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Where does that 0.39 factor come from? You can get a raw idea of the pixel/mm ratio dividing your resolution by the effective field of view (that is, at a GIVEN DISTANCE). But you have to be aware of 2 things: doing this you're not taking into account lens distorsion and as you move away from the center of the image the error increases since your area is flat, so points in the borders are laying at a different (greater) distance from the camera sensor than the ones in the middle. I suggest calibrate your camera for better results on a given plane, but with a single fixed device setup you won't be able to perform measurements in depth.