Suppose we have an object sitting in front of a pinhole camera. Regardless of the distance between the camera and the object, the following equation holds true:

We may use any unit (such as pixels) in the equation's left-hand side and any unit (such as meters) in its right-hand side (on each side of the equation, the division cancels the unit). Moreover, we may define the object's size based on anything linear that we can detect in the image, such as the diameter of a detected blob or the width of a detected face rectangle.

Let's rearrange the equation to illustrate that the distance to the object is inversely proportional to the object's size in the image:

Let's assume that the object's real size and the camera's focal length are constant (a constant focal length means that the lens does not zoom and...