Way before all of the fancy digital technology that’s simplified the calculations needed. Sunny 16 rule explained. The Sunny 16 Rule simply states that in bright sunlight with an aperture of f/16, your shutter speed should be the reciprocal of your ISO. EV 15 in bright sun is ISO 100, 1/125 at f/16, or 1/3 stop different than Sunny 16. The magic of analog photography is back! 3 talking about this. Using the Sunny 16 Rule. The Sunny 16 rule says that the exposure for frontal lighting in bright sun is f/16 at 1/ISO seconds (all Equivalent Exposures work too of course). Really simple isn’t it? This is especially useful if you are shooting with a manual camera that has no built-in light metering. The Sunny 16 Rule is simply this – On a bright sunny day, you set your camera’s aperture to f16 and the shutter speed to the reciprocal of your film speed or ISO. But to better understand how light metering works, it’s helpful to have a look at the Sunny 16 rule. The name is exactly what we’d expect. So, let’s take a look at how the Sunny 16 Rule works in real life. So, Sunny 16 in bright direct sun with ISO 100 is 1/100 second at f/16, or 1/200 second at f/11, or 1/400 second at f/8, etc. So, for example if you have 400 ASA Film, you would set your shutter speed at 1/500 sec and f-stop at f/16 When you do not have a light meter, the "Sunny 16 rule" or "Sunny f/16 rule" is a method to estimate correct daylight exposure. The Sunny 16 rule is a rough rule of thumb that was used back in the film camera days. The Sunny 16 rule can also help to determine aperture and shutter speed settings when conditions are not typical sunny days. Think, Point, Shoot! If you need the "Sunny 16" rule, then that probably means you don't have a light meter, and that probably means that your camera doesn't have a light meter, and that probably means that it is not a digital camera, in which case, you don't exactly set the ISO. The Sunny 16 Rule is a rule of thumb means to find the correct exposure without using your camera's metering.