Face array shot
Industrial lenses generally consist of several lenses with a certain thickness. In most cases, the entire lens can be equivalent to a thin lens for parameter calculation, and the optical path structure of the lens is shown in Figure 1 as the basis for lens selection.
Fig.1 The optical path structure of an industrial lens
As a key optical device in the system, the quality of industrial lenses directly affects the quality of imaging. Industrial lenses are able to collect the light emitted by the object being observed and then image it on the photosensitive surface of the camera's image sensor.
Fig. 2 Imaging principle of industrial lens
1. Classification of industrial lenses
There are many types of industrial lenses, and Figure 3 shows the classification of industrial lenses according to several different classifications.
Figure 3 Classification of industrial lenses
Here we mainly introduce FA lenses and telecentric lenses.
1、FA (Factory Automation) lens
FA lenses refer to industrial lenses designed according to general optical imaging principles, that is, ordinary lenses. FA lenses are generally fixed focal length, can change the size of the aperture, and have a transmitter aperture adjustment ring and a focus adjustment ring on the lens. The function of the transmitter aperture adjustment ring is to change the size of the lens aperture. The function of the focus adjustment ring is to change the relative position of the camera sensor and the focal image surface to make the image clear.
Figure 4 FA lens Figure 5 Telecentric lens
2. Telecentric lens
Telecentric lenses refer to lenses specially designed to correct the parallax of traditional industrial lenses, and the magnification of the image obtained within a certain distance range will not change. The closer the FA lens is to the subject, the larger the image of the subject. Due to the design of the telecentric lens with a parallel optical path, the distance between the lens and the subject will not change in the distance between the lens and the subject within its telecentric range. The telecentric lens with a parallel optical path design on the object side is called the telecentric lens on the object side, the telecentric lens with the parallel optical path design on the image side is called the telecentric lens on the image side, and the telecentric lens with a parallel optical path design on the object side and the image side is called a double telecentric lens.
Fig.6 Optical path of a traditional lens Fig.7 Optical path of a telecentric lens on the object side
Fig.8 The optical path of a telecentric lens like a square telecentric lens Fig.9 The optical path of a double telecentric lens
Fig.10 Schematic diagram of normal lens and telecentric lens
| Pros and cons | Normal lens | Telecentric lens |
| Advantage | Low cost and wide use | The magnification is constant, does not change with the depth of field, and there is no parallax |
| Limitations | Magnification will change, there will be parallax | High cost, large size and heavy weight |
Table 1 Advantages and disadvantages of ordinary lenses and telecentric lenses
2. Performance parameters of industrial lenses
There are many parameters for industrial lenses, and customers can choose the appropriate parameters according to actual use needs. The following describes the critical performance parameters of the lens.
Fig.11 Parameters of industrial lenses
1. Focal Length (FL)
The focal length of an industrial lens refers to the distance from the optical center of the lens (the main point behind the optics) to the focal point of the imaging surface, which is the important energy index of the lens. Parallel beams converge through the lens at a point, which is the focal point. The size of the focal length determines the size of the field of view, the larger the focal length, the smaller the field of view, the smaller the observation range, and the distant objects can be seen clearly; The smaller the focal length, the larger the field of view and the larger the viewing range, but distant objects cannot be seen clearly.
Fig.12 Schematic diagram of focal length
2. Field of View (FOV)
The field of view of industrial lenses refers to the field of view that the lens can clearly image, and the field of view size needs to be greater than the size of the subject when selecting the lens.
3. Field of view (AFOV)
The field of view of an industrial lens refers to the angle 2θ formed by the two edges of the maximum imaging range of the lens with the lens as the apex in the optical system, as shown in Figure 13. The size of the field of view determines the field of view of the lens, and the larger the field of view, the larger the field of view and the smaller the optical magnification. Additionally, the smaller the focal length of the lens, the smaller the working distance required to obtain the specified size field of view.
Figure 13 Field of view and field of view Figure 14 Focal length and field of view
As shown in Figure 14, for a given sensor size, the smaller the focal length, the larger the lens's field of view.
4. Aperture F-number
The aperture F-number (number of apertures) of industrial lenses refers to the ratio of the effective focal length of the lens to the diameter of the pupil, which measures the amount of light transmission in the optical system, usually expressed as F×, F/#, 1:× or f/#.
Figure 15 Aperture
For the aperture F-value, the smaller the value after F, the greater the luminous flux and the smaller the depth of field. The formula for calculating the F-value is F = effective focal length / pupil diameter
| Aperture F-number | luminous flux | Depth of field | Resolution of diffraction limit |
| ↓ | ↑ | ↓ | ↑ |
| ↑ | ↓ | ↑ | ↓ |
Table 2 Influence of aperture F-number on lens performance parameters
5. Working Distance (WD)
The working distance of an industrial lens refers to the distance from the front end of the lens to the object being observed when the lens can be clearly imaged. When selecting a lens, it is necessary to consider whether the allowable installation distance in the real environment is within the allowable working distance range of the lens.
6. Maximum compatible camera chip size
The maximum compatible camera chip size refers to the maximum camera chip size that the lens can be compatible with, avoiding ineffective areas caused by the camera chip size being too large. As shown in Figure 16, Figure A is the image of the lens when it is normally imaging, and Figure B is the image when the maximum compatible camera chip size of the lens is smaller than the chip size of the camera.
Fig.16 Effect of the maximum compatible camera chip size on imaging
Each lens can only be compatible with cameras with a sensor chip up to a certain size. In order to ensure the full use of the camera sensor and the quality of the entire image, the maximum compatible camera chip size of the lens cannot be less than the camera chip size. Like the camera target size, the maximum adapted chip size given by the lens manufacturer refers to neither the side length of the chip nor the diagonal length of the chip, but the diameter of the picture tube equivalent to the area of the chip's photosensitive region. Table 3 shows the side lengths and diagonal lengths of the chips corresponding to common chip sizes.
| Sensor size | Diagonal/mm | Width/mm | Height/mm |
| 1/4" | 4 | 3.2 | 2.4 |
| 1/3" | 6 | 4.8 | 3.6 |
| 1/2" | 8 | 6.4 | 4.8 |
| 1/1.8" | 9 | 7.2 | 5.4 |
| 2/3" | 11 | 8.8 | 6.6 |
| 1" | 16 | 12.8 | 9.6 |
| 1.1" | 17.6 | 10.4 | 14.2 |
| 4/3" | 22 | 17.6 | 13.2 |
Table 3 Commonly used sensor sizes
6. Magnification
The magnification of an industrial lens refers to the ratio of the image size of an object in the focal plane through the lens to the actual size of the object.
7. Minimum Focusing Distance (MOD)
For most industrial lenses, the closer the closest focusing distance, the stronger the lens's shooting ability. When the focusing distance is less than the nearest focusing distance, the camera cannot focus.
Fig.17
8. Pixel size
Cell size refers to the actual physical size of each cell on the chip array. Normally, the larger the cell size of a chip, the more photons a single pixel can receive, and the stronger the photosensitive performance of the chip.
9. Resolution
The resolution of an industrial lens refers to the ability of a lens to distinguish two close points, specifically the logarithm of black and white lines that can be distinguished within a 1mm pitch on the imaging plane, in lp/mm.
Fig.18 Schematic diagram of resolution
Figure 19 shows the comparison of images where black and white line pairs can be clearly distinguished through the lens and black and white line pairs cannot be clearly distinguished.
Fig.19
Lens resolution can also be expressed using MTF (Modulation Transfer Function) curves. The modulation transfer function reflects the lens's ability to reproduce contrast when the spatial frequency changes, and the magnitude is the ratio of the image square contrast to the object side contrast at a certain spatial frequency. Lenses and cameras have their own resolutions, and the larger the spatial cut-off frequency (ultimate spatial resolution) corresponding to the MTF, the better the spatial resolution of the device itself, and the better the smaller details can be seen. The ultimate spatial resolution of the lens must be greater than the ultimate spatial resolution of the camera to achieve the best imaging performance.
Fig.20 Schematic diagram of the MTF curve of the lens
10. Distortion
For ideal lens imaging, the magnification on the object plane and the image plane is fixed, but in fact this property is only available in the small field of view in the central area of the image. The magnification of the image changes as the field of view increases, distorting the imaging. Distortion refers to the degree of distortion of the image formed by the lens on the object being photographed relative to the object itself. There are two types of common distortions in lens imaging: pincushion distortion and barrel distortion. Cucushion distortion refers to the distortion phenomenon of the lens imaging picture shrinking towards the middle, while barrel distortion refers to the distortion phenomenon of the lens imaging image in the form of barrel-shaped expansion, as shown in Figure 21.
Fig.21 Cushion distortion and barrel distortion
According to different calculation methods, distortion can also be divided into optical distortion and TV distortion. Optical distortion is expressed as the percentage of the deviation between the imaginary height and the actual image height; TV distortion refers to the degree of deformation of the image itself when the image is actually taken. In general, there are two ways to improve distortion, one is to calculate the distortion coefficient of the lens through software algorithms and correct it; The other is to reduce the effect of distortion from the lens itself through optical path design.
11、Back Focal Length
The back focal length of an industrial lens refers to the distance from the surface apex of the last lens of the lens to the focal point.
12. Centrifugality
Due to the difference in telecentricity of different lenses, the lens's ability to eliminate parallax is also different. Centrifugity refers to the angle θ between the main light and the optical axis, as shown in Figure 18.
Fig.22 Centricity
For an ideal telecentric lens, θ=0°, the smaller the θ, the better the telecentricity of the lens, and the smaller the measurement error.
13、Depth of Field，DOF
There is a certain length of space in front and behind the focus of the lens, and when the subject is in this space, the image on the photosensitive sensor is within the allowable diffusion circle, and the depth of field refers to the length of the space where the object is located. Depth of foreground refers to the distance from the focal point to the allowable diffusion circle, and rear depth of field refers to the distance from the focal point to the allowable diffusion circle, as shown in Figure 23.
Fig.23 Depth of field
δ—Diameter of the diffuse circle L—Shooting distance ∆L₁—Depth of foreground ∆L₂—Depth of field ∆L—Depth of field
Depth of field∆ L=∆L₁ ∆L₂
The depth of field varies with the lens's aperture value, focal length, and shooting distance.
(1) The larger the aperture, the smaller the depth of field; The smaller the aperture, the greater the depth of field.
(2) the longer the focal length, the smaller the depth of field; The shorter the focal length, the greater the depth of field.
(3) The farther away from the shooting object, the greater the depth of field; The closer you are to the subject, the smaller the depth of field.
14. Optical chief
The total optical length of an industrial lens refers to the distance from the surface of the first lens to the image plane.
15. Interface
Industrial lenses and cameras need to be used together, and the lens is mounted on the camera through the lens interface, and the mounting interface type of the two should be the same. The lens interface is divided into two categories: screw port and mount, the screw port mainly includes M42 mount, M58 interface, M72 interface, C interface, CS interface, etc., and the mount mainly includes F port, V port, etc.
| Interface type | French distance/mm | Thread specifications |
| C | 17.526 | M25.4×0.8 |
| CS | 12.5 | M25.4×0.8 |
| F | 46.5 | φ47mm |
| M42(SLR) | 45.5 | M42×1 |
| M42(T) | 55 | M42×0.75 |
| M58 | 11.48 | M58×0.75 |
| M72 | 11.48 | M72×0.75 |
Table 4 Lens mount specifications
Flange Back/Flange Focal Distance :(when the objective is in focus) is the distance from the mounting datum of the objective lens to the image square focal plane.
Fig.24
16. an adapter ring
The adapter ring is an adapter tool with different interfaces at both ends that allow the lens to be adapted to the corresponding camera.
17. Connecting circles
When the object distance is less than the working distance of the lens, it is necessary to add a contact between the lens and the camera to increase the image distance.
(1) C-mount lens matches C-mount camera;
(2) CS mount lens matches CS mount camera;
(3) C mount lens 5mm connector matching CS mount camera;
(4) The CS mount lens does not match the C mount camera.