pinhole cameras - Why does depth of field occur?

I wonder why depth of field occur in the first place. This is not a question about what changes the effect/size of field. We all know that.

Aperture, Focal Length, Focus Length (distance to subject), sensor size.

But why it happens in the first place?

When we think of pinhole cameras (camera obscura), and when we widen the pinhole, the image that drops to surface will be blurred, and we all know that hole is replaced by aperture on lenses.

Yet this still does not give a proven, explained reason of it.

Any ideas?

Answer

There are a couple of concepts that you really need to understand to grasp depth of field.

First, lenses focus on a plane. You can envisage this as, basically, a razor thin slice of reality and everything in that slice will be perfectly in focus. Everything in front, or behind, will not be.

Second, there is a range out of the focal plane that we will perceive to be acceptably sharp. When we reach a point before or after the focal plane where a point is sufficiently blurry for us to perceive it (a circle of confusion), it is out of the depth of field. So the depth of field becomes where the circle confusion is from the front to behind the focal plane.

Cambridge in Colour has a tutorial on this subject with more depth to it, no pun intended, as does Wikipedia. Wiki even has the physics behind it.