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Knowledge about lens parameters (this article can be divided into one or two according to the content)
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Time: Oct 08,2021
1.Image size
The imaging size is also the screen size;
Image size of sensor:
Continue to use the standard format size of the camera tube, it is the outer diameter size of the camera tube.
2.Focal length
The concept refers to the distance from the center of the lens to the focal point of light gathering.It is also the distance from the center of the lens to the imaging plane of the sensor surface in the module.The focal length is a very important data, and it will be used in the calculation of the depth of field and FOV in the future.
3.Perspective
There are three kinds of lenses: standard, wide-angle, and telephoto lens.
Although the area that the human eye can see can reach 180 degrees, the angle that can really recognize the shape and color is about 50 degrees.Generally, the viewing angle of the touch panel is 55 degrees to 65 degrees. Of course, it should be based on the actual needs of customers;Seesaw principle,Lens manufacturers hope to design a larger field of view that can be suitable for many sensors, but the larger the field of view, the greater the chromatic aberration that needs to be overcome.
4.Chromatic aberration
The photographic lens cannot completely restore a point or a mixed-wavelength light image to a point, but a fuzzy diffuse spot; the image of the object plane is no longer a plane, but a curved surface, and the image has lost the similarity. These imaging defects are called chromatic aberrations.
5.Depth of Field & Depth of Focus
(1) Depth of Field & Depth of Focus
Before and after the focus, the light starts to gather and diffuse, and the image of the point becomes blurred, forming an enlarged circle. This circle is called the circle of confusion.
In reality, the captured image is viewed in a certain way (such as projection, magnification into a photo, etc.). The image felt by the naked eye has a great relationship with the magnification, projection distance and viewing distance. If the diameter of the circle of confusion is smaller than the discriminative ability of the human eye, the blur produced by the actual image in the relative range cannot be recognized. This unrecognizable circle of confusion is called the permissible circle of confusion.
(2) depth of field
There is a permissible circle of confusion before and after the focal point, and the distance between the two circles of confusion is called the depth of focus. Before and after the subject (focus point), the image still has a clear range, which is the depth of field. In other words, the front and back depth of the subject and the degree of image blur on the surface of the film are all within the limit of the permissible circle of confusion.
The depth of field varies with the focal length of the lens, aperture value, and shooting distance. For a fixed focal length and shooting distance, the smaller the aperture used, the greater the depth of field. The principle of myopic love squinting.
(3) Example
Case study,CNF7246. Lens DS628A
Parameter,EFL=2.94mm FNO=2.0 SENSOR PIXEL SIZE=1.75um
(4)Vcm some poor focusing phenomenon
Poor close focus
When designing the Holder, the back focus stroke of the lens from far to near will be within the range of the VCM. If the height of the Holder is not designed well, a holder on the lens near focus will appear, resulting in poor near focus.
6.Distortion
The so-called distortion refers to the degree to which a straight line turns into a curve after shooting through the lens. The degree of distortion is calculated as the percentage of the change in imaging size to the ideal imaging size.The resolution of the human eye to the angle is 1 minute of radian, which is about 1/60 of 1 degree, and it is quite sensitive to the straightness and curvature of the line. Therefore, most optical imaging lenses are very concerned about the deviation of the field angle of the magnification, usually set at 2%.
7.Relative illuminance
Concept, the illuminance ratio of the field of view along the optical axis to the full field of view on the imaging plane, that is, the ratio of the diagonal corners of the image sensor to the intermediate illuminance, this value is restricted by the cos4θ theorem of illuminance, and the corners are unit area The luminous flux of the luminous flux is low, but not so low that there is a phenomenon of vignetting.
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