There’s an old adage in photography that a 3:1 lighting ratio is the ideal ratio between shadow and highlight for a “normal” portrait of a face. It turns out, in most cases, that 3:1 ratio does look pretty good. It’s neither too flat nor too contrasty. It adds shape to the face, without producing such a strong shadow that the subject inquires about the dramatic shadow. In short, a 3:1 lighting ratio really is close to perfect when it comes to normal portrait situations. It’s three times the amount of light on the highlight side as the shadow side; that’s a stop-and-a-half difference. How else can you ensure you’ve nailed that 3:1 ratio but by using a handheld light meter?
With your handheld meter turned on and in Flash mode (assuming you’re working with strobes), set it to a shutter speed of 1/125th and an ISO of 100. This is a good place to start for studio strobe photography, in my opinion. With the white dome diffuser in place (this is used in order to measure incident light, or the amount of light falling on a subject—as opposed to reflected light, which meters light as it bounces back off of a subject), position the dome on the subject’s cheek that faces the key light, with the dome aimed directly at the key light. No other lights or reflectors should be on or in place.
That rounded white dome is a stand-in for the roundness of a face, and it would give you the correct exposure when aimed at the lens. But because we want to calculate the amount of light falling on the highlight side, as well as the amount of light falling on the shadow side, aiming the meter at the light source is important. Point it directly at the key and press the button to fire the strobe and take a meter reading. Let’s say the key light reads ƒ/11.
Next, turn on the fill light (or put the fill card in position) and place the meter’s dome on the shadow-side cheek of the subject, taking care to block the light from the key. This is important because we want to measure only the amount of light filling in the shadows. Let’s say the meter reads ƒ/8. You might think you’re all set and now you know the exposure should be ƒ/11 for the key with shadows just a stop under that to produce a 2:1 ratio. But, in fact, you would be wrong. That’s because the fill light is also adding light to the key side of the face. Maybe as much as a full stop. What you think is a 2:1 ratio is actually probably more like 3:1, or perhaps even 4:1, and the correct exposure for the highlight side is likely ƒ/11.5 or ƒ/16.
To fine-tune the exposure and the ratio, with both the key and fill now in place, re-meter the key. Sure enough, you’ll find it’s no longer ƒ/11 and now it’s even brighter.
Because we started with an ideal 3:1 ratio in mind, that represents a one-and-a-half stop difference between highlight and shadow. With the key now at ƒ/11.5, for instance, that ƒ/8 reading on the shadow side is right in line.
Explain one more time how a stop-and-a-half of light is equivalent to a 3:1 ratio? Because the ratio refers to the amount of light on one side (the highlight) compared to the amount of light on the other (the shadow). It could be thought of, frankly, as measuring photons of light. For every one photon on the shadow side, we want three photons on the highlight side. Sometimes quantifying light as if it were a physical object with mass and dimension makes it easier to understand doubling and halving of light.
A 1:1 ratio is flat illumination. A 2:1 ratio is twice as much light on one side as the other (i.e., a shadow side reading of ƒ/8 and a highlight side reading of ƒ/11) and is a bit too flat, in my opinion, for a typical portrait situation. A 3:1 ratio is 1.5 times the light from shadow to highlight, and as we’ve established, looks great in “normal” portrait situations. A 4:1 ratio is four times the light, or two stops, so it’s a bit contrastier. An 8:1 ratio is eight times the light, or three stops difference between shadow and highlight—very contrasty. Whether you’re going for the pleasing 3:1 ratio or the dramatic 8:1 ratio, the best method for determining the exact amount of light falling on a precise part of the subject is to use the light meter and the basic math of determining appropriate lighting ratios.