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I'm attempting to build something with a Raspberry Pi that can hopefully capture moving vehicle's license plates 50-100 ft away (and at night). I understand very little about photography, but through my own research as I understand it, the focal length needs to be decently large in hopes of getting a high enough resolution at this distance.

If necessary, I can build a panning motor to put the camera on that will constantly scan around (assuming there is no low focal length/large FOV camera that can capture this information). I'm also concerned about if there are headlights next to the plate, wouldn't the aperture have to be low in hopes to see anything?

So far I've been lead to these two products,

A Rapberry Pi HQ 12.3MP IMX477 Camera

A 35mm focal length lens with adjustable aperture

Would the manual focus on the lens be an issue? Is there any better way to achieve this(if this even works), and how would IR affect the situation?

I understand this is a specific situation and probably has a complicated answer, but any type of ELI5 answer as to how I can achieve this would be amazing, I've been fumbling for hours trying to understand how something like this might be achievable.

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  • \$\begingroup\$ Please help clarify: is your problem substantially similar to existing toll and/or red-light camera systems? That is, are you trying to identify license plates through a fixed traffic lane/zone? Or is your problem based on a moving mount (e.g., cameras on a car)? \$\endgroup\$
    – scottbb
    Mar 9, 2022 at 0:07

2 Answers 2

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Assuming:

  • License plate digit height h (4", 10cm: 100mm)
  • Distance D (90', 30m, 30000mm)
  • focal length f
  • Size on sensor: P: at least 20 pixels, from you specs: 1.55µm/pixel, so for 20 pixels: 20*0.00155=0.031mm
P/f = h/D

so

f = P*D/h = 0.031 * 30000 / 100 = 9.3mm

With your 35mm the height in pixels will be

P = h * f / D = .117mm  

In pixels

P = (h * f / D) / 0.00155 = 76px

But if the license plate moves your problem will be exposure time to avoid motion blur while the light on license plates isn't that powerful (in fact license plates are designed mostly to reflect light that comes from an external source). There is a good reason why speed trap radars have a flash... (and why they have to be close to the cars, because the flash output is limited)

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This will not be a full answer but would like to mention some points:

  • manual focus is OK when we talk about static objects
  • the sensor is more sensitive to day light, for night vision you should add strong infrared source. And you may not be happy with the result
  • in the night you should stay with aperture full open to capture as much as possible light

Maybe some security camera (like TAPO C320WS) will be cheaper and better. Moreover this camera offer motion detection which can help you take the image in the right moment.

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