Guide Numbers: What they really mean
Flash devices are built into many lower cost cameras, and higher end cameras have add on flash units. Flash is a useful tool for many photos, even when used simply as a fill flash on a sunny day (fill flash usage is the subject of another article). All flash devices are rated by what's called a Guide Number, or GN. Back in the days of film, when camera users had to know a lot more about their cameras, most people knew what guide numbers meant and how they could be used, but with the advent of point-and-shoot and digital imaging, understanding guide numbers is something that has fallen from common knowledge. Even so, it is an important concept and we'll look at guide numbers briefly here.
A guide number for a flash simply measures the device's ability to illuminate a subject at a given distance (measured by the light falling on that subject at a given distance). Put simply, the higher the guide number, the more output the flash unit has. A flash unit with a guide number of 40 will be able to light a subject twice as far away as a flash unit with a guide number of 20. A flash with a guide number of 20 lights up a subject twice as far away as a unit with a guide number of 10, and so on.
That doesn't mean a flash unit with a guide number of 40 is twice as powerful as a flash unit with a guide number of 20. Because light spreads out with the square of the distance (it comes out as a cone of light and not a directed beam), it really means that a unit with a guide number of 40 puts out four times the light as a flash with a guide number of 20, measured at the same distance. This is why there's a common misconception that guide numbers relate the power of the flash units to one another: a GN of 80 is not twice as powerful as a GN of 40, but it will provide twice the light at any given distance. So, the guide number has to do with the amount of light falling on a subject, not the power of the flash unit directly.
In fact, the guide number is a little more complex than most people think. The guide number is calculated by multiplying the distance from the flash unit to the subject by the f-stop for a given ISO rating required to correctly expose the subject. For example, to photograph a head at 20 feet with an ISO 100 setting and an aperture of f4 will require a guide number of 20x4 or 80. If the lens is stopped down to f8 instead of f4, the GN required increases to 20x8 or 160. (This makes sense when you think it through: an aperture of f8 will let in half the light of an aperture of f4, so to get a correctly exposed image of the subject requires twice the light.)
A typical flash-on-camera unit has a relatively small guide number. Two of my point-and-shoots have GNs of around 15, which means they can illuminate a subject only reasonably close. My bigger Nikon flash devices, on the other hand, have a GN of 125 or higher, allowing them to light up subjects over 100 feet away with a wide aperture.
Guide numbers are really not complicated. Since most cameras work around the default of ISO 100, the GNs can provide a quick measure of how powerful the flash is and whether it can light a subject at a certain distance away. The math is actually pretty easy. Assume a subject 10 feet away with the camera lens wide open at f4: you will need a GN of 40 to light that subject. If the subject is 20 feet away, you need a GN of 80. This means your little point-and-shoot flash just isn't going to provide enough light for your friends standing twenty feet away in the dark! However, the point-and-shoot flash may still be useful for fill flash.
While most people just will never care about guide numbers, when you are trying to figure out how to light a special shot, understanding how to manage he flash is useful. Although to be fair, today's cameras and flash units do most of this automatically, it still is useful to know whether that shot you are trying to get is possible or not based on your flash unit's power.