In this tutorial I demonstrate how you can use expressions to control a drop shadow to give the illusion of depth. It is a pretty simple concept once you think about it. If an object is casting a shadow, as the object moves further from the background the shadow becomes less pronounced. The opposite happens as the object moves closer to the background.

1. Create an object that you want to apply the drop shadow to. (I created a solid 100 by 100 pixels)
2. Now apply the drop shadow effect to the object
3. Add an expression for the drop shadows opacity (alt click the stop watch) and link it to the objects scale value
4. Now divide 10000 by that value so that as the scale of the object increases the drop shadow opacity decrease should look like this 10000/transform.scale[0]
5. Alt click the stop watch next to drop shadow distance and softness and link both of these to scale as well and divide both by 10 should look like this 10000/transform.scale[0]
6. Now all you have to do is keyframe scale and as the object grows the shadow becomes less pronouce and as the object shrinks the shadow becomes more pronounced.