I think the name for this method of photography is a bit misleading or sounds too restrictive. It's not a 'rule' as such, but more a guide to help you get the right exposure settings quickly when you perhaps don't have time to play around or use a light meter.
The joy of this rule is that you don't just have to use it for sunny situations, it will also help you expose on a cloudy day. And, more importantly, if you get a correct exposure, there's less editing required after, trying to make up for lost detail.
So, the rule is, on a sunny day, with a front lit subject, set your aperture to F/16 and then set your shutter speed to the reciprocal of your ISO.
F/16 ISO 100 Shutter Speed 1/100 second
or
F/16 ISO 200 Shutter Speed 1/200 second
F/16 ISO 400 Shutter Speed 1/400 second
(as you can see from the photo's below the exposure is the same despite the different settings because when you change the ISO, you adjust the shutter speed to compensate. Left is ISO 100, then ISO 200 and finally ISO 400)
If you need to shoot on a cloudy day or you want to adjust your depth of field, you can change the aperture but you have to change the shutter speed to compensate. For example;
On a cloudy day or a if a shallow depth of field is needed, if you changed your aperture to
F/4 ISO 100, you would need to change your shutter speed to 1/1600 second and you can do this for settings anywhere inbetween. This is because, with this example, the aperture has changed by 4 full stops, so the shutter speed needs changing by 4 full stops as well.
****Note at this point that some camera's are calibrated in third stops. So three clicks of the wheel on your camera will equate to one full stop.****
The image to the left is F/11 ISO 100 Shutter Speed 1/200. So, I have changed the aperture from F/16 by one full stop, therefore I need to change the shutter speed by one full stop from 1/100 to 1/200.
The image to the right is F/8 ISO 100 Shutter Speed 1/400. This time the settings have been changed by two full stops yet as you can see with both images, the exposure is the same.
Here is a guide for the settings in any weather condition which I hope you find useful:
Snow or Sand: F/22
Sunny: F/16
Slightly overcast: F/11
Overcast: F/8
Heavy overcast: F/5.6
Open shade or sunset: F/4
I hope this blog has helped you understand the rule a little better. Please feel free to comment below and let me know what you think or if you found it helpful. Now go and practise! It's an invaluable tool to have as a photographer as I have learnt in only a matter of weeks.
