A Short Primer On Studio Flash Exposures – Updated

A Short Primer On Studio Flash Exposures – Updated

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.

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 assure 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. It is an INVERSE EFFECT. 

An interesting fact to “know and tell…” photographers like the world renowned fashion shooter Avedon, who often worked with an all-manual 8×10″ Deardorff flat-bed view camera, would measure his “lamp-to-subject” distance and then use a rope with knots tied in it with f/stop labels so he would know exactly what lens apertures to use at which distances. This would enable him to work much faster than if he had to use a flash meter each time he moved his lights. He knew that his studio strobe units always dictated the shutter speed. 

I use this knowledge often when I am shooting in the studio. My flash unit’s power is measured in “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 because I AM ADDING MORE LIGHT from the flash.

In the studio, I like to shoot with an 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.

Note: Studio flash exposures are controlled by aperture settings and ISO. Shutter speeds are controlled pretty much automatically by the constant rate of the flash (usually around 1/500th of a second). If more light is needed, as in this image, use a slower shutter speed to “burn in” the lights. Remember-for any burning, studio ambient lights should be off. The LED lights in this image are visible because of the slower shutter speed and the darkened room:

Here is what my current monolights look like:

And here are some settings on the flash trigger, mounted on the camera:

The numbers here refer to the intensity of the flash. With these Westcott flash units, 9.0 is the most powerful setting. But keep in mind that my main light is always BOUNCED, thus keeping its intensity roughly one-stop less than the accent lights (Letters: B,C and D), which are used direct, with no diffusion. 

This is a normal studio lighting setup for my subjects and I can produce consistently good results. 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 with an added ACCENT LIGHT:

And here is the image:

Since I switched to monolights a couple of years ago, I can now “dial in” the power of each of my lights from the camera position. This provides an almost infinite number of settings to control not only the intensity of my main light, but that of my fill-in and accent lights as well.

One of the main takeaways here is that it is really the flash duration when using these lights that takes the shutter speed out of the equation because it is fairly constant. The thing that changes is the aperture, unless you need to “burn in” any ambient light. Then you would need to slow your shutter speed down a bit (on the camera) to let that light be recorded. But the main exposure is determined by the flash-to-subject distance (lamp-to-subject) and the intensity of the flash.