Calculating the distance between an object and a camera in not an easy task. In theory, you would need two different cameras because one camera only gives you two-dimensional information.
If you only have one camera, you need some tricks to evaluate that distance. If the object (or the camera) is moving you can use some perspective rules to guess the object distance (an object very far away as the background do not change its size while you move the camera, while objects really close to it change their size).
