Note: This article was published about six months ago but I have updated it somewhat.
A Short Primer On Studio Flash Exposures-Update
by William Lulow
I begin my lessons in photographic exposure technique by teaching my students how to get great exposures using their digital or traditional film camera’s built-in light meter. Before they were included as an integral part of the camera, we used hand-held light meters, which were more accurate. Here is an example of one:
The large, light-collecting dome on the front of these meters was great at reading the INCIDENT LIGHT which was actually falling on the subject from its very position. The exposures were extremely accurate. Meters included in most digital cameras these days, can be just as accurate if you know how to use them. However, they are sometimes fooled by bright objects in the frame because they measure only REFLECTED LIGHT. There are also flash meters:
These are incident meters that measure the exact amount of light from a studio flash that falls on the subject. I have found that after many years of shooting in the studio with studio flash units, that I have become very familiar with the power and distance from the subject that I set up my lights. So, exposures don’t really change enough to use one of these. But they are certainly handy tools to have.
The main point about studio flash units is that the flash is what creates the exposure. This may sound simple but the flash is putting out light at a constant rate, usually around 1/400th of a second or so. Because of this, the shutter speed is already set. If you slow the shutter speed down, you will be “burning in” or allowing other ambient light to influence your exposure and your aperture will probably change as well.
It is important to learn the techniques of manipulating the camera settings enough to be able to get a feel for how they actually affect the look of the image. It’s true that sometimes an exposure can be tweaked in post-production software or in the developing and printing stages of continuous tone materials, enough to ensure a correct result, but how can you do this when you use ARTIFICIAL LIGHT sources? It can be achieved by understanding the geometric relationship of how light behaves. Simply put, when using artificial light, moving a light source HALF THE DISTANCE between it and the subject, will DOUBLE ITS INTENSITY. So, if your lamp-to-subject distance is 10 feet, and your exposure aperture is f/8, moving the light closer to a distance of 5 feet will effectively decrease the exposure aperture to f/11 or ONE STOP because you are increasing its INTENSITY! Another way of putting it is that if your exposure is f/8 with a 100watt bulb, using a more powerful source, a 200watt bulb for instance, will add ONE STOP of light to your subject and allow you to use an aperture of f/11, because you now have twice the amount of light on your subject.
I use this knowledge often when I am shooting in the studio. My flash unit’s power is measured in something called “WATT/SECONDS” which represents the amount of power (measured in watts per second of exposure) that the unit puts out in a one second flash duration. (Actually, a powerful studio flash unit will probably output a full-power flash in a fraction of a second). So, if I use a 200 watt/second setting and get a reading of f/8 say, if I increase the power to 400watt/seconds, I will be able to stop my lens down ONE STOP to f/11. It ‘s similar to increasing my camera’s ISO setting by one stop without adding any “grain” or “noise” to the image.
In the studio, I like to shoot with and ISO of 100 which gives me a maximum amount of detail and sharpness in my images. My camera can record 32MP of information which can give me really clear images like the following:
In a portrait like this, I also like to show highlights on the subject’s hair and shoulders. The way to do this is to make sure that the lights producing them are ONE STOP BRIGHTER than whatever main light you are using. (In my case it is a large umbrella into which the main light is bounced). Whenever you bounce a light into an object that then reflects it, the power output of the light is roughly reduced by half. My highlights, or ACCENT LIGHTS are used DIRECTLY on the subject, thereby rendering their appearance as a shade of WHITE. Now, because the monolights I am using allow me to dial in power settings directly, I can adjust these accents with the flash trigger mounted on the camera.
Here are the settings on my old 1000watt/second flash generators (packs). I use them to make the point about various power settings which can be used on virtually any studio flash unit:
This is the setting of the main pack with the main light set at 250 watt/seconds and my fill-in light set at half that or 125 watt/seconds. (Look closely at where the toggle switches are set.) The pack is divided into two banks which can be used together or separately. In this case, I am using them separately. My other pack controls my ACCENT LIGHTS or highlights. Here are the settings:
The left bank controls my background light and the right one, the accent lights. With both accent lights plugged into the right bank, they are sharing the 250 watt/seconds of power giving each 125 watt/seconds. The left one is set at 125 watt/seconds. So, in the image above of the woman, the shine on her hair is produced by the two lights with the 250 watt/second settings, but as I mentioned, these lights are aimed DIRECTLY at the subject with no bounce or diffusion. We sometimes call that a RAW LIGHT.
This is a normal studio lighting setup for my subjects and I can produce the above result consistently. I have arrived at these settings based on a good amount of practice examining what each setting does and the results it produces. The goal is to make the subject look well lit, with that extra sparkle produced by the highlights. The relative power of each light, and the distance from the subject, is what produces these effects. Here’s my MAIN LIGHT and FILL-IN LIGHT set up:
It is important to note the KIND of light that comes from each. The large, round umbrella creates a kind of wrap-around light that is very soft (because of its size and depth) and kind of permeates the set. Note also, that ALL the light hitting the umbrella is bounced back to the subject. There is little or no spill over. The fill-in light is from a softbox which is aimed from below the subject’s face and adds light that fills in whatever shadows the main light creates. This is the best way to create a shadowless and even light on the human face. And, since one light is bounced and the other diffused, they are both less intense than the accent lights. Here are the accent lights set up in my home/studio:
You can see that each of these lights is used DIRECT and because of that, their intensity (also regulated by the settings on the power pack) is ONE STOP brighter than the main. That’s what makes them appear white in the photograph. Here is another example of what this lighting produces:
In these examples, I am using studio flash lighting, but it’s important to note that similar effects (though not quite the same) can be achieved with HOT LIGHTS, or CONTINUOUS TONE lighting. You can vary the intensities of hot lights by using different watt bulbs, for instance. A 500watt incandescent bulb will put out twice as much light as a 250watt bulb. And, if you vary the lamp-to-subject distances, you can achieve similar effects to those obtained with studio flash, but the QUALITY of the light will be different. Light reacts differently with a “flash” than it does from a continuous, steady source. This is something to consider when choosing light sources. These days, most continuous light sources use LED bulbs which don’t really get hot any more. They also produce more intensity than an older, incandescent bulb.
Earlier this year I replaced these older Dynalite studio flash units with a bit more modern Westcott monolights. These lights have the flash generating mechanism built right into the flash heads themselves. and are labeled with the maximum power output of which they are capable. So these units have a maximum output of 200 watt/seconds each and since I was using my older, more powerful lights on the reduced power, these work just fine:
As I mentioned, the power can be adjusted from a flash trigger mounted on the camera calibrated in 1/10th power units. So, full power would be 9.0 and you can dial it down as needed. A power setting of 4.5 turns out to be approximately 1/10th of full power, not 50%! It is important to experiment with any new light source you use so that you understand fully, how it works and what kind of light output it supplies. If your normal studio settings for your camera are in the f/11 range when using a 400 watt/second flash, for example, the 200 watt/second unit, although rated at half of the 400 w/s power might actually necessitate using an aperture of two full stops more. I have found, given my normal studio working distances from my subjects, that whereas my old Dynalites set on 250 watt/seconds yielded perfectly exposed images at f/11, the new, 400watt/second monolights yield a setting of f/9 at a setting of only one-and-a-half units of power from 9.0 to 7.5. So the settings from flash unit to flash unit can differ greatly and must be tested to see what works in your particular studio setup. This is the smaller, 200watt/second unit:
This is the larger, 400 watt/second unit:
So, to sum up, when you are adding artificial light to any subject you need to consider both the relative power of each light and the distance the lights are to the subject they are illuminating. As I mentioned, you can also influence your overall exposure by changing your ISO settings, but you need to realize that the higher you set them, the more you will introduce some “noise” or “grain” to your images, making them a bit less sharp. But note that an increase of ISO from 100 to 200 represents an increase of ONE STOP of light. Likewise an increase of ISO from 100 to 400 represents TWO STOPS and from 100 to 800, THREE STOPS! In a normal studio situation, you shouldn’t have to vary your ISO (sensitivity) settings at all. If you need more light, just increase the power or shorten the “lamp-to-subject” distance.
