You can find the setting (aperture) for flash using an old fashion method. We are talking about “guide number”. This is a value associated with your flash that can be used to compute the aperture setting. Consult the manual that came with your flash for the guide number ,or you can run a simple test to discover it.

How it works: To use your flash in manual mode, set the shutter to a designated speed required from flash synchronization. Compose and focus your shot and note the subject distance as expressed by the camera’s focusing scale. Actually you are after the flash to subject distance; likely your ability to estimate distance will suffice.

Suppose your guide number is known to be 120 for a setting of 100 ISO, and after composing your shot, the distance flash-to-subject is estimated to be 15 feet. Now you simply divide --- 120 ÷ 15 = 8. This tells us to set the aperture to f/8. Voilà! Your exposure will be spot on.

Seems too simple to be true? You need to know, a guide number takes into account the ISO, flash intensity, and the division works the flash-to-subject distance. The guide number method is not perfect. Variables like the color and height of the ceiling can frustrate however this method works.

Your flash manual will likely publish the guide numbers for different ISO’s and different numbers for those folks that work with metric measurement. Suppose you can’t’ find the published value for your unit? Now we run a simple test. Select a room with a light colored ceiling that is moderate as to height 8 – 10 feet. Set your test subject 10 feet distant and shoot a series at 100 ISO using all the middle aperture settings. It won’t hurt to place a placard to be sure of the test aperture setting. Examine each image for best exposure. Suppose the f/11 image is best. Now multiply distance by f-stop --- thus 11 X 10 = 110. That’s’ the guide number to use for 100 ISO.

Once the guide number for a typical shot is known, you can modify, best to work in 1 f-stop increments. The modifiers are 1.4 and 0.7. Suppose your guide number is 110 for 100 ISO. What will it be for 200 ISO? Multiply guide number by 1.4. Thus for 200 ISO, the revised guide number is 110 X 1.4 = 150. For 400 ISO, it is 150 X 1.4 = 210. For 50 ISO, it is 110 X 0.7 = 75. If outdoors or in an auditorium with high ceiling, multiply guide number by 0.7.

If you think this is too much work, you can simply wet your finger and hold it and make an estimate (gobbledygook).

If you will be using your flash to fill shadows in sunlit vista, this method wills work provided you know the guide number. Again, suppose guide number is 120. Compose your group shot. Set the shutter to a speed that synchronizes. Aperture setting is per camera’s metering, suppose f/8.

Now divide guide number by f-number -- 120 ÷ 15 = 8. This is the distance in feet flash-to-subject whereby flash intensity and sunlight intensity are identical. The fill flash wants to be subordinate. Multiply distance by 1.4 --- 8 x 1.4 = 11. This tells us to place the fill flash 11 feet from subject to fill with flash 1 f-stop subordinate.