How Much is Enough?
A safe generalization is that the more light that's available the clearer the picture and the more saturated the colors will be, although too much light is almost as bad at too little.
If there's adequate base light, you may only have to move a few of the light sources in the room to create a pleasing picture: for example, moving a lamp closer to your subject to serve as your key light, adding a little fill and bouncing light off the ceiling for your back light.
With regard to how much is enough, bright sunny days are especially difficult for videographers. The bright sunlight may wash out the shadows which help us define objects, and may actually overload the camera's electronic circuitry. Conversely, bright sunlight may cause dark shadows, again creating loss of definition. Sunlight falling on the glass of the camera's lense can cause annoying flare.
Whenever possible, avoid shooting in harsh sunlight. Try to find a location which is illuminated by reflected light, or in which the sunlight is broken up, as by leaves overhead. Consider purchasing a polarizing or daylight filter for your camera if you anticipate doing lots of summertime outdoor videography.
The Iris -- consult your manual to learn about your camera's iris. The iris is used to control the opening behind the lens that allows light to enter the camera. By adjusting the iris you control the amount of light that falls on the image sensing device. When lighting is low the iris may be opened to let in more light. And when the light level is high the iris can be closed down to let in less light. Always check the monitor to see what your adjustments do to your video image.
And although it's confusing, try to remember this: The smaller the iris number the larger the opening. Iris numbers are indicated as "f-stops." An f-stop of f2.4 lets a great deal of light onto the sensing device; an f-stop of f16 lets in very little light.
