Painting with Light
If you'll be shooting with something like a basic smartphone, please feel free to ignore most of this; the controls on most telephones don't allow you much freedom.
Though you might consider an application like DSLR Pro if you've got an Android machine. There's probably a similar thing for an iPhone.
A modern compact or telephone camera will give good results about ninety percent of the time, but if you can control things, you should get good pictures one hundred percent of the time.
The main things you have to worry about with light are:
- Exposure - getting enough light on the scene
- The colour of the light - the yellowness or blueness of a shot
- Contrast ratio - cameras don't like deep black or shiny white
- Light source - what light(s) to use
- Naturalness - does it seem 'right' or artificial?
Well without any light at all you wouldn't be able to shoot. At its simplest, you need to put enough light on the subject to enable the camera to expose the film, chip or whatever.
And don't be fooled by manufacturers' specifications that include things like, "Shoot in only 1.5 lux - imagine shooting your daughter blowing out the candles on her birthday cake with no extra lighting". Well the shot will be crap for quite a few reasons; it will be grainy and noisy (I'm sure you've seen a shot where the gain has been turned right up and there are little white and/or coloured dots cluttering up the scene). And almost certainly, there will be a vertical white bar top to bottom of the frame on each candle. Worse - when she blows out the candles she'll vanish!
White is white is white. Or is it? Well, not quite. The sun is white - but when it's on the horizon it looks red. And anyone standing facing it will look a bit reddish-orange If you walk from a room lit by ordinary domestic sixty watt bulbs to one flooded by fluorescent light, the 'feel' of the light is cold, hard, blue-green (and maybe a bit flickery).
In general you're not too aware of the colour temperature of the light, because your eye compensates for it in the same way as your iris opens and closes to suit the amount of light incoming. If you're talking to a friend in a white shirt outdoors and you move indoors your brain knows that it's the same shirt and compensates for the colour of the indoor lighting without you having to do anything.
Scientists measure the redness or blueness of light by temperature. Heat a bit of steel up to 5,600 degrees Kelvin (whoever he is) and it will glow a very blueish white - approximately the same colour as average sunlight. Let it cool a bit to about 3,200 degrees and you've got the same colour as ordinary household lighting. Just in case you're wondering, Kelvin equals Centigrade plus 273 degrees.
There are many variations: sunlight at noon in some countries can reach twenty thousand degrees; photofloods are slightly hotter (more blue) than ordinary household tungsten lights; a light on a very long extension cord would give a more orange light, etc., etc.
Of course, the light from the metal bar or the sun isn't of one particular wavelength; sunlight has rays of every shade from infra-red to ultra-violet. It's just that the sun has more of the blue end - a domestic bulb gives more red.
More on Colour Temperature
Hollywood films have taught us that moonlight is rather blue. Actually, it's reflected sunlight and, unless the moon really is made of green cheese, the light arriving on Earth will be about the same colour as daylight. It looks blue-ish because your eye is comparing it with (for instance) light from the window of a house.
If you take a picture of someone facing a sunrise, he or she will appear rather pinky-yellow. But take that person to a location where there is no direct light from the sun (maybe the western side of a house) and the shot will be very blue. The light from the sun is scattered by dust and things in the air: the red is scattered less, so direct light is red. The blue end of the spectrum is much more scattered, and it's that light that illuminates the western side of the house.
What about Indoors?
Fluorescent tubes give a very strange light. Instead of a nice gentle curve showing variations in the red-blue continuum, most fluorescents give an uneven, spiky sort of light. Two pieces of orange cloth that, in sunlight, are only a few shades apart can look totally different in supermarket lighting. There are fluorescents with a response curve that's much less jaggy, but they tend to be rather expensive.
Colour temperature can be measured in Mireds instead of degrees Kelvin. Engineers say that using mireds makes for a much more scientific and accurate result. For example, when you're trying to match the light coming through a window to the lights you've set up to film in a room, mireds can help you work out exactly which filter to use. On the other hand I don't know any director of photography who uses mireds in practice...