When you switch your flash to manual, you have full control of its power output.
With external flashes, you can typically set the power from 1/1 (full power) all the way down to 1/128 power, which can be useful when you have very specific needs: A lower flash power setting will give you a faster recycling time–helpful when you want to shoot a rapid sequence–and it will also reduce the duration of the flash, which enables you to freeze exceptionally brief moments in time. Whatever your reason for controlling your flash manually, you will need to set the necessary aperture and ISO yourself. Do not forget: The shutter speed is less important as it only affects the ambient exposure, not the flash exposure.
Determining the exposure can be done in a number of ways, depending on what you’re shooting and how you prefer to work, but perhaps the simplest solution is to use a handheld light meter that can take flash readings. This is typically the way that most pro photographers would work with large studio strobes, and in most instances it’s the quickest and easiest way of nailing your exposures.
However, you could also determine the exposure based on your flash’s guide number. Basically, to determine the flash exposure you use one of these formulas:
guide number / distance = aperture
guide number / aperture = distance
So, if you had a flash with a guide number of 36 metres (at ISO 100) and your subject was three metres away, you could use the first formula to determine that the aperture setting you’d need to use would be ƒ/12 (36/3=12). In practical terms, ƒ/11 would be the closest full aperture setting.
Alternatively, if you wanted to use an aperture setting of ƒ/4 with the same flash, you could apply the second formula, which will tell you that your flash-to-subject distance needs to be 9m (36/4=9). In both cases, it’s important to note that they only apply if the flash is used at full power, its zoom head is set at the focal length used to determine the guide number (where applicable), and the camera is set at the guide number’s ISO setting.
Complete Photography is Chris Gatcum’s 400 page guide to take you from beginner to expert photographer. Take the guest work out of your picture-taking and understand not just what to do with your camera, but how and why this affects the photos that you take. Whatever your camera, learn how to make a bigger, better impact with your photos.