The "Sunny 16" Rule
We should begin this treatise by clearly stating that the Sunny 16 rule is nothing new, and if anything it’s a formula that’s been kicking around since the early days of picture taking. It’s also worth noting that the Sunny 16 rule has held true through every technological breakthrough from tinplates straight through modern-day digital imaging. It held true way back then and it holds true today.
In a nutshell, the Sunny 16 rule is a simple method of establishing a correct exposure when taking pictures outdoors without using a light meter. The premise of the Sunny 16 rule is that sunshine is a constant source of illumination, which depending on a short list of variables, is easy to classify. You have clear, sunny skies, hazy days, slight overcast, heavy overcast and precipitation in the form of snow, sleet or rain. Add to the above parameters the reflective nature of the surroundings, i.e. city/suburban streets, the beach, snowscapes and high/low altitudes.
The starting point for establishing the correct exposure is to set the only non-variable part of the equation, specifically the f-stop of the lens, which as you might guess when talking about the Sunny 16 Rule, is f/16.
Note: Once the correct exposure is formulated you can change this to any combination of shutter speeds and apertures, but for this exercise, stick to f/16.
Once the lens is set to f/16, we now have to establish the correct shutter speed, which on a clear sunny day should correspond to your working ISO speed. What this means is if you’re shooting a landscape or a portrait of your cocker spaniel, for that matter, on a sunny day with your camera’s ISO set to 100, the correct exposure should be f/16 @ 1/100-second. Similarly, if your ISO is 400 or 4000, the shutter speed should be 1/400- or 1/4000-second respectively. Easy… no?
The thinking part comes into play when it’s not a bright, sunny day, and here too we’re not talking rocket science. If you’re shooting on a hazy day, open up a half stop to f/11.5. Cloudy? Open up a full stop to f/11. Light rain? Open another half stop to f/8.5, which is a stop and a half wider than the base f/16 we use on sunnier days. And if it’s pouring cats and dogs or the skies are heavy overcast, open up a full 2 stops to f/8 and you’re good to go.
The opposite of urban haze, clouds, rain and snow are sunny days at the shore, snow-capped landscapes and higher altitudes, each of which tends to be brighter due to the reflective qualities of sand, water, snow and the thinner atmospheric qualities of high altitude mountaintops. In these cases, instead of f/16, set your lens an additional stop smaller to f/22. If your lens doesn’t stop down further than f/16, simply bump your shutter to the next faster shutter speed, which from an exposure standpoint, is effectively the same.
Now for those of you aching to know if it’s possible to use apertures other than f/16, rest assured, you certainly can. F/16 is merely the designated starting point, partly because the “Sunny 5.6 Rule” doesn’t have quite the same ring to it, and more importantly, decimal points tend to confuse we “creative” types.
As for the best combination of f/stop and aperture, your final choice should be determined by the nature of the photograph you’re capturing, i.e. a landscape, sporting event, portrait, etc. That said, an exposure setting of f/16 @ 1/100-second can be easily converted to f/22 @ 1/50, f/8 @ 1/400, or f/2 @ 1/6400-second. The depth of field and blur of moving objects within the frame will vary, but the exposure density of each of these exposure variations of the original f/16 formula will remain identical regardless of your final choice of f/stop and shutter speed settings.