[Be nice to Jordy. He serves an important purpose here.] I guess one would "care" if they were designing a 3D program to simulate more realistic camera shots. Or, if they were designing lenses or cameras. All worthwhile ambitions. I suspect the answer is in the mathematics in the above links, but 'prolly not stated as a ^2 or ^3 as this thinking is more related to things like light falloff. With DOF, the "circle of confusion" is going to get bigger as distances go back, and forward!, from the exact focus point. I'm guessing, but as a first approximation I'd bet that the radius increases linearly as the distance moves away from exact focus. For 3D modeling, one could perhaps use a Gaussian Blur, with the radius varying linearly with distance? In reality, details like the shape of the aperature matter; resulting in distant highlights taking on a funky shape. I also wonder if the new "digital" lenses have different DOF characteristics because of the colluminating rear element?