Shelton Muller discusses the principles once used by all photographers to determine flash exposure…
These days, flash photography has been made so easy with TTL flash technologies that mean photographers don’t need to think at all.
The camera and the flash talk to each other in a language most photographers used to speak themselves. However, successful photography is based upon an understanding of those languages, even if it is a matter of principle. One component of this language is understanding guide numbers for flash exposure.
Determining your f-stop setting for flash is not that hard. The first place to start is the guide number of your flash. This is often reflected in the model number of the flash, but not always. Your instruction book will no doubt give you the flash guide number details.
The calculation is easy. It is “Guide number divided by flash to subject distance equals aperture”. To explain – if your flash has a guide number of 40 in metres at 100 ISO in manual mode, and your flash to subject distance is 10 meters, your f-stop setting would be about f4 – if you wereÂ shooting at ISO 100Â of course.
The calculation would look like this:
Always ensure your settings are in either feet or metres to ensure correct distance and guide number relationships, and understand that doubling the ISO will mean closing down one more stop – and vice versa. For instance, If you were using 200 ISO film, your f-stop in this instance would be f5.6 and f8 if you were using 400 ISO etc.
If you are bouncing your flash, you have to calculate for the distance from the flash to the reflective surface and then from there to your subject. Then, you might want to open another stop to allow for light absorption.
If you reduce your manual flash output to half power, then you will need to open a stop. If you reduce it to Â¼ power, then open another stop â€“ and so on.
Understanding these principles will enable you to make fairly accurate flash exposure settings. They also enable you to understand ambient/ flash exposure relatioships. This knowledgeÂ is also handy for studio flash exposure if you don’t happen to have a flash meter but do know the guide number of your flash or studio flash head.