DOF = C*N*(1+M)/(M^2 * (1-C*N/F*M), where C -- circle of confusion N -- F-stop F -- focal length M -- magnification
when you double the focal length and double the diameter of the circle of confusion,
everything else held constant, the DOF doubles (which makes sense -- if you keep
the same magnification, with a longer lens you have to step back).
if you also double the magnification, it's a bit more tricky and non-linear, but
DoF roughly halves (M^2 factor in the denominator).
best, Mishka
