Without a properly calibrated camera and a reference object of some sort this is not possible. You can estimate the distance with some tricks, but with a single camera its not easy.
Right now im writing my final year project about exacly that topic, took me 2 months and a special designed marker to achieve the measurement of distance and angle from the camera to the object. If i dont have the marker the results are wrong. If i want to measure a distance behind or in front of the marker: wrong.
If you take a image with a single camera you only have a 2D picture, if you have a reference you can restore 3D information to the reference, but everything else without reference cant be restored properly. For that you would need another picture from a second camera (stereo vision).
EDIT: I should note that i have a moving camera and the object (marker) is stationary, so i cant just calibrate the camera once for a set distance and then use that as reference, since my distance changes.