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is there a formula to calculate the pixel size of a object at long distance? Here is the scenario, I have a very small object 1cm by 1cm at 200m away from my camera. The camera has a resolution of 640x480. So I want to find out the pixel size of my object in the camera screen. What about parameters would I need as well? Thanks.

  • Check first answer, just rearrange the equation: photo.stackexchange.com/questions/12434/… – Romeo Ninov Jul 10 '15 at 4:09
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    If you do not define the focal length and the sensor size there is no way to define it. Besides that I don't know what you mean exactly by pixel size. A pixel mesures... one pixel. – Rafael Jul 10 '15 at 4:20
  • This is a really neat problem to figure out. I think I understand you want the area in pixels the object might take up in your final image? Rafael is spot on with needing to know the focal length and sensor size and crop factor. I also believe that the color, lighting, and focus would determine the final pixel size, but I would also assume you are talking about perfect conditions rendering your 1cm^2 object perfectly visible with good contrast in your final composition. – cliffclof Jul 10 '15 at 6:43
  • yes i am trying to figure out the image area in pixels. as mention by romeo, i got the equation: distance to object (mm) = [focal length (mm) * real height of the object (mm) * image height (pixels)]/[object height (pixels) * sensor height (mm)] may i know what is meant by image height in pixels and object height in pixels? – nee meng Jul 10 '15 at 7:06
  • object height in pixels is the number of pixels of object projection to the sensor. – Romeo Ninov Jul 10 '15 at 9:17
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You must know sensor size (mm) and focal length (mm), and the pixel dimensions of your sensor (pixels). If it is a compact camera or phone, you may not know much of this.

If you know, you can go to http://www.scantips.com/lights/fieldofview.html to determine the field of view at 200m.

Lets assume a DSLR with sensor size 24mm wide, and focal length is 100mm and the sensor is 6000 pixels wide.

First, it computes the horizontal field of view at 200m is 48m. This distance through this lens onto this sensor will show this width.

Then we know the sensor width is 6000 pixels which covers the 48m.
1 cm would cover 6000 pixels width x 1cm/4800cm = 1.25 pixels. A pixel cannot be divided, so call that 2 pixels.

Or, you could take the picture, zoom in on it really large, to say 1600% size, so you can see the pixels well, and then count them on this object.

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The simple way is to measure it. Take a picture of a ruler at a known distance from the lens, with the ruler extending out of the frame. Say the ruler is 10m from the camera and you can see 1.37m vertically. Then at 10m, each pixel represents 1.37/480 m. At 200m, each pixel represents 20 times more size because you are 20 times further away, so it represents 20*1.37/480 meters

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