Setting your flash manually

You can manually set the 430EX and 580EX II to fire at a wide range of settings. For example, you can set output on the 580EX II anywhere from 1/128 power to 1/1 (full) power at 1/3-stop increments. If you are doing close-up photography (such as nature, food, or macro subjects), being able to set your flash manually and override the ETTL settings is important.

You can use your flash’s guide number (GN) to determine correct manual flash settings. The 580EX II has a guide number of 58 (meters) at ISO 100 and the flash head at a 105mm setting; the guide number drops as you increase focal width. This number is used, then, with a standard guide-number formula for calculating manual power settings:

Guide Number = f-stop + distance

or

Distance = Guide Number / f-stop

or

F-stop = Guide Number / distance

Usually you will know your f-stop number because you will be deciding if you want to shoot a narrow depth-of-field shot or not. Using the guide number formula, then, you can determine how you can set your power manually and how far you can be from your subject — something over which you may or may not have control.

Of course, a guide number is just that: a guide — it’s not precise and won’t work in every photographic lighting scenario. Guide numbers are very good for comparing the relative power of various flashes when you’re in the market for a new model. And, while this number gives you some specifics regarding distance, you may, in fact, not want to be the exact distance from your subject that this formula calculates for you. For example, you are doing a shoot of food at a Japanese restaurant, and you want a nice shallow depth-of-field shot at f/2.8 of a plate of sushi and you need to use a flash. If you’re using a 24-70mm lens, and you want to be about five feet from your subject, using the GN formula, here’s how the situation calculates:

Guide Number = f-stop X distance

f/2.8 X 5 feet = a Guide Number of 14

A guide number of 14 represents substantially less power than how the camera is rated at its default settings of GN 28 (24mm) to GN 42 (50mm).

So what does that mean? You’ll have to lower the power on your flash to a setting that won’t overexpose the subject. If you decide you want to shoot the plate of sushi at your lens’s wide setting of 24mm, you’ll at the very least want to manually set your flash to half of its full power (the 1/2 setting) and then shoot to see if that produces the image you want.

Here’s another way to think about this, and a good way to achieve a quick calculation to determine the right aperture for a shot. If you have a flash with a guide number of 43 (such as the 430EX), and you are 10 feet from your subject, your aperture should be f/4.5 (at ISO 100): 43 * 10 = 4.3 (rounded to f/4.5).

While there are ways to perform calculations for the precise power settings you should use, I have found that using this system to know if I’m within range or not at the default ETTL settings, and then experimenting with manual settings from there, is sufficient to achieve very good results. Many of the complicated photographic calculations were more useful in the time of film, where knowing the math and using it meant using less film to achieve a good photo; today it’s acceptable to waste some digital shots instead of taking the academic approach.

Category: Flashes

Comments are closed.