The Sunny-16 rule has absoulely nothing to do with shooting at f/16. The rule is this:
For a frontlit subject on a bright sunny day, proper exposure is approximately f/16 at a shutter speed equivalent to the reciprocal of the ISO.
So, for an ISO of 100, the shutter speed would be approximately 1/100s; for an ISO of 200, 1/200s; etc. This sets up the following "proper" exposures.
........ISO 100...ISO 200...ISO 400...ISO 800...ISO 1600
f/2...@ 1/6400s
f/2.8 @ 1/3200s...1/6400s
f/4...@ 1/1600s...1/3200s...1/6400s
f/5.6 @ 1/800s....1/1600s...1/3200s...1/6400s
f/8...@ 1/400s....1/800s....1/1600s...1/3200s...1/6400s
f/11..@ 1/200s....1/400s....1/800s....1/1600s...1/3200s
f/16..@ 1/100s....1/200s....1/400s....1/800s....1/1600s
f/22..@ 1/50s.....1/100s....1/200s....1/400s....1/800s
f/32..@ 1/25s.....1/50s.....1/100s....1/200s....1/400s
f/45..@ 1/12.5s...1/25s.....1/50s.....1/100s....1/200s
Bear in mind that this is for a front-lit subject. For a side-lit subject decrease exposure by one stop, and for a back-lit subject decrease exposure by two stops.
The sunny-16 rule can be used in other lighting ituations as well with the following guidelines:
SUNNY means a bright sunny day with sharp shadows.
BRIGHT: bright sunny day on sand or snow increase exposure by one stop.
HAZY: sun through haze or thin high clouds with distinct but soft shadows, decrease by one stop.
CLOUDY: bright coudy day with very soft shadows, decrease by two stops.
OVERCAST: overcast day with no shadows, or open shade on sunny day, decrease by three stops.
HEAVY: heavily overcast day, decrease by four stops.
DEEP: deep shade or deep woods on sunny day, decrease by five stops.
STORM: just before heavy storm or deep woods on overcast day, decrease by six stops.
OFFICE: brightly lit office or store interior, decrease by seven stops.
STAGE: well-lit stage or sports arena, decrease exposure by eight stops.
HOME: well-lit home, decrease by nine stops.
SOFT: softly lit home, decrease by ten stops.
Have fun, folks.