If you know the size of the reference object you could work out the distance to it using the variables $CameraObjectWidth and $CameraObjectHeight the camera produces.

There will never be effective distance measurement on a single 2d camera. For distance measurement using cameras, there needs to be at minimum 2 (Stereoscopic) - that is how the xBox Connect works, by having two cameras. Well, that is also how you work! That is why you have two eyes.

Assuming the distance is constant, or slow moving, you could have the camera on some kind of sliding mount and take two pictures from two positions and compare them to get depth perception. That is how the new "Lensblur" feature in the Google Android camera software works. However, having a second camera like my HTC One_M8 to capture depth data is a whole lot easier and faster.

Alan

Thanks for the replies.

  I thought I remembered DJ mentioning it before so I thought I would ask. I'm fine with my pot and gearbox for now.