The Sunny 16 Rule is a great addition to any photographer’s toolbox. Basically it means that when shooting on a sunny day @ISO100 you’d be pretty close if shooting @1/100 and f/16. It is a clever rule because it is very easy to remember. 100 @ 1/100.
Photographer Neil van Niekerk points out that it is pretty easy to complement this rule when trying to overcome the sun with an external strobe. And his method means you can get a great exposure with no metering. The idea is pretty simple: setting your strobe to full power and using the strobe’s GN (Guide Number) to figure out where to place the strobe. This would get a pretty good first exposure.
Since Guide Number is basically (distance X f-stop), using the strobe at full power (i.e. maximizing the GN) will let you know where to place the strobe.
Since you know the aperture and the Guide Number getting the distance is pretty easy: GN/aperture=strobe distance. Neil shares a useful example:
The background exposure would typically be: 1/200 @ f/11 @ 100 ISO. This is based on the Sunny 16 rule, where we shift the aperture and shutter speed up and down in correlation.
The Sunny 16 Rule says that for bright sunlight, our exposure would typically be: 1/100 @ f/16 @ 100 ISO OK … so now we have 1/200 @ f/11 @ 100 ISO. That’s the ambient exposure for the background. Your subject is shaded, or in the shade … and you need to dump f/11 amount of light on them from the flash.
We can put the flash in full manual output. This makes sense in bright light.
If the guide number is 110 … (which is very close for the Nikon SB-900 / SB-910 and Canon 580EX II and Canon 600EX-RT for the flash-head zoomed to 35mm.)
Now, with the GN = aperture x distance, then the Guide Number of 110 implies that at full power (with the flash-head zoomed to around 35mm), we need: 110 = 11 x distance
The 11 is the f/11 for the bright background, as implied by the Sunny 16 Rule. So now we see we have to hold the flash 10 feet away from our subject #
Now, I am not saying that this would be the perfect exposure, but it should be pretty close and a very good set of settings to start tweaking from.
You can read more details and a few more tricks over at Neil’s full tutorial on the subject.