The Sunny 16 Rule

The ‘sunny 16’ rule has been around for years. It’s a way of calculating exposure when you don’t have a light meter.

Modern cameras have light meters built into them. Casual photographers might not even think that the camera is calculating how much light it is ‘seeing’, but the camera is making that calculation to work out the exposure.

As you might know, exposure is a function of three things. The first is how sensitive the sensor in the camera (or the film in a film camera) is.

The second is how big the hole or aperture is through which the light is hitting the sensor or film.

The third is how long the hole or aperture is open for.

That’s why you will see an exposure stated as something like this:

1/50th second at an aperture of f2.8 at ISO 100

The ‘sunny 16’ rule is a way of calculating exposure when you don’t have a light meter. The rule is that on a sunny day, for a given ISO the correct exposure will be f/16 at a shutter speed that is the reciprocal of the ISO.

In other words, at ISO 100 the shutter speed will be 1/100th of a second.

1/100th second at an aperture of f16 at ISO 100


So now let’s see what that means about a setting of

1/50th second at an aperture of f2.8 at ISO 100

Let’s see how ‘unsunny’ the light is.

Let’s break it down, looking at the shutter speed first:

1/50th second means that it is half as sunny as at 1/100th second

Apertures following these settings, with the next one in the series allowing twice as much light in as the setting before:

f16, f11, f8, f5,6, f4, f2.8

So f2.8 means that it is letting in five times as much light as f16. Or in other words, the light is 1/5 as sunny as at f16

So combining the shutter speed and the aperture we can see that the light is 1/10th as unsunny as on a sunny day.

Looking out of my window just now, that is exactly the reading for the aperture on this miserable winter’s day here in Edinburgh – where only one tenth of the light is coming in through the window compared to what would come in on a sunny day.

By the way, I wrote an article about lens apertures and how high-end modern cameras can do more than measure how much light is hitting the sensor. They have scenes built into their memory chips and they measure them against what they are looking at.

The Limitations Of The Sunny 16 Rule

The rule says that on a sunny day the correct exposure for a film (or the sensor in a digital camera) will be a speed that is the reciprocal of the ISO at an aperture of f16.

In other words, f16 at 1/100th second at ISO 100.

But is it true that this is the amount of light on a sunny day?

Yes, it is true for temperate latitudes around the world. But for the Tropics, where the sun blazes down from a cloudless sky – the Sunny f22 rule is more accurate.

6 thoughts on “The Sunny 16 Rule

  1. I did know about Sunny 16. Most of the time I shoot in Aperture priority mode, but also Manual. I really like to shoot birds, and I don’t feel that I have all that much time to fiddle around with the M mode then. Sometimes I ask myself; ‘Why don’t I just shoot in A all the time, and let the camera take care of it all?!’ Quite often it seems to do a better job on it. I think I’m losing it…

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    1. I doubt you are losing it 🙂

      The famous black cat sitting on a heap of coal, or a white cat in the snow. How does the camera know the exposure when it is not within the ‘normal’ range?

      I shoot Aperture Priority all the time and when I think there is a problem I either change to spot metering or use the exposure compensation dial.

      I am so hesitant with it, though. I try 0.7 or something like that and sometimes finish up with 2.0 stops before the histogram looks correct.

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      1. That’s true, of course … and even just _snow_ itself, without the cat 🙂 I use the exposure compensation a lot. The histogram is a great tool.

        I will get back into it … perhaps when we’re out of the deep freeze. -22ºC and windy!

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