In an effort to explain how some light plus a little more light becomes even more light, let me explain how this might matter in the first place. Let’s say you’re photographing a portrait in a studio setting, so you set up a light to illuminate the subject. (It could be a strobe, a hot light, an LED, in a softbox, with a snoot—as long as it’s a light, it applies.) This light, which provides the primary illumination for the subject is the key light, or main light. In a picture made outdoors, the sun is almost always the key light.
So now you’ve got a light set up and aimed at your subject. You could take a meter reading of the amount of light illuminating your subject, and let’s say your main light reads 1/125th at f/8 and ISO 100. Set your camera this way and you’ll have a nice, normal, well-exposed photo.
Now let’s say you add to this scenario a fill light. This is a secondary light that’s reaching the subject with less intensity than the main light and is intended to illuminate shadows—maybe a little bit or maybe a lot, depending on the look you’re going for. Without fill, shadows are deep and dark and dramatic, and that’s not always the look you want. Especially for, say, a nice, friendly portrait. So you add a fill light, placed as close to the camera as possible so as not to create a new set of competing shadows from a secondary main light—a phenomenon known as “cross lighting.” Adding a fill light from the camera’s position is ideal because, after all, we don’t care about shadows the camera can’t see. A light from the camera will fill in all the shadows the camera can see.
This fill light can be a big soft source or a tiny, specular light; either way, as long as it’s coming from the lens axis, it won’t likely compete with the main light for attention unless it’s way too strong. You can determine its intensity by looking at the viewfinder, sure, but also by using a handheld light meter.
Let’s say this fill light measures only ƒ/5.6 when metered at the subject. It’s less powerful than the key light (which was ƒ/8), so as long as we expose for the key light at ƒ/8, the exposure will be appropriate and the fill will brighten up the shadows, a stop less than the key light, right? Wrong. Because when you add the fill light you add more light to the whole scenario, and light adds up. The correct exposure for both lights together is actually ƒ/8.5, or ƒ/8-and-a-half. The best way to tell is to meter it.
Why is a handheld light meter important for lighting, and not just the eyeball test? Because the meter allows you to precisely measure the key light and the fill light independently, and then take a reading of the two lights together, which has the bonus effect of proving what I’m telling you—that light adds up.
So, back to the start of our scenario, where we’ve got one key light set up. With the light meter at the subject (on the cheek, for instance, in a portrait) aim the white dome diffuser of the light meter directly at the light source. This will meter the amount of illumination falling on the subject from the key. As mentioned, this metered at ƒ/8 in the example here. When you add the fill light to the scenario, to meter it correctly you’ll want to meter only the fill light without the key. So you can turn the key light away from the scene, block it with your hand, or turn it off completely. Then aim the dome diffuser of the meter directly at the fill light and take a new reading. In the example here, it was ƒ/5.6—a full stop under the key light.
The primary purpose of metering lights independently and with a handheld incident meter is to determine the lighting ratio—or the contrast between the highlights and the shadows in the scene. This is actually where most people go wrong with metering and lighting ratios. And it’s because they don’t account for the fact that light adds up.
The stronger the ratio between highlight and shadow, the more contrasty and dramatic the scene will be. This is expressed as a 4:1, 6:1 or even 8:1 lighting ratio. A one-stop difference (twice the light) would be 2:1. A 1.5-stop difference (three times the light) would be 3:1, and so on. These ratios are an effective way to achieve specific lighting effects, from flat (1:1) to very contrasty (8:1) and to communicate those looks to the people working with you, clients or subjects. A 1.5-stop difference between highlight and shadow, otherwise known as a 3:1 ratio, is a good place to start for a nice, attractive portrait ratio that’s neither overly flat nor overly contrasty. It’s the classic portrait lighting ratio, and therefore a useful target for balancing fill with the key light.
Because light adds up, however, we don’t simply meter the lights independently to determine their ratio. We have to meter the total amount of illumination falling on the highlight side of the subject with just the amount of fill that’s illuminating the shadow side. So, with both the key and fill lights in position, we aim the white diffusion dome of our handheld meter directly at the camera and take a reading. What was ƒ/8 for the key alone and ƒ/5.6 for the fill alone now reads ƒ/8.5. Because we gave some light a little more light, the total quantity of illumination is greater than the key or fill lights alone. It makes sense when you think of it this way, yet many photographers meter the key alone to determine exposure and then add the fill without considering its overall impact. This results in overexposed images.
If you’re having trouble grasping the concept, think of each “stop” of light as one single “unit,” then it makes sense that one unit plus another unit would be two units. Somehow photographers don’t always think of light this way. But they should; it adds up.
So, because the fill light alone measures ƒ/5.6, let’s call that one unit. Then if the key light alone measures ƒ/8, that’s 2 units (a full stop more light—twice the light). One unit plus 2 units is 3 units. So that’s the total amount of light falling on the highlight portion of the subject. And the shadow side just gets the one unit from the fill, so we have a highlight-to-shadow ratio of 3:1, or the stop-and-a-half difference between highlight and shadow that’s often ideal for portraits.
You can check your work by looking again at the apertures. The highlight side is now getting exposed at ƒ/8.5, while the shadow side only gets ƒ/5.6. That’s a stop-and-a-half difference between highlight and shadow. That math—much like light—adds up.