For photographic purposes, light possesses several properties. The first is intensity, which will be discussed later. The second is colour, which in photographic terms is measured by colour temperature. This temperature scale is based upon the concept of a ‘black body radiator’. In essence, this means that if we take, for example, a cold black iron bar and heat it, we will eventually reach the point where it begins to emit light. The temperature required to make this body emit light is measured in Kelvin degrees, the scale of which begins at absolute zero or minus 273 degrees C.
Consequently, the light emitted by a tungsten light source (for example, studio flood lights), is said to have a colour temperature of 3,200 degrees or the equivalent in light spectrum emission to a black body radiator heated to this temperature. The higher the temperature, the bluer and less red the light emitted until eventually, at very high temperatures, the light moves towards the violet end of the visible spectrum. Normal daylight is measured to be 5500K degrees.
is important in colour photography as film is manufactured to give accurate colour images based on this system of measurement. Tungsten film will produce false colour in daylight and vice versa. Some sources of light (eg. Fluorescent tubes) produce light through the injection of electrical energy into an excitable gas. Such sources are given a value called a correlative colour temperature. Since the gas does not behave as a black body radiator, the value cannot be seen as having the same reliability, since these types of light sources do not give a consistent colour output and can include spectral spikes which cause colour casts with some films.
Incident or Reflected Light
The other two major properties of light are whether the light is incident or reflected. Incident light is measured from its source as it lands on or hits the surface of a subject. Reflected light is the light that comes from a surface, or in other words, the incident light measured as it bounces off the surface of the subject. In general, incident light readings for photographic purposes are a more accurate manner of determining correct exposure than reflected light readings. However, there are obvious situations (eg. landscapes) where it is impossible to take an incident reading due to the scale of the subject being photographed. In these cases, reflected readings are our only options.
Often you can be misled when photographing an object such as people. A light meter can read off the background light rather than the subject...this is the most common problem. Although less common, a dark background can also mislead the camera. To overcome these problems, you should move your light meter close to the subject and take the reading from it. If you can't get close to the subject, take a reading off a grey piece of card held in the same light intensity as the light on the subject. You might change the exposure to achieve special affects at times.
Even if a camera is automatic, you can still adjust aperture using either an exposure compensation control (if it has one – usually a dial marked with +1 and -1 near the rewind knob on a 35mm camera) or by using the film speed setting control the camera’s exposure system. To increase exposure one stop, set the ASA or ISO rating on the camera at half the film's actual ISO (or ASA) rating. This will not be possible on cameras with DX coding for film speed – a good reason to buy a camera with at least some manually adjustable features.
For general photography, where depth of field is of limited importance, professional photographers will often try to use the highest possible f stop to achieve as sharp a picture as possible. If you are forced to use the largest aperture (f2.8 or larger), then you are almost certainly in dim light and will also be using a long shutter speed (1/30th or longer). A tripod will be useful to minimize camera shake and vibration. In brighter light, when seeking to maximize depth of field, the larger the f stop is, the smaller the aperture will be; hence you will need a longer shutter exposure. Again, shake may become an issue and you might need to compromise. Experiment and compare your results with photos taken by professionals and other photographer friends.
Management of lighting is an essential skill in photography. To manage light properly requires planning a photograph before it is taken. You need to consider how you want subject matter of the photograph lit.
1st - Consider all the components of the photograph
- What is the focal point, or main thing that you want people to see? This might be a face, or perhaps just part of a face; or perhaps a building or part of a building.
- There will be other components in any photo, even if no more than a dark background.
2nd - Consider how you want the various components lit.
- The intensity, colour and direction of light hitting various components may create shadows, and make some parts of the image stand out more or less than what they perhaps do in real life. In this way, by managing the light, the photographer can have a strong influence upon the image that is created.
- Stronger light will obviously make the focal point more prominent
3rd- Decide on your light sources
- If using natural light, you can manipulate the light source by choosing to photograph at different times of day; or by using things to block or reflect the natural light (eg. A cover of trees, moveable screens, mirrors, reflective surfaces like metal or water)
- If using artificial light, you can choose the colour, intensity and positioning of any light source.