A camera takes a perfectly exposed picture when the lens diaphragm is set to f/4 and the shutter speed is 1/250 s. If the diaphragm is changed to f/11, what should the new shutter speed be so that the exposure is still correct? (Standard camera shutter speeds include 1/250 s, 1/125 s, 1/60 s, 1/30 s, and 1/15 s.)