[Practical sharing] This article will help you understand the camera module CMOS image sensor (Sensor) and lens

1.CMOS image sensor (Sensor)

1.1 Sensor size

Sensor size is a key indicator to measure the size of its photosensitive area, usually expressed in diagonal length in inches. The aspect ratio is generally 4:3, but for historical reasons, 16mm in the sensor is defined as 1 inch. The larger the sensor size, the stronger its ability to capture light, thus generating higher quality images.

Common sensor sizes include 4/3 inch, 1 inch, 2/3 inch, 1/1.8 inch, 1/2 inch, 1/2.5 inch, 1/3 inch and 1/4 inch. As the size decreases, its actual width and height will also decrease accordingly, affecting its ability to capture light and generate images.

Size (inches)

Actual width mm	Actual height mm	Diagonal length
4/3"	18	13.5	22.5
1"	12.8	9.6	16
2/3"	8.8	6.6	11
1/1.8"	7.11	5.33	8.89
1/2"	6.4	4.8	8
1/2.5"	5.75	4.32	7.19
1/3"	4.8	3.6	6
1/4"	3.2	2.4	4

1.2 CMOS Image Sensor interface

The interface types of CMOS image sensors mainly include MIPI, DVP and LVDS. The MIPI interface is a high-speed serial interface used to transmit image data and control signals, with the advantages of low power consumption, high bandwidth and strong anti-interference ability. The DVP interface is a parallel interface that transmits image data simultaneously through multiple data lines. It is relatively slow, but the cost is low. The LVDS interface is a differential serial interface used to transmit high-speed image data, suitable for long-distance transmission and high-speed applications.

1.3Pixel

Pixel is the basic unit of the image sensor, which determines the resolution and clarity of the image. Generally speaking, the higher the pixel, the higher the image resolution and the stronger the ability to express details. However, too high a pixel may also lead to increased image noise and reduced image processing capabilities.

2.Lens

2.1Detailed explanation of optical lens parameters

EFL (Effective Focal Length) Effective focal length

The distance from the center of the lens to the focal point. Image focal length refers to the distance from the image principal surface (rear principal surface) to the image focal point (rear focal point); object focal length refers to the distance from the object principal surface (front principal surface) to the object focal point (front focal point).

●Notes: A focal length that is too short will result in a large field of view, making it difficult to control distortion and the main light angle, low relative illumination, severe lens bending, and difficult to correct aberration. If the focal length is too long, the lens will be too long, which is not conducive to system miniaturization, and the field of view is too small to meet user needs.

TTL (Total Track Length) Total length of the lens

The total optical length refers to the distance from the first surface of the lens in the lens to the image surface. The total length of the mechanism refers to the distance from the end face of the lens barrel to the image surface. In lens products, it generally refers to the mechanism TTL.

BFL (Back Focal Length) Optical back focal length

The distance from the last surface of the lens in the optical system to the image surface.

FFL (Front Focal Length) Optical front focal length

The distance from the first surface of the lens in the optical system to the object surface.

●Note: It should be distinguished from the mechanism back focal length (FBL, sometimes mixed with FFL, but here it specifically refers to the mechanism back focal length) or flange focal length.

FBL/FFL (Flange Focal Length) Mechanism back focal length (flange focal length)

The distance from the last mechanism surface of the lens group to the image plane.

FOV (Field Of View) Field of view

The maximum field of view that the lens can capture. The field of view can be divided into diagonal field of view (FOV-D), horizontal field of view (FOV-H), and vertical field of view (FOV-V). The diagonal field of view is the largest, the horizontal field of view is the second, and the vertical field of view is the smallest.

●Calculation formula: FOV-H=2tan(H/2D), FOV-V=2tan(V/2D), FOV-D=2tan[sqrt(H²+V²)/2D], where H is the horizontal dimension, V is the vertical dimension, and D is the distance from the center of the lens to the object.

F/NO. (F-Number) Aperture (Relative Aperture)

The ratio of the effective focal length to the entrance pupil diameter.

●Function: Determines the brightness of the lens.

●Remarks: Under the premise of ensuring the same aperture, the shorter the focal length, the smaller the relative aperture should be. Generally, F/#=2.8, but F/#=3.2 for single-chip design. The smaller the F/#, the larger the aperture.

RI (Relative Illumination) Relative Illumination

The ratio of central illumination to peripheral illumination.

●Notes: Too low relative illumination is manifested as a brighter center of the image and darker surroundings, which is a vignetting phenomenon. Too low relative illumination can also cause color distortion. RI is proportional to COS⁴ (semi-FOV). When RI < 50%, the human eye can distinguish the difference. In severe cases, the "missing corner" phenomenon will appear with all four corners of the picture completely black. Therefore, the basic requirement for RI is RI> 50%.

CRA (Chief Ray Angle) Chief Ray Angle

The chief ray angle is the angle between the chief ray and the parallel ray. The chief ray is the ray emitted from the edge of the object, passing through the center of the aperture diaphragm and finally reaching the edge of the image.

● Notes: An inappropriate chief ray emission angle will lead to severe vignetting, decreased contrast, and color cast.

MTF (Modulation Transfer Function) 

Optical modulation transfer function Modulation is the ratio of the brightest light minus the darkest light to the brightest light plus the darkest light. The result M is the contrast of the light.

● Note: The required calculation formula for Sensor MTF is Sensor full-frequency resolution = 1000/2.8/2 = 179lp/mm (2.8um Pixel Size) or 1/(2×Pixel Size). Resolution is the number of resolvable line pairs per 1 mm, in units of lp/mm.

TV-Line Scan Line

The number of resolvable lines in the horizontal image of the screen can be converted by resolution. TV-Line=lp/mm×2×Sensor width.

 Flare/Ghost

Refers to the opposite miniature or foggy image formed by the random scattering of light on the image plane in an optical system, that is, the non-imaging beam in the optical system.

These parameters jointly determine the performance and application range of the optical lens. When selecting and using an optical lens, these parameters need to be comprehensively considered according to the specific application scenarios and requirements.

This is a parameter on a fixed-focus lens:

16mm: focal length 16mm

IR: infrared lens

1/2'': target size 1/2''

5MP: lens resolution, 500w pixels

F4.0: also written as F/4.0, aperture coefficient, 4.0 aperture, focal length/aperture; under other conditions unchanged, the smaller the aperture coefficient, the greater the luminous flux and the brighter the image.

Target surface size: corresponds to the size of the sensor, and cannot be smaller than the sensor size. If it is smaller than the sensor size, there will be dark corners.

Focal length: The focal length of a fixed-focus lens is constant; the more convex the convex lens, the smaller the focal length; the flatter the convex lens, the larger the focal length. A zoom lens changes the focal length of the lens by changing the relative position of the lens inside the lens. The focal length of the lens is defined as the distance between the optical center of the lens and the image sensor (or film plane) of the camera when focusing at infinity. In some cases, the image distance is equivalent to the focal length.

Aperture value: F1.4, F2.0, F2.8, F4.0, F5.6, F8.0…, the next one is 1.4 times (square root of 2) of the previous one, and the luminous flux is half of the previous one.

2.2 The influence of lens focal length and aperture on depth of field

When the aperture remains unchanged, the focal length of the lens is inversely proportional to the depth of field:

The larger the focal length, the shallower the depth of field

The smaller the focal length, the greater the depth of field

When the focal length remains unchanged, the aperture size is inversely proportional to the depth of field:

The larger the aperture, the greater the depth of field

The smaller the aperture, the greater the depth of field

2.3 Lens interface

Threaded interface

It is divided into C-mount and CS-mount, the difference is that the flange distance (the distance from the lens mounting base to the focus) is different, that is, the distance between the bottom of the lens and the Senor is different.

The flange distance of the C-mount is 17.526mm, while that of the CS-mount is 12.5mm.

According to the interface diameter, it can be divided into M12, M42, etc. M12 refers to the interface diameter of 12mm.

Battery mount

Generally used in SLR cameras. It is divided into E-mount and EF-mount, and the difference is also that the flange distance is different.

2.4 Total Lens Length TTL

The total optical length refers to the distance from the first surface of the lens in the lens to the image plane

The total length of the mechanism refers to the distance from the end face of the lens barrel to the image plane

When changing lenses, consider whether the total length of the lens is appropriate.

TTL affects the installation position. When TTL is too long, the sleeve may not be long enough, resulting in failure to install; this situation occurred when we changed a lens.

This is the specification of a lens, TTL = 16mm.

TTL: Total optical length, the distance from the first surface of the lens to the image plane

TFL: Total lens length/total mechanism length, the distance from the end of the lens barrel to the image plane

EFL: Effective focal length

F/NO: Aperture size

FBL: Mechanism back focus (flange focal length), the distance from the last mechanism surface of the lens group to the image plane

BFL (Back Focal Length): Optical back focal length, the distance from the last surface of the lens to the image plane

FOV: Horizontal FOV, vertical FOV, diagonal FOV

Chief Ray Agnle: Main incident angle

Thread: Screw size, M8 diameter 8mm, P0.35 thread pitch 0.35mm

Construction: Structure, 4G+IR+Metal, 4 glass 4 glass lenses, infrared filter, metal housing

Resolution: Resolution, LP/mm, Line pairs per millimetre, line pairs per millimeter; for example 10 lp/mm means that there are 10 pairs of black and white lines in a length of 1mm, and there are 20 black and white lines in total, so the width of each line is 1/20 = 0.05mm

2.5 FOV field of view

FOV is affected by the aperture size; for the same lens, the larger the aperture, the larger the FOV, and the smaller the aperture, the smaller the FOV

The influence between field of view, aperture, and depth of field.

The larger the FOV, the smaller the image at the same distance, and the shallower the depth of field.

2.6 Equivalent focal length

When the size captured by the current sensor + current lens is the same as the size captured by the full-frame camera + lens, that is, when the field of view is the same, what is the focal length of the lens when shooting with a full-frame camera? This is the equivalent focal length, that is, the focal length of the lens used to capture an image with the same field of view with a full-frame camera.

2.7 Aperture

The aperture affects the amount of light entering. The larger the aperture, the more light enters, and the brighter the image.

Aperture affects depth of field. The larger the aperture, the smaller the depth of field.

When choosing a lens, you need to balance the amount of light entering and the depth of field according to the scene and choose a lens with a suitable aperture.

The reciprocal of the relative aperture (f'/D) is called the F number, also called the aperture number, recorded as F/….

The amount of light entering is inversely proportional to the square of the F number, that is, the amount of light entering the F4.0 aperture is half that of the F2.8, and the exposure time needs to be doubled.

3. Lens imaging principle

Image distance v: distance from lens to sensor

Object distance u: distance from object to lens

Focal length f: lens focal length parameter; fixed-focus lens, fixed focal length; zoom lens, variable focal length

Autofocus and autofocus: autofocus AF, focal length unchanged, adjust image distance and object distance (mainly image distance); autofocus zoom, adjust focal length, object distance and image distance unchanged

3.1 Convex lens imaging principle

The convex lens imaging principle refers to the imaging using the principle of light refraction.

u>2f, f<v<2f: inverted and reduced real image (camera application)

u=2f=v: inverted and equal real image

f<u<2f, v>2f: inverted and enlarged real image (projector)

Imaging formula: 1/u + 1/v = 1/f, only when this condition is met can a clear image be formed.

In cameras, usually u >>> v, the object distance is much larger than the image distance. Therefore, in the AF process, a small change in the image distance v can cause a large change in the object distance u. Therefore, the distance pushed by the VCM is usually very short, but it can control a large clear imaging range.

Image size:

When f remains unchanged, the image distance v increases, the object distance u decreases, and the image becomes larger (u+v decreases) (When the AF VCM is pushed outward, the clear point is closer and the object image becomes larger)

When f remains unchanged, the image distance v decreases, the object distance u increases, and the image becomes smaller (u+v increases)

If you change to a lens with a longer focal length, the image distance remains unchanged and the object distance must also be reduced; if you do not want to reduce the object distance, you must reduce the image distance.

3.2 The influence of different light on focal length

Light of different wavelengths has different propagation speeds and refractive indices in the medium, so the same lens has different focal lengths for light of different wavelengths. This is called dispersion, which is the same as the dispersion of a prism. The longer the wavelength, the larger the focal length.

The wavelength range of different colors of visible light:

Red: 770~622nm; Orange: 622~597nm; Yellow: 597~577nm; Green: 577~492nm; Blue, Indigo: 492~455nm; Purple: 455~350nm.

Let's delve into the details of the operating mechanism of a surveillance camera: During the day when there is sufficient light, the camera captures and presents color video images; when night falls, it switches to infrared mode to capture night images. It is worth noting that during this IR CUT (infrared cutoff filter) switching process, the focus of the camera often adjusts due to changes in the nature of light, which makes it necessary to refocus to ensure the clarity of the picture.

In order to meet the needs of all-weather, uninterrupted monitoring, many places have put forward higher requirements for the performance of cameras – not only to be able to complete the monitoring tasks excellently during the day, but also to provide clear and high-quality images at night. In recent years, with the widespread popularity of infrared cameras and infrared lights, as well as the gradual decline in the prices of day and night cameras and high-definition color cameras, lens manufacturers have ushered in unprecedented market opportunities. It is in this context that IR (infrared) surveillance camera lenses came into being.

IR lenses, which are lenses designed specifically for infrared surveillance, use unique optical glass materials and combine cutting-edge optical design concepts to successfully eliminate the problem of visible light and infrared light offset on the focal plane. This means that both visible light and infrared light can be focused on the same plane in the lens, ensuring that all images can present excellent clarity. In addition, IR lenses also use special multi-layer coating technology to significantly improve the transmittance of infrared light. Therefore, cameras using IR lenses can not only achieve a longer monitoring distance when monitoring at night, but also bring better monitoring effects.

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