If you look here, you'll find the equation: c = m*A*|S2-S1|/S2, with c being the circle of confusion in the focal plane (i.e. the film plane), m being the magnification, A being the diameter of the aperture, S2 the distance in perfect focus and S1 being the object distance.
Originally Posted by JBrunner
What does this mean: for equal subject framing and equal DOF impression on the final print you keep c/m constant. The larger the film, the larger the circles of confusion may become before a section looks blurry, but at the same time the larger the magnification m becomes. After all is said and done, only the aperture diameter determines your DOF. And that's exactly where the myth comes from that long focal length lenses or large format have narrow DOF: it's easy to have A=1mm with a 14mm lens (just dial in F/14), but next to impossible to find a 300mm lens with that aperture diameter.
When it comes to really narrow DOF, 35mm cameras rule at the moment. You get normal lenses with A=50mm, portrait focal legths (85, 135mm) with A=70mm, and longer focal lengths with A>100mm, even A>150mm. No medium or large format camera can offer you that. Your 8x10" normal focal length lens may be f=300mm, but with F/8 you get A=37.5mm, which is nice but not extraordinary.
All you have with large format is DOF too thin for reasonable hyper focal distances, that's why you need camera movements to get a landscape shot in focus, where 35mm cameras would just stop down a little.