Let’s wrap this whole section up by talking about how the 4 characteristics of light are related. Going right back to the beginning, any light you use will have a:
Direction (which also incorporates distance)
Quality (which relates to size)
You can’t escape that, whether you like it or not. You also can’t escape the fact that they’re all related, and that altering one will affect the others, particularly the first three, which are very closely interlinked. To begin with, if you’re using just one light, this is all fairly easy to deal with. The challenges occur when you either change that one light in some way, or bring in more than one light source.
For example, in the video I’m shooting with some flashguns outside on a sunny day, and using the light from them to fill in the shadows on my subject. I want a soft light, so I’ve bounced the flashguns into a large silver umbrella. This makes the small light source from the flashgun much larger in relation to the subject (Quality) as the light has to bounce around the silver fabric before travelling to my subject. This bouncing around increases the light to subject distance, and therefore reduces the quantity of light hitting the subject, so to compensate for that I’m using not one, but three flashguns to give me more power. You can see how changing one aspect has affected other aspects.
Let’s say I’m shooting in a dark and dingy gym, and want to freeze the movement of people training. The gym is lit by tungsten lights, so I set my camera white balance to tungsten. The catch is, there’s not enough light (Quantity) in the gym to give me a usable shutter speed to freeze motion, so I want to add flash to allow me to do this. Flash is a different colour to tungsten, so if I add flash there will be some weird colour clashes going on. By adding little sheets of transparent plastic (called gels) to my flashes I make them the same colour as the tungsten lights, and everything now looks uniform. Of course, I’ve also reduced the Quantity output of the flashes, because their light now has to travel through a couple of sheets of plastic, but since there’s so little light in the gym in the first place, this isn’t a big problem.
Don’t forget – every light has these 4 characteristics, and all 4 of them are related. Changing one will almost certainly mean a change in one of the others.