Back in the days of film, there was really no LCD and no histogram to review your photo for proper exposure. Photographers relied on their exposure rules. One of those rules that is still applicable today is the "Sunny 16 Rule."
Basically, the sunny 16 rule (also known as the sunny f/16 rule) is an easy method of dialing in the correct exposure in sunny environments without having a light meter. The rule is based on incident light (hitting your subject), rather than reflected light "reflecting from your subject).
The rule states that nn a sunny day you can set your aperture opening to f/16 and your shutter speed to whatever ISO you have chosen for any subject in direct sunlight. So, on a sunny day:
- With your ISO set at 100, set the aperture at f/16 and the shutter speed to 1/100th of a second
- With your ISO set a 200, set the aperture at f/16 and the shutter speed to 1/200 or 1/250
- With ISO 400 and aperture at f/16, set shutter speed to 1/400 or 1/500
That said, Sunny f/16 rule at 1/100th of a second can be converted to f/22 @ 1/50, f/8 @ 1/400, or f/2 @ 1/6400th of a second. The depth of field and blur will vary, but the exposure correctness of each of these variations will remain identical regardless of your final choice of f/stop and shutter speed.
We can then deduce that there are other rules for different shooting conditions but those are not as easy to remember:
- The snowy/sandy F/22 rule
- The overcast F/8 rule
- The slightly overcast F/11 rule
- The heavy overcast F/5.6 rule
- The sunset F/4 rule
Luckily for you though, you don't need to memorize all of the above. Just start with the base Sunny f/16 Rule and all you need to do is to keep moving our aperture up by one stop and our shutter speed down by one reciprocally stop until you reach f/22.
The idea with the Sunny 16 rule is that you can apply it without worrying about having a blurry photo that is under or overexposed. It’s worth noting that the Sunny f/16 rule has held true through photography technological advancement from the early days straight through modern-day digital photography. It held true then and it holds true today.