Unicycle, Rome, 2009, photograph by Angela Kleis
Back in the olden days, portraits took a long time since they had to be carved in stone. Then, cameras were invented. It's hard to believe these primitive cameras used an archaic medium called film, and there was no "auto" setting, so exposure had to be set by hand. Thank goodness we left the dark ages for digital cameras and their on-board light meters, phew!
Except that people still use those primitive cameras, film is still the same fantastic medium it always was, and there isn't always time to stop and take a light reading before that dog riding the unicycle gets away. How can you be sure to get the right exposure? (And, you ran out the door without your cell phone, what are the chances?!)
The Sunny 16 Rule is a method to estimate the correct exposure without a light meter: On a sunny day, set the aperature to f/16 and the shutter speed to the 1/ISO (film speed or digital ISO setting) for a subject in direct sunlight.
This means that if the dog riding the unicycle is also wearing sunglasses because it's so bright and sunny, and the film loaded in your camera is ISO 100, set the aperature to f/16 and the shutter speed to 1/100 (or 1/125 if that's the lowest setting on your camera). If the film is ISO 400, then the aperature is set to f/16 and the shutter speed is set to 1/400 or 1/500. Pretty simple, huh?
Of course, an aperture of f/16 may not always work for your needs - and, the Sunny 16 Rule can be adjusted. Think of f/16 as a starting point - as long as the aperture and shutter speeds are equivalent, they can be changed: at ISO 400, with each decrease in aperture number (from f/16 to f/11), double the shutter speed (from 1/500 to 1/1000). Taking this in the other direction, increase the aperture number by one stop (from f/16 to f/22) and decrease the shutter speed by half (from 1/500 to 1/250). This basically keeps the amount of light hitting the film or sensor the same since the proportions between the size of the opening (aperature) and the time it's open (shutter speed) are consistent.
If the weather isn't sunny, there are other aperature starting points to guide you - just combine with 1/ISO shutter speed:
Bright sand or snow: f/22
Sunny, hazy day: f/11
Bright, cloudy day (faint shadows): f/8
Heavy clouds/overcast (no shadows): f/5.6
Heavy shade (shadows from tall buildings): f/4
Hopefully this will help estimate the correct exposure starting point when using a camera without an on-board or hand-held light meter, or to even creatively adjust your DSLR.