200bu200badjusted by the entire block. Today's digital SLR cameras also allow photographers to change the aperture value by 1/2 or 1/3 block adjustment, so that more accurate exposure can be achieved.

   The f-number represents the focal length divided by the diameter of the aperture. For example, f/2 means that the diameter of the aperture is half the focal length of the lens, f/4 is 1/4, f/8 is 1/8, and so on. Then for a 50mm focal length lens, the aperture value is f/2 and the aperture diameter is 25 mm, and the aperture value is f/4 and the aperture diameter is 12.5 mm.

The range of    lens aperture refers to its maximum aperture and minimum aperture. The maximum aperture is the state in which the aperture is fully opened; when the aperture is closed to the minimum, the least amount of light passes through. Lens design Many zoom lenses have two maximum aperture stops, such as 55-200mm f/4-5.6. This means that the maximum achievable aperture of the lens changes according to the change of the focal length of the lens.


  f-value will make us confuse some concepts. Because the maximum aperture is represented by the smallest number, such as f/2.8 or f/4; the small aperture is represented by a large number, such as f/22 or f/32. This may be completely opposite to what you think. Therefore, in order to help you remember their size, we might as well understand it like this: the number after f is actually the denominator, and the numerator above is all 1. In other words, f/8 is actually 1/8, and f/4 is 1/4. Obviously, 1/8 is less than 1/4.

   The aperture and shutter speed of the lens are in a complementary relationship. Adjusting the aperture size and changing the shutter speed will affect the intensity of light passing through the lens. To put it simply, choose a larger aperture (the f-number is smaller), the light will pass quickly, and the shutter opening time should be shortened accordingly; then choose a smaller one

   aperture (large f-number), the exposure time needs to be longer, and the shutter opening time should be increased. This will have an important impact on the visual effect, because the length of the shutter opening time will change the imaging effect of moving objects, and the size of the aperture can determine which parts of the recorded image are clear. These clear optical lens areas are called depth of field.

   In recent years, companies that often do precision measurement with machine vision will hear some relatively new terms, such as bilateral telecentricity, unilateral telecentricity, and object-side telecentricity , Fang Yuanxin, etc. These optical concepts that were not often mentioned before, make people confused and do not know how to understand. The information collected is often highly specialized and difficult to understand. Today, from the perspective of practical application Set out to briefly describe the relevant principles of bi-telecentric industrial lenses.

  1) Principle of bi-telecentric lens and problems that can be solved:

  1. Principle of convex lens imaging

  Character 1: All light passing through the optical center does not change its propagation direction

  Character 2: Convex lens has a converging effect on parallel light, The imaging is to take advantage of this.

  2. Bi-telecentric lens imaging principle

   principle: by placing the light in the middle of the lens The diaphragm makes the light entering and exiting the lens are parallel light, and other light is blocked by the diaphragm and cannot reach the imaging chip. Each side of the imaging chip is the object-side telecentric lens and the image-side telecentric lens. The object side solves the problem of depth of field, and the image side solves the problem of magnification changes.

  The resolution of ordinary industrial lens cannot keep up with the improvement of chip resolution. It is subject to the principle of optical imaging. The best can only be about 10um, at most It can be used with a 1000W pixel camera, which cannot meet the current requirements of high-resolution cameras and high-precision measurement and inspection.

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