Back in the days of film cameras, being able to accurately judge a correct exposure without having to fish around for a light meter was extremely handy. To do this, photographers would use the Sunny 16 Rule. Now that we have light meters and histograms built into our digital cameras it’s easy to dismiss it as a relic of a bygone era. However, knowing a simple and accurate formula to calculate a good exposure can save you time and be very useful. And if you want to try film photography, it’s a must-know rule.

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Defining the Sunny 16 Rule

The Sunny 16 Rule says that on a clear, sunny day you will achieve a good exposure shooting with an aperture of ƒ/16 and a shutter speed that’s the inverse of your ISO. If your ISO is set to 100 then your shutter speed should be 1/100 seconds. Change the ISO value to 400 and you should adjust your shutter speed to 1/400.

Aperture = ƒ/16; ISO = 100; Shutter speed = 1/100 seconds

Why do I need the Sunny 16 Rule?

It’s easy to understand why the Sunny 16 Rule was so useful for film photographers: cameras didn’t have integral metering systems and your ISO was your film speed. When you put ISO 200 film into your camera, that was your ISO and you had to alter the shutter speed and aperture to get a good exposure. But with digital cameras that have light meters and you can adjust your ISO? It’s still useful.

1. You can set your exposure quickly when you’re in manual mode.
2. If you’re not sure how accurate your light meter is, it gives you a good baseline exposure.
3. If you want to try film photography, you know an easy means to set your exposure.
4. Light meters measure reflected light not incident light, which in some situations can provide an inaccurate reading. The Sunny 16 Rule works on incident light not reflected light, giving you a more accurate exposure setting.

To expand on point 4 a little more, while light meters are now far more accurate than ever before, they can be thrown by either very bright or very dark subjects in a scene. For example, you might need to deliberately overexpose an image with a bright white wedding gown in it, because your camera’s meter can’t process it can be that bright. Or maybe you are photographing a black dog or cat and the exposure setting from the light meter makes it look grey. In which case, you need to deliberately underexpose according to your light meter. The Sunny 16 Rule gives you a bit of a reality check that you’re getting it right.

What if I want to use a different aperture to ƒ/16?

An aperture of ƒ/16 creates a very deep depth of field. Lots of your scene will be in focus. If you want a shallower depth of field with a narrower plane of focus, you use a larger aperture. Given it’s the Sunny 16 Rule, how do you adapt it for other apertures? By using the exposure triangle, you can adjust the shutter speed or the ISO variables according to the number of stops you change the aperture.

For example, by opening up your aperture 1 stop, to ƒ/11, you can maintain the balance of the Sunny 16 Rule by adjusting either the ISO or shutter speed by one stop. That would mean the ISO changing to 200 or the shutter speed to 1/200 second.

If you were to open up the aperture by 2 stops, to ƒ/8, then you would need to adjust the ISO or shutter speed by 2 stops. You could do that by changing each by 1 stop. Or you might choose to alter the shutter speed by 2 stops, making it 1/400 seconds.

Assuming you maintain ISO 100, this table gives you a quick guide to adjusting your shutter speed according to your aperture, when it’s sunny.

Aperture	Stop change increase	Shutter speed and ISO	Stop change decrease
ƒ/16	0	1/100 second / ISO 100	0
ƒ/11	+1	1/200 second / ISO 100	-1
ƒ/8	+2	1/400 second / ISO 100	-2
ƒ/5.6	+3	1/800 second / ISO 100	-3
ƒ/4	+4	1/1600 second / ISO 100	-4
ƒ/2.8	+5	1/3200 second / ISO 100	-5

If you know that you prefer to shoot with an aperture of ƒ/5.6, then remember that a shutter speed of 1/800 second and ISO 100 should give you a good exposure on a sunny day.

What if it isn’t sunny?

Of course, it isn’t always sunny with a bright blue sky. What do you do if the weather isn’t adhering to a photographic rule and you have a heavy overcast sky? You rewrite it slightly!

Weather condition	Shadow	Aperture
Snowy or sandy	Very marked	ƒ/22
Clear and sunny	Distinct	ƒ/16
Slightly overcast or some cloud	Soft	ƒ/11
Overcast	Barely noticeable	ƒ/8
Heavy cloud	None	ƒ/5.6
Open shade, sunset	None	ƒ/4

You adjust the aperture according to the weather but retain the reciprocal of the ISO and shutter speed element of the rule. This means that you stick to ISO 100 and 1/100 second shutter speed. Or ISO 200 and a shutter speed of 1/200 seconds. If you’re quite sure how to judge the weather conditions, look at the shadows and use those to help you decide on an aperture.

Looking at the shadows

Using the weather conditions to select your exposure can be a little inaccurate and rough. After all, it can be a day with bright sun, but quite a few clouds. Instead, look at the shadows in your scene to help you determine a proper exposure.

Using the Sunny 16 Rule in real life

Remember: the light conditions that you meter for are the result of lots of different factors. There’s sunlight, cloud, foliage, shade from buildings. You might be shooting in bright sunlight, but if there’s lots of foliage in the scene it could create deep, dark shadows. This will make the scene much darker, in which case a larger aperture would be a good idea to prevent underexposure. There’s an element of feeling for what’s right.

Wrapping up

The accuracy of today’s light meters makes it easy to think that you don’t need to know the old rules. And no, you don’t need to know the Sunny 16 Rule. But having a quick and easy means to choose your camera settings and achieve an accurate exposure is really handy. And it’s not exactly a difficult one to remember, either!

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