Canon Professional Network - Tools: Canon flash power calculator

October 2009

In the latest of a set of tools for Canon photographers CPN brings you the online flash power calculator. This easy-to-use calculator allows you to input a variety of variables so that via a simple calculation photographers can determine flash powers should they wish to use manual flash power settings.

With the advent of E-TTL metering, and the ability to get flash exposures correct without too much thought, it may seem that having the ability to set flash powers manually on some of the Canon Speedlite flashguns was a throw-back to the old days. However, while E-TTL will work most of the time, there are still those who prefer having complete control over flash in some situations.

The image on the left was shot with an incorrect flash exposure whilst correct flash exposure ensured the image on the right was properly exposed when using flash.

The flash power calculator makes it easy for you to implement the equations to work out either your aperture or range. The equation can also be rearranged to calculate the guide number of a flashgun but since you can tell the Guide Number of a Canon Speedlite by its model name (580EX II = GN 58, 430EX II = GN 43) this has not been included.

For those of you who aren’t fully au fait with guide numbers - the Guide Number (GN) is a measure of the power of a flashgun. The higher the GN, the more powerful the flashgun is. A guide number is found by multiplying the flash-to-subject distance by the aperture for a well-exposed photograph. For example, if good results are produced by using f/11 for a subject that’s 5 metres away, the guide number is 55. You need to know the film speed used, and that the distance was measured in metres, so guide numbers are usually written as ‘55 (ISO 100, metres)’.

To make use of the calculator, simply input the variables you know and the calculator does the rest. If you are calculating the aperture, then ensure you round up the result to the next nearest aperture, so for example, if the result is 8.6, use f/9.

You can find out much more about flash, the Canon flashgun range and using flash with Canon cameras in the CPN Infobank.