Exercise 2 solution

f/8 to f/4 is two stops more light (f/8 -> 5.6 -> 4). Cut the time by two stops: 1/125 -> 1/250 -> 1/500 s.
Check: EV f/8, 1/125 = log2(64*125) = log2(8000) = 12.97. EV f/4, 1/500 = log2(16*500) = log2(8000) = 12.97. Same.
(Run ev(8, 1/125) and ev(4, 1/500) in the chapter script.)