Doremus,

If you take a 150mm lens and focus at infinity using for example f5.6, the physical aperture diameter is 26.8mm. That physical aperture will, for any point in the subject, throw a cone of light from the rear lens element to the film plane. The distance of the base of the cone to the film plane and the diameter of the cone base will determine the angle of incidence of the outer edges of the cone of light on the film plane. Now extend the bellows to 300mm instead of infinity(150mm). The diameter of the cone base remains the same. But since the distance has changed then the angle of incidence on the film also changes. It gets less and since it becomes less the depth of focus is increased because the film can be further out of position from perfect focus whilst still being within the deired circle of confusion. At infinity the depth of focus is at its smallest so if the film is not in the correct plane the error will be maximised.