Depth of field and the subjects distance

Why is there less depth of field when an object is closer to the camera?

Thanks,
Kkamin

Might be an odd way of thinking about it, but as macrophotographer I just think in terms of magnification rather than focal lengths and distances. DOF is inversely related to magnification. So if an object is close you are magnifying it more in the image.
Obviously focal length , focus distance and magnification are related see http://en.wikipedia.or​g/wiki/Magnification

Brian V.

It deals more w/ perspective control, but DoF is discussed.

Here is some light reading for you.