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A Photographer's Guide to Portrait Lighting Ratios

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This post is part of a series called Studio Portrait Lighting.

Artificial lighting is intimidating for many photographers. It’s easy enough to know good natural light when you see it (though this too takes practice), but artificial light requires a few extra skills to create the look you want.

To rapidly improve your artificial lighting skills, it helps to understand lighting ratios, or the relationship between how much power (ie., how much light) each light emits.

In this tutorial we'll start from scratch, looking first at what key and fill lights are, then I'll move on to how they relate to each other, how to be precise about this relationship, and why it makes a difference where your lights are positioned.

Then we'll see how these concepts apply to the ideas of "high key" and "low key" lighting, as well as adding other lights into the mix.


What Are Key and Fill Lights?

When talking about studio lighting, there are two main lights to think about: the key light and the fill light.

  • The key light is so-called because it's the brightest light of the two, and is doing all the heavy lifting on the illumination.
  • The fill lights "fills-in" the shadows cast by the key light.

When the key light has illuminated one side of the subject, it’s natural that the other side of the subject is cast into shadows. Adding the fill light adds light and stops the shaded side of the subject from disappearing into blackness. So the fill light also helps to define the shape of your subject and provide extra detail.

There Can Be Other Lights, Too

The key and fill lights make up the classic two-light setup, but you can add other lights too. These are things like rim lights—used to provide extra separation of the subject from the background—and hair lights or other lights. Additional lights and modifiers can accentuate details.


How to Calculate Lighting Ratios

Now that we've reminded ourselves what the key and fill lights are, how do they work together?

I generally do key on camera right fill on camera left but this may be different for different models
I generally do key on camera right, fill on camera left, but this may be different for different models.

Basic Scenario: Same Lights, Same Distance, Different Power

Let's assume you're using two identical lights, placed the same distance from your subject with the same light modifiers. In this example, we can find the lighting ratio by calculating the how much light each light sends out compared to the other. Like so:

  • If the key light is on 1/2 power, and your fill light is on 1/4 power, the key to fill ratio is 2:1: for every two rays of light key lights emits, the fill light emits one ray of light
  • If the key is 1/4 power and the fill is 1/16 power, the ratio would be 4:1. The key light emits four rays of light for every one from the the fill
caption

I'm getting the zone system information from Norman Koren's work on digitising it.

All Light Modifiers Change the Amount of Light

If we're using bare light bulbs it’s easy to calculate lighting ratios. Common light modifiers like softboxes or umbrellas, however, make it more challenging because in changing the light they all absorb a little bit of it, and thereby change the amount of light that actually reaches your scene.

So light modifiers alter the effective power of your strobes: they spread the light around, softening and dimming it. Just because the dial says 1/4 power, doesn't mean you're actually getting that much light onto the subject. Ultimately, you just need to know how much each modifier knocks off your power setting, and adjust accordingly.

What This Means: You Need to Measure to Actually Know

When it comes to artificial lighting, what really matters isn't the power ratio, or "incident" ratio, but the "reflected" ratio. This the measurement of the amount of light that bounces off your subject and back to your camera. Light is altered by all kinds of factors, everything from skin tone, to ambient light, to how much dust there is in the air. You need to use a light meter to find the reflected ratio for your lights as they are really working in your actual scene.

When we're looking at the reflected ratio, what are we measuring? In short, it's the ratio of the amount of light reflected by the highlights versus light reflected by the shadows. So regardless of the exact power levels on your lights, if the light meter or your histogram is saying the highlights are two stops brighter than the shadows, you have a 4:1 lighting ratio.

41 lighting ratio

This is around a 4:1 lighting ratio.

Incident Ratio: Don't Forget about Overlapping Lighting

If the light from your key light and fill light overlap on the subject, then you have an area of highlight that is brighter than either of them alone. This is often called an incident ratio.

11 incident ratio The bright line down the face is around 220 the sides which should be the same as the front are around 170- a stop and a half below

This image demonstrates a 1:1 incident ratio. The lights are set up at the same power, distance, and modifiers, however, the light from each head overlaps slightly. The bright line down the face (look at the ridge of the nose and on either side of the nose), is around 220. The sides, which should be the same as the front, are around 175—a stop below!

To account for this overlapping light, you can either reposition lights so that it changes the area that is "doubly lit", or adjust the power down a little, proportionally.

Distance Matters

So far we've kept our hypothetical lights at the same distance but, of course, that distance plays a part in all of this. Light dissipates as it travels: it spreads out in all directions. This means the further a light is from the subject the less light falls on the subject.

The inverse square law shows us that a minor change in the distance of a light greatly impacts the amount of light that hits the subject. It's too complicated to get much into here, but here are two handy rules of thumb:

  • Increase the distance between the light and subject by double and your subject will receive one quarter the light (from that light)
  • Reduce the distance between the light and the subject by half and your subject will receive four times the light (from that light)

Lighting Ratios In Practice

In short: the power of the lights is just a starting point. The light that actually reflects off the subject and onto your film or sensor is what matters for the exposure. Start by choosing the lighting ratios on your lights, and then measure to account for reflectors, skin tone, and more. Then observe your lighting and adjust the power, position, or modifiers to get the look you want.

The terms high key and low key describe two popular styles of lighting. They're only two of many lighting styles, but they are good ones for beginners because you can do a lot with them and they each teach you a lot about how light works. Photographers of all experience levels can apply and adapt these styles to their own purposes, bend the rules, and play with them as visual starting point.

High Key Lighting

High key is bright, soft, low-contrast imaging, where the lighting ratio is around 1:1. This virtually eliminates shadows and a lot of small detail; all that's left is strong details and outlines of forms. The eyes, bottom of the nose, and lips can all stand out. It's usually strongly illuminated from behind, too, whether through backlighting or a bright solid white backdrop.

Average professional portrait High key backlight fairly flat and boring
Average "professional portrait." High key, backlight, fairly flat and boring.

The term "high key" comes from early cinema and TV. The film and sensors used couldn't deal with strong contrast then, so the actors had three lights on them to kill the shadows. We've come to associate this look with cheerfulness, even as many TV shows try to move away from the "cheap" look. Fortunately for us photographers, it's not considered cheap-looking in still images.

Making a high key image is not very hard. Even without arranging artificial lights, a combination of sunlight and reflectors can create a high key look.


Low Key Lighting

Low key is the opposite, stylistically: the lighting ratio is unbalanced, usually heavily, producing a strongly affected image. A ratio between 4:1 and 8:1 will usually achieve the low key feel. 

Low key lighting creates deep shadows around all forms and structures of the subject. It's a moody, sometimes gritty look used for dramatic portraits.

Low key good in the right scenario but something of a clich on the internet in recent years
Low key: good in the right scenario, but something of a cliché on the internet in recent years.

While we often now generally associate it with film noir, the effect ultimately really comes from the Renaissance painting effect of chiaroscuro (Italian, "light-dark"). Chiaroscuro could be hard or soft lighting, but the intent was always to bring the three-dimensionality of the subject's form to life through directionality and shading.

So while the effect has history in various art media, in stills it sometimes comes across as cheap because of the simplicity of the setup required to achieve it. When done well, however, low key creates a very dramatic portrait.


What About Other Lights?

So what about those other lights that aren't the key and fill? Primarily what we're talking about here is the backlight or rim light, used to create a strong outline of light around the subject to pull them off the background and into the image. These lights aren't considered a part of the lighting ratio we've discussed so far, as they add supplementary light and don't usually change the light falling on the main focus of the image.

However, stray light and reflected light can change the highlight to shadow ratio, so you do need to pay attention to how much light your extra lights are contributing. Mostly you want them at least two stops under your key light (that's 1/4 the power), because the hardness and angle of reflection mean that they can create flare in your photos, even at low power.

Even against two 160Ws strobes on 12 power the 60Ws speedlight only had to be on 132 due to the efficiency of reflective surfaces
On the high-key example, even against two 160Ws strobes on 1/2 power, the ~60Ws speedlight in the back only had to be on 1/32 due to the efficiency of reflective surfaces.

Why Focus on Lighting Ratios? Speed

On every paying photo shoot, time is money.

You don't have to be a technical expert to create well-lit photos. Many skilled photographers never think about lighting ratios at all. Does that mean you shouldn't have a good grasp of lighting ratios? No!

Just like the inverse square law and other rules of lighting, the more you know, the better and faster you can prepare. Here's what will happen if you get comfortable with lighting rations:

  • Your shoots will be more directed, with less trial and error, and far fewer mistakes
  • You'll start to notice little things about your lighting that you didn't notice before
  • You'll start trying to figure out how other photographers did their lighting
  • You'll start to imagine new ways to light

And then, then the really cool things will happen. Your shoots will start to move smoothly and feel easy. You'll you'll be able to communicate efficiently with your assistants, to tell them exactly where and how to set the lights. You won't waste any of your client's time.

When you're trying to create a specific lighting look, it helps to have knowledge of how to arrange and power your lights. Using your light meter, the camera's histogram and reviewing your lighting results as you go are an easy way to create exciting looks.

Keep on Learning

Lighting is a fascinating, fun thing to learn. Here are some more tutorials on lighting from Envato Tuts+:

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