Answer
$f/8$
Work Step by Step
Example 25–2 shows that the exposure is proportional to the product of the lens opening area and the exposure time.
The time has been decreased by a factor of 4, so to keep the picture properly exposed, the area of the lens opening should be increased by 4.
Page 715 discusses how the square of the f-stop number is inversely proportional to the lens opening area. To increase the area by 4, we cut the f-stop number in half, from f/16 to f/8.
