A Study on the Problems Related to the Resolution of Lens

  First of all, there is no so-called concept of pixels for the lens itself. When the lens is out of the factory, the manufacturers usually use the equipment of "lens detection projector" to test the resolution of the lens. The resolution index of lens detection projector is expressed by lp/mm, i.e. line pair, which can be analyzed per millimeter. This index has nothing to do with other imaging units except lens.

  But the general lens manufacturers will not clearly mark this index in the specifications, but replace it with a more intuitive logo such as 1MP, 2MP or 3MP. Because lp/mm is too professional, the general user may not understand it. It is not as intuitive as the direct pixel. So, how does lp/mm correspond to the pixels of the lens calibration? From the lens manufacturers, they usually use 1/2.8 "sensor as the target to convert, such as: IMX222. IMX222 has a pixel size of 2.8um and a maximum effective pixel of 1920x1200, so we can calculate the target size of IMX222.

  1920 x 2.8 um = 5.376 mm, 1200 x 2.8 um = 3.36 mm

  With IMX222 as the target, we can roughly calculate the corresponding relationship between lens pixels and lp/mm as follows:

  For a lens nominally 1MP, its corresponding resolution should be 1280x720 = 921600, so that its corresponding lp/mm value can be roughly calculated.

  720/2 x 3.36 mm= 107 lp/mm

  A lens labeled 1.3MP should have a resolution of 1280x960 = 1228800, and its corresponding lp/mm value is 1,280x960 = 1228800.

  960/2 x 3.36 mm= 143 lp/mm

  A lens labeled 2MP should have a resolution of 1920x1080 = 2073600 and its corresponding lp/mm value

  1080/2 x 3.36 mm = 160 lp/mm

  The lens labeled 3MP should have a resolution of 2048x1536 = 3145728, and its corresponding lp/mm value is 2048x1536 = 3145728.

  1536/ (2 x 3.36 mm) = 228 lp/mm

  A lens labeled 5MP should have a resolution of 2592 x 1944 = 5038848, and its corresponding lp/mm value is

  1944/ (2 x 3.36 mm) = 289 lp/mm

  A lens labeled 6MP should have a resolution of 3096 x 2048 = 6340608, and its corresponding lp/mm value is

  2048/2 x 3.36 mm) = 304 lp/mm

  ...

  Through the above quantitative calculation, we can roughly understand the corresponding relationship between the lens pixels and the resolution force.

  2. The Corresponding Relation between the Resolution Force of Lens and the Number of Television Lines

  So what is the relationship between the resolution of the camera and the number of TV lines? To answer this question, we need to clarify the classification criteria of the camera. According to the provisions of General Technical Requirements for Security Video Surveillance Cameras - Draft for Consultation - 2011.12:

  Cameras can be divided into:

  Class A: Define the camera, the number of horizontal and vertical pixels in the image format is less than or equal to 768x576.

  Class B: Quasi-HD camera. The number of horizontal and vertical pixels in image format is greater than 768 x 576 and one is less than 1920 x 1080.

  Class C: HD camera, the number of horizontal and vertical pixels in image format is greater than or equal to 1920 x 1080.

  According to the image size category, the central resolution of the camera should satisfy:

  Class A: Horizontal resolution (> 400 lines) and vertical resolution (> 380 lines)

  Class B: Horizontal resolution (> 600 lines) and vertical resolution (> 600 lines)

  Class C: Horizontal resolution (> 800 lines) and vertical resolution (> 800 lines)

  At present, the resolution of the two commonly used high definition cameras are 1280x720 (1MP) and 1920x1080 (2MP). According to the classification criteria of cameras, they belong to category B and C products respectively, and their central resolution needs to be more than 600 lines and more than 800 lines respectively. The line here refers to TV Line, which is used in the field of television. It refers to the number of lines that the picture can analyze in a certain direction. It is related not only to the resolution of the lens, but also to the resolution of the sensor. The corresponding relationship between TV Line and lp/mm is as follows:

  Target height of TV Line = lp/mm x 2x sensor = vertical resolution X pixel size of lp/mm x 2x sensor

  The reason for X2 is that lp/mm refers to the logarithm of lines per millimeter, while black and white counts for a pair, while TV Line counts for two lines. The reason why it multiplies the height of the sensor rather than the width is that the index is a TV line rather than a column. The target height of the sensor can be obtained by multiplying the vertical resolution of the sensor with the size of the pixels, so that we can get the corresponding relationship between the resolution of the lens, the image resolution and the number of TV lines.

  III. Relevant calculations

  Next, we verify it by quantitative calculation. Taking MT9M034 as an example, the sensor has a target height of 3.6mm, a maximum resolution of 1280x960 and a pixel size of 3.75um.