Detailed Explanation of Industrial Lens Parameters

In the machine vision system, the lens is the eye of the machine, and its main function is to focus the image of the target object on the photosensitive device of the image sensor (camera). All image information processed by the data system needs to be obtained through the lens, and the quality of the lens directly affects the overall performance of the vision system.

This lens school will give a brief introduction to the relevant parameters of machine vision industrial lenses:

01 Detailed lens focal length (EFL)

Definition: The distance from the center of the lens to the focal point of the image space.

The focal length in an optical system is used to measure the ability of an optical system to converge or diverge light.

Focal length diagram

How to understand the relationship between focal length and field of view? For the same photosensitive element, the longer the focal length of the matching lens, the smaller the field of view, and vice versa (excluding the influence of pincushion distortion). You can intuitively feel it according to the following group of pictures

Note: use the same photosensitive chip

02 Detailed lens aperture number (F.No)

Definition: The ratio of the focal length (EFL) to the entrance pupil (D, the image of the aperture in the object space), that is, F.No=EFL/D.

How does the F.No number affect the screen brightness, depth of field and assembly sensitivity?

a. screen brightness

The size of the F.No number determines how much energy enters the optical system.

The larger the F.No, the less light enters the optical system and the darker the picture; the smaller the F.No, the more light enters the optical system and the brighter the picture.

As shown below:

Note: The default camera exposure time, gain and other parameters are constant for this item.

b. depth of field

On the premise that all other parameters remain the same, the larger the F.No, the larger the depth of field; the smaller the F.No, the smaller the depth of field.

c. Assembly sensitivity

The larger the F.No, the lower the sensitivity of the image to installation tilt, and the easier it is to assemble; the smaller the F.No, the higher the sensitivity of the image to installation tilt, and the easier it is to assemble.

03 Detailed explanation of lens distortion (Distortion)

04 Detailed Explanation of Lens Field of View (Field of View)

Definition: The spatial angle range that the lens can see on the photosensitive component, that is, the opening angle of the center of the entrance pupil of the optical system to the object or the opening angle of the center of the exit pupil of the optical system. The relationship between the field of view and the focal length (EFL): If Y is the half-diagonal length of the Sensor, without considering the distortion of the optical system, the diagonal field of view (2θ)=2*arctan(Y/EFL) ,As shown below:

Note: The field of view is related to the focal length of the lens, the size of the Sensor target surface and lens distortion:

a. When the focal length of the lens is the same, the larger the sensor target surface, the larger the field of view;

b. In the case of the same sensor target surface, the smaller the focal length of the lens, the larger the field of view;

c. When the focal length of the lens and the target surface of the Sensor are the same, the larger the pincushion (negative) distortion is, the larger the field of view will be.

05 Detailed lens definition (Resolution)

Definition: The result of the comprehensive performance of resolution and contrast. Resolution, also known as resolution, resolution, discrimination rate, resolution, refers to the ability of the lens to clearly reproduce the details of the scene being photographed. The higher the resolution of the lens, the clearer and more delicate the captured image. Clarity can also be expressed by the number of line pairs that can be resolved by the lens on the image surface within the unit of MM. The unit is LP/MM. The more line pairs that can be resolved, the higher the resolution of the lens. As shown below:

Contrast is the contrast, which is used to describe the difference between the brightest part and the darkest part of the image; expressed by MTF (ModulationTransfer Function), MTF describes the transfer characteristics of the optical imaging system to the contrast of each frequency component, as shown in the following formula :

Among them, I'max and I'min represent the maximum and minimum gray value of the image, and Imax and Imin represent the maximum and minimum gray value of the object. Obviously, the modulation degree is between 0 and 1, and the higher the modulation degree M, the greater the contrast; when the maximum brightness and the minimum brightness in the image are equal, the contrast completely disappears, and the modulation degree is 0 at this time.

The following figure is a comparison picture with a resolution of 10LP/MM and an MTF between 0.05 and 0.4. It can be seen from the figure below that the minimum resolvable MTF value acceptable to the human eye is between 0.15 and 0.2.

How does MTF affect the overall imaging effect of the lens?

The MTF value at low frequency determines the contrast of the image, and the MTF value at high frequency determines the resolution of the image. The quality of MTF directly affects the overall imaging effect of the lens, as shown in the figure below:

A curve: The curve declines smoothly, indicating that the resolution and contrast of the image have reached a good level, see Figure A;

B curve: The MTF value at low frequency is continuously high, indicating that the contrast of the image is good; but the MTF at high frequency is very low, indicating that the resolution of the image is poor, see Figure B;

C curve: The MTF value at low frequency drops faster, indicating that the image contrast is poor; the MTF at high frequency is higher than that of B curve, indicating that the image resolution effect is better than B, see Figure C;

06 Detailed Explanation of Relative Illumination

definition

The ratio of image edge brightness to center brightness. The illuminance at different positions on the sensor surface is proportional to the Cosin fourth power of the incident angle of light incident on the image plane.

Relative illuminance <1?