Guide to Guide Numbers

Guide numbers are the common method of comparing the relative powers of different flashes. The value is found experimentally by firing the flash at maximum power at a subject which is a fixed distance from the flash. With no other source of light, a flash meter is used to obtain an optimum aperture for a "correct exposure", which is of course slightly subjective. Similarly a camera could be used, and photographs compared for the best exposure, although there would be significant complication introduced by having to take additional account of reflectance of the subject being photographed and the distance from the camera to the subject.

One area of common confusion is that Guide numbers can be specified as either feet or metres. Sometimes a flash will be quoted in feet, which makes it seem ever so much more powerful than if quoted in metres, but there is of course no difference.

A second common confusion is use of ISO speeds. Most Guide numbers are calculated for ISO 100, which is traditionally a very common film speed. However, quoting at ISO 200 can make a flash seem more powerful, so it's important to check what ISO speed is being quoted.

So, getting back to the experiment, say we set up our flash 10 metres from our flash meter (and subject). We then fire the flash at full power, and measure the light output. Our flash meter, set for ISO 100, gives a reading of f/8.

To calculate the guide number for our flash, we simply multiply the aperture value with the distance to our subject.

So we get:

Guide Number = Distance x Aperture
Guide Number = 10m x 8
Guide Number = 80m

So our flash has a Guide Number of 80 metres at ISO 100.

For another quick example, lets say we've bought another flash that is rated at a Guide number of 360 feet @ ISO 200. Given this incredible Guide number, it had to be a good deal, and much better than the flash we already had at 80m @ ISO 100, right?

Well let's work it out:

Since quadrupling ISO speed means we double the Guide number due to the Inverse Square law, doubling ISO speed means we multiply the the square root of 2 to get the Guide number. However, since we're wanting to half the ISO speed, we therefore divide by the square root of 2.

Converting to ISO 100, we therefore get

360 feet @ ISO 200 = 254.6 feet @ ISO 100

Converting feet into metres, we get

254.6 feet @ ISO 100 = 77.6 metres @ ISO 100

So despite the very high quoted guide number, it turns out this flash isn't actually as powerful as our first flash.

So what other trick do manufacturers have up their sleeves?

Besides using feet instead of metres, and higher ISO speeds, to make numbers sound more impressive than they are, manufacturers also have a couple of other possibilities for their marketing departments to make use of.

The major one is the hazy description of "correct exposure". In general, a manufacturer will opt for a slightly dark exposure, because this will make the Guide number of their flash higher. You, on the other hand, might feel this is too dark, and would want a brighter photo, so your Guide number wouldn't be nearly as high to make the photo the way you like it.

Another one is only an option for manufacturers of small flashguns. Most of the half decent flashguns on the market now have a zoom capability. As you zoom your lens, your camera sends signals to your flash to tell it to zoom in. What this does is modify the way the light is directed, and make the output of light more efficient for a given power of flash burst, simply because more of the light given out is directed towards the subject, rather than lighting all around the subject.

So why is this important? Well when the flash is zoomed in, it has an effectively greater guide number than when it is zoomed out. So you can guess that the manufacturers will make use of this, and quote the Guide number of the flash at its best zoom position.

The best advice therefore is to do your own tests when you get a flash, if it matters to you to know for sure. That way you know there hasn't been a marketing department involved in the Guide number you arrive at.