Exercise 2 solution

DOF changes with the square of the distance. Doubling 2 m to 4 m multiplies it by 4, so 0.27 m becomes about 1.08 m.
Script output confirms: u=2 m f/2.8 -> 0.27 m; u=4 m f/2.8 -> 1.08 m.
Note: stepping back also makes the subject smaller in the frame, so this is not the same picture; framing was not held constant.