The “Fourth” Side Of The Exposure Triangle

The “Fourth” Side Of The Exposure Triangle

by William Lulow

Well, I tried to think of a clever title for this article and this is what I found. The “fourth” part of any exposure with a camera is, of course, LIGHT! The “Exposure Triangle” refers to the three elements of any exposure: the ISO (or sensitivity) setting, the APERTURE and the SHUTTER SPEED. But these are all assuming that enough light is present to adjust all three of these to acceptable levels in order to make a correctly exposed image. When you have adjusted each of the three settings to allow enough light for picture making and you are STILL getting underexposed images, you then need to add the FOURTH element which is LIGHT in sufficient quantity to raise the overall illumination in order to produce an optimal exposure.

There are many ways to do this. You can use a stronger light source, a portable flash, sometimes referred to as a SPEEDLIGHT or you can actually set up some kind of stronger light like a studio flash unit, which will bring the light levels up to a point where you can then use the “TRIANGLE OF EXPOSURE” again to obtain a proper image. One of the simplest ways to do this is to use the “pop-up” flash that comes built-in to most cameras these days. It is referred to as the “built-in” flash to distinguish it from the kind you mount on your camera’s hot shoe or on a light stand.

What you are trying to do here is to bring the overall light level up. These days, digital cameras are built to include much higher ISO settings than were ever possible with film. Today’s digital sensors are way more sensitive to light than film ever was, even with push-processing. But once you begin to use ISO numbers much above 2500, you will begin to see a degrading of the image referred to as “noise.” It was called “grain” in the days of film and too much of it would cause the image to lose sharpness to the point of defeating the purpose. Too much grain or “noise” is just as bad as having a severely underexposed image. You can’t see enough detail in either case.

If you must record something in extremely low light conditions, maybe it’s better to have a grainy image than none at all! But, most of us are looking for images we can use for something, images where we can see enough detail to make the whole experience satisfying. So, the next, logical step in image-making is the ability to ADD LIGHT to the scene to enable making the exposure you want. One of the best examples I have for this is the following image of a guitar:

This was an image I was making to illustrate extreme depth-of-field, so I wanted to use a very small aperture (in this case f/32, shot with my 60mm f/2.8 macro lens). This required that I place the camera on a tripod because it forced me to use a very long shutter speed (in this case about 30 seconds). But even then, there wasn’t quite enough light, so I added a 500watt photoflood bulb to bring up the overall light level to obtain this image.

Another example of this kind of technique of adding light to a setup is the following:

This is an interior shot in which I also needed some depth-of-field, so I had to add some lights to the setup. First, the camera was placed on a tripod. Next one light was added to the back to light up the glass doors, another to light up the last conference area and a third was placed to light up the first table. There was also a fluorescent light under the red credenza which had to be filtered with magenta gels since this was recorded on color film. Exposure was f/22 @ 30seconds with two flash exposures added to increase the effect of the studio flash units. 

So, the “fourth” part of the EXPOSURE TRIANGLE is light! Sometimes you need to add extra light if there isn’t a sufficient amount to make the kind of image you want. Light is probably the most important element in any image. After all “photograph” means light picture.