The Sunny 16 Rule is a photography principle many people swear by, as much as the rule of thirds for composition. Although it hails from film photography days, the Sunny 16 Rule still applies to combining the three elements of exposure (shutter speed, aperture and ISO) to get a well exposed photograph when using manual mode.
This is because the Sunny 16 rule follows a pattern of setting the shutter speed and ISO to best match the f-stop (the number that measures aperture). This means the amount of light that enters through the camera lens will be appropriate in relation to how strong the sun is.
Take a look at the chart below to see how the Sunny 16 rule works in a number of different environments:
Getting completely correct exposure requires you to do more than just use the Sunny 16 rule. You need to also check your light meter and histogram while you are shooting, because a photograph’s exposure can be affected by lots of external factors (such as the clouds moving slightly, or light being reflected off another surface into the frame). However, the Sunny 16 rule gives you a good starting point for selecting your settings for each of the three exposure elements, when working in most types of light. You can then tweak the settings to better suit the light, if necessary.