Getting to grips with artificial lighting seems to be quite the headache for many photographers. What I found aided more rapid improvement was gaining a better understanding of lighting positions and ratios. In this article, I'm going to be addressing the latter (though it does involve a little of the former, as they can interact).
I'm going to start from scratch, looking first at what key and fill lights are, then I'll move on to how they relate to each other and why it makes a difference as to where they're 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 These Light Thingies?
When talking about studio lighting, there are generally two main lights to consider first: 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 and dimensional rendering of your subject. I'd say that's pretty key to the whole lighting thing!
The fill is, as you've probably already figured out, filling in the shadows cast by the key light and generally making your subject more visible against the background. It stops the shaded side of the subject from disappearing into blackness, helping to model the shape of it of your subject and provide extra detail. In some situations, particularly in outdoor shoots, the fill is often a reflector, bouncing back light from a big softbox or the sun.
There Are Other Lights Too
We're not really concerning ourselves with other lights right now, I'll bring them in a little later. These are things like rim lights, used to provide extra separation of the subject from the background, and hair lights or other snooted lights, illuminating details that the photographer wants to make more prominent.
Ratios and Other Mathematical Conundrums
Now that we've reminded ourselves what the key and fill lights are, what's the ratio between them? The ratio is based upon lighting power, rather than EVs or f/stops. Let's assuming you're using two identical lights and they are the same distance from you subject with the same modifiers, 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, as the key is putting out twice the lighting power. If the key is 1/4 power and the fill is 1/16 power, the ratio would be 4:1, the key is four times brighter than the fill.
Remember that in terms of exposure, however, this must be converted to EVs: eight times brighter is three stops brighter (double, doubled, and doubled again). So if you want the shaded side of the subject to be two stops below the "lit" side, you would use a fill light four times dimmer than your key.
If we're using bare lights, that's all well and good. Realistically, we're usually not. Modifiers are going to alter the power of your strobes, because they're spreading the light around, softening and dimming it. So just because the dial says 1/4 power, doesn't mean you're getting that much light onto the subject if you're then spreading it all over a big softbox.
Then there's the fact that silver interiors will push out more power than white interiors. Ultimately, you just need to know how much each modifier knocks off your power setting, and adjust accordingly.
Actually, That's Not Quite The Case
Unfortunately, I'm about to confuse matters even further. Because what the lights are putting out isn't necessarily what the camera's picking up, and that's what counts for your exposure. What really matters isn't the power ratio (or "incident" ratio), but the "reflected" ratio. This is what's bouncing off your subject towards your camera.
Now you not only have to account for what's getting lost in the modifier, but how much your subject is absorbing too! It's difficult to really know at this point what's going on with the power of each light, the only way to be really be sure is to use a flash meter.
What This Means
Now we're looking at the reflected ratio, what are we measuring? Well, exactly what we're looking at, actually, highlights to shadows. So regardless of the exact power levels on your lights, if the lightmeter or your histogram is saying the highlights are two stops brighter than the shadows, you have a 4:1 lighting ratio.
If you were actually setting out to achieve this, you'd most likely set this ratio into the lights, adjusting for properties of modifiers, and then work from there based upon what your flash meter or histogram are saying.
But There's More!
If the light from your key and fill is overlapping on the subject, then you have an area of highlight that is brighter than either of them alone. So a 2:1 incident ratio on the subject becomes a 3:1 reflected ratio, because the highlights aren't just where the key is hitting, but where it's overlapping with the fill. The appearance will be slightly harder than you intended, with shadows appearing slightly deeper and sharper.
If you're shooting an 8:1 incident ratio and the reflected H/S ratio is 9:1, will a small fraction of a stop make much of a difference here? Nope, not really. So know when your power settings are going to be important, and when it doesn't matter so much.
Don't forget, of course, that distance plays a part in all this. I mentioned the inverse square law earlier, and this can easily come back to bite you if you're not precise when setting up sources close to a subject.
If two equally powered strobes were shot through three-foot umbrellas, but one was placed one foot away from the subject on one side, and the other was placed closer to two feet away on the other, the incident ratio immediately becomes 4:1 (double the distance, quarter the power).
If there's any overlap in the front, your reflected ratio is now 5:1. Either way, your bright soft high-key fashion headshot has suddenly become a moody low-key portrait, just in unnecessarily cramped conditions.
As you can see, it pays to pay attention to your lighting ratios!
I just mentioned high-key and low-key, which are probably terms you've heard before and may not know the meaning behind them. Essentially, they're styles of photography based upon the lighting ratios used to light the subject.
High key is bright, soft, low-contrast imaging, where the lighting ratio is around 1:1. This virtually eliminates shadows from subtle form and a lot of small detail, all that's left is strong details and outlines of forms. Fhe 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.
The term comes from early cinema and TV, as the film and sensors couldn't deal with strong contrast then, so the actors had three lights on them to kill the shadows. It became a staple of lower-budget soaps and sitcoms from around the 60s onward, as a simple light setup that could be rigged quickly. We've come to associate it 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 stills, possibly due to the cost of the lights, modifiers and studio grip required to pull it off well! That said, sun-backlighting and a couple of reflectors can achieve it just as well in some cases.
Low key is of course the opposite, where the lighting ratio is much higher, maybe 4:1 or even 8:1. This creates deep shadows around all forms and structures of the subject, throwing it all into sharp relief. It's a moody, sometimes gritty look and can often be used in conjunction with Rembrandt lighting, forming the triangle of light under the shaded eye.
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 often comes across as cheap because of the simplicity of the setup required to achieve it. So be careful when you're going for low-key lighting, and ensure your subject is interesting enough to carry it. The best ones I've seen recently have been a single beauty dish from above. It's like a modern take on the idea.
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. How does this contribute to the lighting ratio? Well, it doesn't really affect the general illumination of your subject, so it's not part of the key to fill ratio.
However, it can affect the highlight to shadow ratio, and of course you don't want the edges of your subject to blow out, so you do need to pay attention to how much light your rimlights are contributing. Mostly you want them at least 2 stops under your key light, because the hardness and angle of reflection on the subject to create the rim mean that they can easily flare up, even at very low powers.
If you're introducing other lights like top lights or snooted hair lights/eye lights, etc. then you have to figure out whether in your use they're key or fill lights and adjust accordingly.
Does Any Of This Actually Really Matter?
Well, yes and no. Do you need to make a note of the light ratio for every shot, carefully calculated and calibrated with a spot flash meter off the subject? You can if you want, of course, but not really.
Since cameras include a screen and histogram now, it's often just as easy to use the camera to gauge your lighting setup. Many swear by their lightmeters, and consider not metering to be lazy and inaccurate, but to me the proof is in the picture.
Taking a couple of test shots and making adjustments will take no longer than metering properly, and should yield more information about pose and light placement into the bargain. What it looks like matters much more than what the numbers say, and a technically flawless shot that's boring because you spent more time metering than interacting with and directing your subject is a pointless shot anyway.
Does that mean you shouldn't have a good grasp of lighting ratios? Not at all. Just like the inverse square law and other rules of lighting, the more you know, the better and faster you can prepare. Your shoots will be more directed and less trial and error.
The better you understand how your lights are working for you, the more you can previsualise how the result will look, and the more creative you can be without worrying about technicalities in front of your subject. If you make the most amazing photograph in the world, but it comes out badly exposed, that's no better!
Questions? Comments? Hit up the comments below!