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I have an HDL-20D digital camera with 1000TVL horizontal resolution, S/N ratio 54dB and 2200k-pixel 2/3-inch 2x IT-CCD and these are the specifications:

enter image description here

How do I calculate PPI from these details?

Context: Trying to recreate the camera parameters for simulating images from the camera. I need focal length to set in the Camera object for Unity renderer. I have a Camera matrix (intrinsic matrix), obtained through Calibration using OpenCV. Camera matrix has focal length in pixel units. My plan is to calculate PPI from Camera specs, and then use that to figure out focal length in mm, and set that on the Unity renderer.

Thank you!

  • Can you explain the photographic context of this question? – mattdm Jul 2 '19 at 22:02
  • That is, this problem does not seem relevant to the art, science, or business of creating images with light. Is there a connection I'm missing? – mattdm Jul 2 '19 at 22:03
  • Honestly, I was looking for a "Camera Stack Exchange" but this was the closest I could find. Is this out of scope for this forum? – Ambareesh Jul 3 '19 at 2:04
  • It's... borderline. There isn't a "camera stack exchange" and I kind of wish there were. Because so many people on Stack Exchange have an engineering bent, it's easy for the actual questions about photography to get overwhelmed by these highly technical questions that have to do with cameras but not with making photographs. I'd like to have a space for photography itself. – mattdm Jul 3 '19 at 3:00
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    I'm voting to close this question as off-topic because this is about building a (simulated) camera, not about taking images. – OnBreak. Jul 3 '19 at 23:22
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From https://en.wikipedia.org/wiki/Image_sensor_format#Smaller_sensors ...

the 2/3 inch sensor size is 8.8 x 6.6 mm.

I don't know your image size, but from the 2,200,000 pixels, the calculator at my site at https://www.scantips.com/mpixels.html says it has to be about 1712 x 1288 pixels if 4:3 aspect ratio.

(or 1976 x 1112 if a native 16:9 chip, but which would likely be 1920x1080 then, but that's 2.0736 megapixels).

1712 pixels / 8.8 mm is 195 pixels per mm (at the sensor). The focal length is surely found in the image exif.

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  • Maybe slightly more straightforward math: but since the count of pixels is essentially an area measurement, the pixel-per-linear-measure should just be the square root of the quotient of pixel count over sensor area (i.e., length * width). So px/mm = sqrt(2.2e6 / (8.8 * 6.6) mm^2) ~= 195 px/mm. Neatly avoids the aspect ratio stuff. – scottbb Jul 3 '19 at 20:53
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    Agreed, that is another way. However, since the pixel dimensions are often the only numbers we actually know (four significant digits), it always seems a good plan to multiply image Width x Height to double check the spec megapixels. I don't know the manufacturers method for megapixels, maybe they are counting the non-image pixels too, but they usually seem a little too large to be actual image working dimensions. – WayneF Jul 3 '19 at 23:11

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