# The Sunny 16 Rule in Cinematography

If you’ve done much still photography, particularly on celluloid, you will probably have heard of the Sunny 16 Rule. It’s a useful shortcut for correctly exposing bright day exteriors without needing a light meter. Is it of any use in digital cinematography though? Yes, and I’ll explain how.

### How the rule Works

Sunny 16 is very simple: if the sun is out, set your aperture to f/16 and your shutter speed denominator to the same as your ISO. For example, at ISO 100 set the shutter to 1/100th of a second. At ISO 400 set the shutter to 1/400th of a second – or 1/500th of a second, if that’s the closest option the camera permits – and so on.

You can use the rule to work out other combinations from there. Say your ISO is 100 but you want the sharper, less motion-blurred look of a 1/400th shutter. That’s two stops slower, so open the aperture from f/16 to f/8. (Check out my exposure series if this is all Dutch to you.)

The Sunny 16 Rule works because the sun outputs a constant amount of light and is a constant distance from the earth – at least constant enough to make no significant difference. The sun’s illuminance at the earth’s surface is about 10,000 foot-candles. The following formula relates illuminance (b) to f-stop (f), shutter speed (s) and ISO (i):

Using Sunny 16 in the case of ISO 100 and a shutter speed of 1/100th of a second, this formula gives us…

… 6,400 foot-candles. Less than 10,000fc, certainly, but remember this is only a rule of thumb – and one designed for film, which isn’t hurt at all by a little over-exposure. The rule probably accounts for the fact that you may want to see into the shadows a bit too. (See my article “How Big a Light Do I Need?” for explanations of illuminance and foot-candles and more on the above formula.)

Anyway, you can see from the equation why the shutter speed denominator and ISO cancel each other out if they’re the same.

### Using the rule in cinematography

A few weeks ago when I was on the banks of the River Cam setting up for a scene in Harvey Greenfield is Running Late, my 1st AC Hamish Nichols asked which ND filter I wanted in the matte box. It was 5:30am; the sun had barely risen and certainly wasn’t high enough yet to reach me and my light meter over the trees and buildings on the horizon. But I knew that it would be hitting us by the time we turned over, and that the weather forecast was for a completely cloudless day, indeed the hottest day of the year at that time. So I was able to predict that we’d need the 2.1 ND.

How did I work this out? From the Sunny 16 Rule as follows:

• I was shooting with a 1/50th of a second shutter interval (a 172.8° shutter angle at 24fps), so the Rule told me that f/16 (or T16) at ISO 50 would be the right exposure.
• I was actually at ISO 800, which is four stops faster than ISO 50. (Doubling 50 four times gives you 800.)
• I wanted to shoot at T5.6, which is three stops faster than T16.
• That’s a total of seven stops too much light. To find the right optical density of ND filter you multiply that by 0.3, so 0.3 x 7 = 2.1. (More on this in my ND filters post.)

Everything on a film set sucks up time, so the more you know in advance, the more efficient you can be. Little tricks like this mean you don’t have to do a last-minute filter swing and waste five minutes that the director could have used for another take.