Four important things you should know about light
In photography, light is the key. It's the photographer's raw material, as photography literally means drawing with light. Here are the four characteristics of light.
Color temperature
Daylight, moonlight, horizon daylight, incandescent lamps, studio lamps, tubular fluorescent lamps, xenon short-arc lamps, LCDs, a clear blue poleward sky, etc. All these light sources don't share the same color temperature as some of them look orange, and others look blue. Color temperature is conventionally stated in the unit of absolute temperature, the kelvin, having the unit symbol K. If the human brain adjusts brilliantly to see the right colors in different light conditions, a camera in auto white balance mode can have trouble getting things right. Note that white balance is not defined in a RAW file, so if the camera misses it, the right white balance can easily be applied to the picture during post-processing. However, JPEG files don't share the same flexibility as RAW files and the photographer will want to make sure that white balance is right when shooting in JPEG. Fortunately, most cameras allow color temperature to be set manually, by photographing a white texture or by selecting the correct temperature in kelvin. Learning how to approximate color temperature depending on light sources can really help getting more realistic colors during shooting if you shoot JPEG or during post-processing if you shoot RAW. A simple tip is to know approximately what is the color temperature for typical daylight. It is around 5,500K, that's why professional studio flashes produce 5,500K light. Below, light looks yellow or orange, and above it looks blue. To show the influence of color temperature on an image, I took three pictures with my camera in the same light conditions (under an incandescent light) at different color temperature settings:
There are two incandescent lamps in the room, so color temperature should be around 2,700K - 3,300K (a yellow tinted light). For the first picture, I selected color temperature manually and set it to 2,000K. It means that I told my camera that color temperature was 2,000K (very orange) which is wrong, as it should be around 2,700K - 3,300K. As a result, the picture looks blue because the camera thought light was orange so it produced a blue-tinted image to reach the right balance. For the second picture, I set color temperature manually to 13,100K (a very blue color) and the picture looks orange because the camera produced an orange-tinted file to reach proper balance. For the third picture, I set color temperature manually to 2,950K (approximately between 2,700K and 3,300K) and colors look way more natural. This table can be helpful:
The quality of light
Light can either be hard, or soft. Light is hard when the light source is small relative to the subject. A hard light casts crisp, hard-edged shadows. For instance, a flash directly oriented towards the subject generates a hard light, because the light source (the flash) is very small compared to the subject (your aunt Paulette), and this hard light source casts a hard-edged shadow on the wall behind. A light is soft when the light source is large relative to the subject. A soft light casts diffuse shadows with soft edges and tend to wrap around objects.
By orienting your flash towards the wall beside your aunt Paulette, you can soften your light source because the light source (which is now the wall reflecting light from the flash) is big compared to your aunt Paulette.
For instance, on a bright summer afternoon, light is hard. Even if the Sun is not exactly a small light source, it generates a hard light because the closer the light source, the softer it becomes and 150 million kilometers away makes the Sun appear small in the vast blue sky which makes up for quite a hard light source. On a cloudy day, light is soft because light does not come from a single point in the sky anymore as the layer of clouds diffuses it across the whole sky. Generally, soft lights are more flattering and more easy to work with.
Intensity
Brightness as seen by the human eye is relative. Long story short, a flashlight looks more luminous in the dark than in plain daylight. Use a light meter to make sure that light is intense enough and that light is more intense on your subject than in the background when shooting portraits. In the studio, a light meter will also be useful to make sure that light intensity is homogenous in the background.
Light direction
It has quite a big impact on the image. Depending on where light comes from, an image will have a totally different look. There are no particular rules about light direction. A photographer simply should be conscious about where light comes from and take it to his/her advantage.
