"Writing with light" is what photography is all about. So, let's base our lighting discussion on five essential properties of light: direction, intensity, color, contrast, and hardness. These properties determine how a photographer can tell what is going on in an image.
Direction tells us whether objects are being lit from the front or back. If they are being lit from the back, they will appear darker than if they were illuminated from the front. This is because they are not being illuminated by direct sunlight, but by reflections from surrounding surfaces.
Intensity determines how much light is falling on an object. High-intensity lights such as sunlights produce very bright objects while low-intensity ambient lights such as those from lamps or candles produce very soft results. The word "contrast" means the difference between dark and light areas of an image. Strong contrasts make for dramatic photographs that "pop." We will discuss ways to create strong contrasts later in this lesson.
Color is the third property of light. Objects appear different colors depending on what type of light they are being illuminated by. Colors in nature are always blended; there are no pure colors in reality. But photographers use artificial means to reproduce specific colors within their images.
Understanding the four aspects of light—quantity, quality, color temperature, and direction—will not immediately transform you into a great photographer. Sure, the information is vital, but it is meaningless unless you are willing to use it in the real world. Only then will it begin to give you an edge over your peers.
The first aspect of light is its quantity. Light comes in degrees from full to nothing by which I mean 100% sunlight to 0% twilight. Between those two extremes is a huge range of percentages less than full daylight but more than darkness.
In terms of photos, I divide up the degree range into three categories: full, moderate, and slight. Images taken under full daylight have a very high exposure ratio (often 1/100th of a second or faster) and are usually very bright. Those taken under clear skies with no clouds and direct sun have a zero exposure ratio and are always dark. In between these two ends of the scale are images that get some amount of light even during cloudy days or at night. These exposures can be as long as several minutes if not hours. Finally, there are images that are only exposed for a fraction of a second in order to capture fast-moving objects like birds flying away or animals reacting to sounds. Such shots require careful planning and execution; we'll discuss techniques and equipment for taking them later in the book.
Intensity, shape, color, direction, and mobility are the fundamental properties of light. Intensity refers to the measure of light's power per unit area. The intensity of light waves is always measured in watts per square meter (W/m²). The brighter the light, the more intense it is. Light that falls on an object is either reflected back out into space or absorbed by the object itself.
Shade means "covering," and dark night skies are called such because you can't see very far beyond the nearest star. Dark clouds block out much of the sunlight that reaches us from the sky; during a total solar eclipse, the only parts of the Earth that see any direct sunlight at all are around the location where the moon blocks out most of the sun. Even then, only about 1% of the sunlight that strikes the surface goes into our eyes because most of it is reflected away.
Color refers to the range of wavelengths emitted by a single source. Blackbody radiation is radiation that is uniformly distributed across the surface of a hot body like the moon or planet Venus. It consists of photons with energies proportional to their frequencies. Higher-frequency photons are blue-shifted by the effects of special relativity, while lower-frequency photons are red-shifted.