Kinds of light in photography
Еhere are three fundamental kinds of light that affect photography. There is diffuse light, which comes from sources such as a white wall or a cloudy day. This kind of light illuminates the whole object evenly and does not create shadows.
Then there's direct light, which creates strong shadows because it tends to be more intense. The last kind is reflected light, caused by objects close to the subject reflecting onto it. It is also known as bounced light and diffuses the primary source of lighting in the photograph.
As common knowledge will tell you, the key light is one of these three types; diffuse light ("the key"). In general terms, it is "any main source of illumination", but there are several kinds of keys. For example, in portraiture, the key light is typically placed above and to the side of the subject's face, as this provides a good illumination on most people. In a portrait of a young lady, an overhead key also tends to smooth the skin and hide any wrinkles.
So what is the key light exactly?
As mentioned earlier, it is the main source of illumination (the light that illuminates the whole object evenly; diffused light). Technically speaking, the key light is always the brightest light in a scene. How to recognize this? It's simple: look at your subject and decide which part of him or her is lit upmost. This is precisely where the primary source of lighting should be placed for you to achieve professional-looking results.
What if there are two areas with equally bright illumination? Then you need to first imagine what kind of mood do you want your picture to give out. Do you wish to use hard-angle dramatic lighting or soft ambient lighting? Depending on that, you will then figure out where should each key lies - and hence, your subject's illumination.
In photography, there are two types of key light: the main one and the fill. The main key is the most important source of illumination in your photo, while the others provide secondary illumination. In this case, you can use up to three sources of light.
For example, if you want to create a romantic atmosphere with a woman at night, you can have a streetlight that illuminates her face from below – just as long as it doesn't cast too much shadow – then there's going to be an artificial "key" coming from above her head, which will create a halo effect on her hair and clothes, and finally, you can add another to give more detail on her face.
What factors should you consider when setting up a key light?
There are two ways to answer this question: the first one is objective, the second is subjective. In objective terms, after careful analysis of your subject's features – be it a person or an object – you need to decide which part of him needs more illumination. If there are shadows on his face and clothes, for example, a source of light from above will help. Moreover, always keep in mind what mood you want your photo to convey.
Simply put, if you want to create a happy atmosphere with your picture, place your main source of lighting from above as well; otherwise, use any other angle that suits the mood you're aiming for.
In subjective terms, never place your key light too high; it will give the person you're photographing an unnaturally tall appearance. The same goes for putting it right in front of him or her (which is why you should use a fill); doing that will make the subject look like he's facing "into" the camera, rather than looking at you.
How to take a photo?
Key light doesn't need to come from a single source; it is okay as long as it's the most intense light in the scene. It is also possible to bounce it off other objects such as walls or ceiling (see top image), but make sure that there is no clear-cut shadow cast by those surfaces onto the subject. If necessary, use paper or another white material to diffuse them.
Finally, pay attention to where exactly your key will be placed; don't make the mistake of over-illuminating something that won't contribute much to your overall shot (e.g., brightening the sky in a picture of just your friend sitting on a bench), as this will only result to unnecessary noise or pixelated bits that are difficult to remove.
And lastly, always keep an eye on lighting colors. It is common for amateur photographers to overlook this part, but lighting can greatly change the mood of an image. For example, if you choose a tungsten light color (orangey-yellow) for your scene's key, it will have a warm/romantic feel, which may not be what you are aiming for if it isn't suited to the kind of shot you're attempting. Similarly, using colder hues such as blues and purples tend to make photos look surrealistic.
Sources of the key light
Sun
The sun is the primary key light in almost all outdoor scenes, which is why it is best to take these photos either early in the morning or late afternoon. If you are shooting during other times of day (e.g., at noon), another strong source of illumination should be present in your photo already, such as artificial lights (see below).
Artificial lights
If you want to try something new that will allow you to create more dramatic effects, artificial lighting can work wonders. However, keep in mind that fluorescent and LED bulbs tend to give out colder colors (i.e., less yellow) than regular ones thus making it harder for you to achieve a romantic look. For this reason, halogen spotlights are the best choice if you want to mimic natural light.
Flashlights
If you are in a place where artificial lights are either unavailable or too dangerous to use, flashlights can work. However, remember that these are on the shortlist for main sources of lighting because they also emit colder colors. If possible, get one with tinted white LED bulbs so it will have more yellow than normal flashes. The best bet is to try experimenting with them first before using them on important shoots.
Window light
Windows have some potential as key lights, but only if there's an object in front of it that will reflect onto your subject, Ie .., something that's already generating shadows (i.e., another person). Also, note that polished windows act like mirrors; they produce a strong glare that will make it appear as if your subject is staring at his or her own reflection.
Reflectors
Reflectors are the best artificial light sources for beginners, as they emit less noise than flashlights or lamps, and can be used to create multiple effects. Ideal if your subject is placed near a window facing outwards where there's another strong source coming from.
Sunlight reflected by objects
If you happen to find yourself in an environment with many heavily-polished surfaces (e.g., a large office full of metal chairs), it would be best not to use them as key lights because their reflections will add significantly to the overall lighting in your scene. However, you can still try using your camera's flash together with these lights when trying to take dark shots of people wearing black clothes in front of blue or green walls.
Composition and setting a key light
Once you have a key light, it's time to set up your scenery. First, get rid of all objects that you think will only interfere with the main dish (i.e., your subject). Then, position yourself and your camera so that it will be level or slightly above your model. Finally, take the shot from a far distance but don't forget to check its look on different monitors/viewfinders to make sure everything is in order.
The above guidelines are certainly not comprehensive but should give you a general idea of what you need to do when taking photos using artificial lighting. Be sure to practice these tips before taking photos for important purposes such as for photoshoots since they may prove helpful later on.
The mood of the photo and key light
If you want to create a romantic mood, your key source (i.e., the light that comes directly from the largest object in the scene) should be coming from behind your subject. This is because light from the back tends to add a yellowish tone to your photo, thus making it warmer. In addition, when shooting directly in front of the main source of illumination, you will end up with a photograph that's mostly illuminated from above. This means it will also have colder colors and thus feel more distant.
This creates a sense of intimacy in the minds of viewers since they will be reminded of times when they were held closely by someone else in their arms (e.g., during childhood). It's worth noting though that if you want to simulate this feeling using artificial lights, green or blue walls or backgrounds need to be present.
For instance, if one person is sitting on an office chair and looking at another person, the latter should be standing behind a blue wall while the former is sitting on a green one.
If you want to take a shot that feels more distant or aloof, then it would be best to have your model looking up, as this will add a feeling of smallness in viewers. In this case, make sure the key light (i.e., the biggest source of illumination) is coming from beneath his or her chin, plus walls and backgrounds with stronger colors. If neither option works, try setting up an additional flashlight over your subject's head.
The key here is to place your camera lower than eye level while pointing it upwards towards your mod el's face so that they end up looking down via the perspective. This will not only make them appear small, but if you position the key light correctly, it will also blend into their eyes, giving off a romantic tone.
If you want to create an effect where your subject looks like he or she is staring intensely at his/her lover (e.g., during Valentine's, or when both of them are taking part in a heated argument ), then make sure the key light is coming from below towards their chins just like what was mentioned earlier.
The color should be relatively cold (i.e., blue) and walls and backgrounds should also reflect this trend (e.g., they're painted green or brown). Just remember that you can adjust these depending on your tastes and what you want to achieve. As long as the results look good enough, then it should be fine.
Bottom line
The key light is the biggest source of illumination in your shot. Use it to create different moods on what you're trying to take a photo of. If you want to take a photo that's distant, use light from below. If you want to take a photo that looks warmer, have the key light coming from behind your subject.