The Sunny 16 Rule:

How does the f/16 rule work and how can you adapt it to suit your needs?

The Sunny 16 Rule, or Sunny f/16 Rule, in its simplest form allows a photographer to accurately estimate camera settings (shutter speed, aperture, and ISO) on a bright and sunny day where a light meter is not available. These days it is not used as much given the availability of high tech digital cameras but back in film days it was critical to ensure you had correct exposure. Because digital cameras allow us to chimp, or effectively shoot via trial and error by checking for blinking highlights on the camera’s LCD until we have correct exposure, the rule is not as important. However, back in the days of film when a light meter was not available you needed to ensure you had spot-on exposure and this is where the Sunny 16 Rule gave the photographer a base set of figures from which they could work around to suit the particular circumstances. So, how does it work?

The rule states that on a sunny day, you should get correct exposure with camera settings of aperture f/16 and shutter speed as the inverse of the ISO (film speed). So if you have an ISO of 100, then the shutter speed should be 1/100 (or its closest conservative setting of 1/125s). At ISO of 200, the shutter speed would be 1/200 (or its closest conservative setting of 1/250s). At ISO 400, the shutter speed would be 1/400 (or its closest conservative setting of 1/500s) and so on.

Generally speaking, if shooting in bright sunlight with a digital camera, it is best to choose the lowest native ISO setting on your camera which is normally ISO100 or ISO200 depending on camera model. Doing this will ensure your images will have the least amount of ISO noise and you will have the widest aperture available to you. Notice I said 'native'? Choosing an 'extended' low setting is not advisable as it reduces the dynamic range of your camera sensor. The exception to choosing the lowest base ISO value would be when using flash. In this case you would choose a higher ISO to ensure your flash doesn’t have to work as hard and will recycle faster. Though you need to be aware that by choosing a higher ISO you will have to compensate on aperture and shutter speed.

The Sunny 16 Rule can be manipulated to suit your specific artistic taste, you are not just limited to using f/16. As the rule hinges on the exposure value (EV) system you can choose a different aperture as long as your ISO and shutter speed are also compensated. So for example, if we choose an aperture of f/8 (2EVs greater than the base f/16) and we set our ISO to ISO 100, the Sunny 16 Rule states that we should invert the ISO, giving a shutter speed of 1/125s (closest setting). But we also need to compensate for the 2 EVs we added on our aperture. So a shutter speed of 1/500s will satisfy the rule (1/500 is 2EVs lower than 1/125s using the exposure value system).

Another way of calculating the correct camera settings is by using a simple formula. Keeping your ISO speed constant, if you modify your aperture by X number of EVs (whole numbers), then you need to modify your shutter speed by 1/X². So in our case above we modified our aperture by 2EVs. We then need to modify our shutter speed by ¼. From the example above, 1/500s is ¼ of 1/125s, or in terms of the EV system, 1/500s lets ¼ or 25% of the light in compared to a shutter speed of 1/125s.

In simple terms, every time you adjust your aperture up, you need to adjust your shutter speed down by the square of your aperture adjustment. This might all seem very complicated but once you get to know how the EV system works it is reasonably simple to calculate camera settings to suit your own artistic taste. Here are a number of aperture/shutter speed variations of the Sunny 16 Rule:

Aperture (f-stop)	ISO (film speed)	Shutter Speed
f/22	100	1/60s
f/16	400	1/500s
f/11	100	1/250s
f/8	100	1/500s
f/5.6	800	1/8000s
f/4	200	1/4000s

The third entry in the table is an important one when using flash in bright sunlight. It is important because at a shutter speed of 1/250s, normally the maximum flash sync speed of your flash, the widest aperture available to you is f/11. If, like me, you prefer shallower depth of field then you have no other option available to you unless you either go into your extended low ISO range or use an ND filter which will allow more flexibility with aperture choice.

As mentioned earlier, most digital cameras these days have a built in light meter and when in Auto or Program mode for example the camera settings are calculated for you. Similarly when in Aperture Priority or Shutter Priority modes the camera metering system will calculate either the shutter speed or the aperture automatically. But all camera metering systems work off reflected light, whereas the Sunny 16 Rule is based on incident light only and is far more accurate as it compensates for very bright and very dark subjects. So if you are shooting in Manual mode on a bright day, then quickly adjusting to the Sunny 16 settings (or an adjusted form of the settings – f/8, ISO 100, 1/500s for example) will ensure you are very close to correct exposure without having to think about it. I call it autopilot for manual mode! It is extremely handy when for example taking photos at an event, a wedding for example. A typical scenario is as follows: When inside the church/wedding venue you will have a particular set of camera settings. Once the ceremony is over it will be out to the grounds for group photos and if you know your group size you can estimate a conservative aperture to ensure correct depth of field for the group and background. If the day is bright and sunny you can use a form of the Sunny 16 Rule based on your chosen aperture and as you are walking outside you can simply go straight to these settings without having to fumble around with the settings when the group is in position. Using this nice autopilot function, you will have ballpark settings before you even get outside the door, a huge timesaver, allowing you to spend the time saved by being creative in the positioning of the people in the group. When you get really good you can count the clicks on your aperture dial to get to your chosen aperture and do the same for shutter speed. Doing this you can get to your chosen settings without even looking at the camera!

There are other variations of the rule that take into account scenes with more or less incident/reflected light. So for example, a beach or a snowy setting will have a lot more reflected light so your aperture needs to take this into account. As a result, for such scenes, the Sunny 16 Rule becomes the Sunny 22 rule. If your lens doesn’t go beyond f/16 then you can compensate using shutter speed, adding a stop. Similarly, when there is less incident light, for example when it is very cloudy, the Sunny 16 Rule becomes the Sunny 8 Rule. It all boils down to available light so if you use the Sunny 16 Rule as a baseline, then you can adjust the aperture to suit based on the available lighting conditions.

The Sunny 16 Rule is best used as a guide only and the aperture f/16 is only to be considered a starting point. Personally, I rarely shoot above f/8 and never shoot above f/11. Apart from the fact that I prefer wide apertures where possible it is also because I want to minimise any appearance of sensor or lens dirt and also because I want to stay well away from my lens diffraction limit. It all boils down to what you are trying to capture. Portraits, action shots, landscapes are all completely different in terms of camera settings and artistic taste, but using the rule allows you configure camera settings very rapidly that will yield images with close to correct exposure without having to fire the shutter.

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