Use camera depth texture to find distance from camera

I realize that this must seem extremely basic, but I cannot seem to find any relevant information on the subject. If you believe this has already been answered, please read the rest of the question to make sure.

I am attempting to create my own Image Effect which is similar to the DoF effect, but there are some large enough differences that I cannot simply modify DoF shader. I need to find the distance between the fragment and the camera. The tricky thing is that I also need to find the angle between that fragment and a specific fragment. This means that the distance must not be in the 0-1 range, but in the NearClip-FarClip range.

In short, I need to know how to get a specific pixel from the depth texture, the camera frustum information, and the approximate world-space position of the fragment. Thoughts?