This is such a powerful tool for both amateur and professional photographers that I decided to dedicate a page solely to it. 
Sunny 16...you'll remember this tip not only because it has a cute name, but because it is your exposure sidekick; your backup when you don't have a digital light meter or your camera's light meter is not giving you the correct exposure for the shot. The key to using this rule is that it works only during daylight hours, but is effective to use with the different lighting conditions you may experience in a given day.
Here are the steps you'll need to follow to make it work for you.
Make your shutter speed equal to your ISO. (When shooting in full sun an ISO of 100/200 is best.) If you choose an ISO of 100, then your equivalent shutter speed will be 1/125 (number closest to 100). If you choose an ISO of 200, then your shutter speed will be 1/250
(number closest to 200.)
If you want to change the aperture to achieve a different effect in your photo, (blur the background or bring more of the shot in focus) then use the chart below to help you determine the aperture and shutter speed combo, but keep in mind your available light. You can also make the adjustments by remembering that whenever you move your aperture up, you have to move the shutter speed down the same number of stops to have a balanced exposure. Likewise, when you move your aperture down, you need to move your shutter speed up the same number of stops.
Let's put this into practice: It's a sunny day and you set your ISO to 100. You are shooting a landscape and want to bring everything into focus by using an aperture of f/22. Adjust your aperture to your starting point f/16 (remember sunny 16!) and move it up until you arrive at f/22 (count the number of stops you make as you go up the scale). Next, adjust your shutter speed by making the same number of stops down on the scale until you arrive at 1/50. This will give you a balanced exposure. Super easy!
The chart below is a guide for use under different lighting conditions during daylight hours. It uses a combination of aperture, shutter speed and ISO to estimate the correct exposure for the shot. This is a great chart to print and take with you.
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| Table contents courtesy of Guide to Film Photography |
Remember:
1. Sunny 16 is just a guide, a starting point to help you achieve a balanced exposure. It will not produce perfect results every time. You may have to adjust exposure compensation if you feel the shot is over or under exposed.
2. There will be times when you may choose to underexpose or over expose a shot to achieve a particular artistic effect which is perfectly fine!
Wait! There's more!
Visit this site for a few other helpful rules for photographing in challenging situations: http://www.digital-photo-secrets.com
