If you are an experienced or professional photographer you will immediately understand how to use Sunny16 and how it can enhance your creative process. For everyone else, or the curious, please read on.
The Sunny 16 exposure rule allows you to determine a correct exposure under any naturally occurring lighting condition without the use of a light meter. Here's how it works: under bright, direct sunlight set your aperture (f/stop) to f/16 and your shutter speed to the reciprocal of your ISO setting. In other words, if your ISO is 100 set your shutter speed to 1/100 and your aperture to f/16 and you will get a correct exposure under direct, bright sunlight. And the rule is adaptable to any naturally occurring lighting situation you might find yourself in but it gets a bit difficult to remember how the rule shifts under different light. Sunny16 implements this exposure rule in a big way. It will show you all the correct exposures for nearly any naturally occurring lighting condition.
An exposure is a certain amount of light falling onto film or a digital camera's sensor. The amount of light is controlled by two things. One is the camera's shutter which controls the amount of time that light is allowed to pass. The other is the opening in the lens, the aperture, which controls how much light is allowed to pass. A correct exposure is achieved when these two elements, working together, allow the correct amount of light to pass. There are many ways these two elements can work together to obtain a correct exposure. A small lens opening with a longer shutter speed can result in the same exposure as a large lens opening with a shorter shutter speed. For the conditions you specify, Sunny16 allows you to see all the combinations of these two elements that result in a correct exposure.
When Sunny16 first starts you are presented with the default settings and exposure. From that default state you can change anything you like by just tapping. Go ahead and experiment. You cannot end up with an incorrect exposure with Sunny16.
Think of a reciprocal exposure as an equivalent exposure. Sunny16 always shows you all of the possible reciprocal exposures for the conditions you specify. Take note of the current shutter speed and f/stop. Tap a shutter speed above or below and you'll see that the matching f/stop follows. This new shutter speed and f/stop combination will result in the same exposure as the combination selected a moment ago. But maybe not the same image--more on that below. Keep tapping or swiping up or down until Sunny16 doesn't let you go any further. Now go the other direction all the way.
All of these shutter speed and f/stop combinations will result in a correct exposure under the conditions you have specified. But they won't result in identical images. Much of the creative, and fun, part of photography is achieved by understanding the relationship between shutter speeds and f/stops. Do you need deep or shallow depth of field? Do you need to freeze action or do you want to blur or pan? Sunny16 always shows you all the possible combinations of shutter speeds and f/stops so you can decide which one is best for your situation.
Select a new shutter speed. Then change the exposure compensation. You'll see the f/stop change and the shutter speed you selected will remain. Now select a new f/stop and change the exposure compensation again. This time the shutter speed will change and the f/stop will remain the same. Sunny16 will do it's best to keep your last selected shutter speed or f/stop if you change the ISO, the light, or the exposure compensation.
Tap the shutter speed below the selected shutter speed. Keep tapping below or swipe up (swiping is faster). Eventually the shutter speed will reach a limit. See the screen shot. In this case there are more shutter speeds available but you can see that the limit for the f/stop has been reached. Tap one of the grey shutter speeds. Sunny16 rolls it back. Remember that for the conditions you specify Sunny16 will not allow you to create an incorrect exposure.
Exposure compensation allows you to quickly override an exposure. For example, if your subject is backlit you may want to increase the exposure and create some highlights in the scene. Another important use for compensation is bracketing. Bracketing is a technique photographers use to increase the chances of capturing an image with the perfect exposure. A common method is to take one picture at the default exposure, another with an increased exposure, and another with a decreased exposure. Sunny16 allows you to do this quickly and easily.
If you want to pursue photography as a hobby it is important to develop an understanding of what stops mean. But for now, if this is an unfamiliar term, for the purpose of using Sunny16 there are only two things you need to know. The first is that the 1/3 stop increment provides more options for a particular exposure than Full stop increments. Three times more options to be precise. And the same math applies to the 1/2 stop increment. In the end, this is a personal preference. If you are unsure just leave it at 1/3 for now. The second thing to remember is that whichever increment you select on Sunny16 you need to select that increment on your camera as well.