How do you convert pixels to inches?

How to convert pixels of an image to obtain the original size of the object?

• If a camera is used to take a picture how to does one convert the pixels in the image to the original size of the object, if the focal point of the camera is 3.85mm and a 1/4" (.25in X .25in) sensor is used which is divided into 2,000,000 pixels?

• Answer:

  I'm not sure if the calculation will be exact when using a lens. For a pinhole camera, you could divide the width by the focal length and get (25.4*.25) / 3.85, or about 1.65 for the ratio of width of an object to its distance from the camera. So an object 5 meters across would cover the whole field if it's about 3 meters from the camera. Then each pixel would represent about 1/√2000000 of the 5 meters or about .0035 meters (3.5 mm).