I did a small proof of concept to calculate distance in 3D using a greyscale 2D image.

Here is the sketch on openprocessing
http://openprocessing.org/visuals/?visualID=19725

I assume the two items are on the same plane (i.e. looking directly at one another) so the camera can see both along the z-axis, sorta like a slice of an apple where you cut along the z-axis that connects the two points.

Here is a screencap


Here is the writeup

An experiment using a greyscale image of ellipses reducing in grey intensity. The brightness is used a test of distance. This is to be translated to 3D where each ellipse represents a sphere.

As the items goes further away i.e. down the z-axis , up the y axis or across the x axis it is assumed the two objects are on the same plane.