Exposure Settings With Studio Flash
By William Lulow
We have spoken about what is often referred to as the “exposure triangle” of ISO, APERTURE & SHUTTER SPEED, but when we begin to use electronic flash with umbrellas, softboxes and other light modifiers, we introduce a fourth element to the equation: Flash Power.
Now in the days of film, especially sheet film which was notoriously slow, but rendered superb detail, photographers often needed as much power from their electronic flash units as they could get. So big, bulky flash generators often were able to output 2000 or more “watt/seconds” of power. (A “watt/second” is a measurement of the power of a flash generator. In other words, a 2000 watt/second unit could put out 2000 watts of power in a one second exposure. That is a lot of light.
There is a caveat here before I delve into this topic: You need to have some understanding that the light generated from an electronic flash is just that: electronic. So, this article is a short introduction to what that means.
FLASH GENERATORS
A flash generator like the one below, stores electricity from an ordinary socket and through a number of capacitors and what is known as “flash triggers,” is able to deliver a brief flash of light when the photographer takes the picture.
Another factor to keep in mind is that when using these units, the flash itself takes the place of the shutter speed. The flash is so brief that it provides light on the subject for maybe 1/1000th of a second. So, when using these units, all you need to be aware of is the APERTURE, the power of the generator itself and the ISO setting. Here is what the control panel of one of these units looks like:
These “packs” (short for flash generator pack) have several settings on them. You can usually use them on full power, half-power, quarter power or several combinations. The power output can be controlled by the switches on the pack’s control panel. On this particular version there are four places that flash heads (heads) can be plugged into, two on one side and two on the other. The switches directly below them control their use. So the pack can be divided equally between four flash heads if they were all plugged in at the same time, and the pack was set to full power, each head would receive 250 watt/seconds of power. If only one head was used on Bank A, and the pack was set to Bank A+ Bank B, the total output from that head would be 1000 w/s (watt/seconds) . That would be the entire power of this pack delivered to only one head. If you added a second head to the configuration, and left the power switch to A+B, each head would receive 500 watt/seconds of power.
Now each step down in power represents one f/stop of exposure. So if that one head was then divided between both banks using the setting “A:B”, it would deliver 500 w/s of power to each bank. Since no other light was plugged in to Bank B, it would deliver 500 w/s of power to Bank A. The white switches just below the plugs for the heads also control the power of a head or heads plugged into that bank. So you could have 500 w/s or 250w/s or 125 w/s delivered to one or two heads plugged into that bank. If you used two heads in Bank A, each head would divide the power equally, 250 w/s to each would be the maximum power
In other words, if one head in Bank A could have 500 w/s of power, that might give you an exposure of something like f/11 @ ISO 100. (The shutter speed here doesn’t matter because it is being controlled by the flash pack). If you used two heads in Bank A, it would divide the power equally between them and because the power would be reduced to 250w/s, it would affect the exposure by decreasing it from f/11 to f/8. Therefore, each halving of power, is equivalent to a one f/stop decrease.
Photographers often have to change the settings of their power units in order to arrive at a good exposure for what they want to shoot. Many of these packs are capable of being adjusted in all kinds of ways. Keep in mind that today’s sensors are way more sensitive to light than film used to be. So there just isn’t the requirement for a lot of power these days. I have two of these 1000 w/s packs and they provide more than enough light for whatever I need to shoot. I used to have several 1600 w/s units for jobs where I needed to use my view cameras. But that just isn’t the case anymore. As a matter of fact, many photographers are using what are referred to as “monolights.” These are flash heads with generators built right in to them. They are handy because you don’t need wires running on the floor of your studio. But they have one major drawback. If one goes down, you have to send the whole thing in for repair, depriving your studio of one of its lights. If one of my heads goes down, I simply have to plug in another. Here is an example of a monolight:
I am a strong proponent of the notion that “light is light.” It doesn’t really matter what piece of machinery produces it as long as you know how to control it for your purposes. As I might have said, my units are around thirty years old and have been providing excellent light for all that time. I maintain them properly (about every thirty days I run electricity through the capacitors to keep them alive and I have had them serviced when needed).
Electronic flash units can be expected to provide years of great images.
