~Starting with Ambient Light~

Understanding flash begins with ambient light. Flash is needed when ambient light is either too low, contrasty or nonexistent to properly expose the subject. Low light can come from shooting in the shade of a tree or late evening. Contrast seems to haunt our every exposure as we fight the battle between highlights and shadow. And no light for photography can be encountered in our own homes when shooting a birthday party. In any case, understanding these deficiencies is the first step in properly using flash.

 

The next step is understanding how we properly expose film for ambient light. Ambient light is the light that comes from the sun (99% of the time, artificial lighting such as from household lamps could be considered the other 1%) that bounces everywhere in and out of doors. To properly expose for ambient light, we have available to us shutter speeds and apertures. We must use the correct combination of shutter speed and aperture to properly expose film for the existing light based on the film's ISO (film's ISO to light index). A proper exposure in general terms is when detail can be seen in the highlights and shadows of a photograph. (If detail is not seen in either the highlight or shadows, then the range of exposure is beyond the ISO of the film.) The shutter speed and aperture each have a separate yet combined roll to play in proper exposure.

 ~ISO in DSLR~

The shutter speed determines how long the film sees the light. Shutter speeds are described in terms of time. Current Nikon cameras have shutter speeds with durations of 1/8000 of a second to a full 30 seconds (this doesn't include manual speeds/times fired using the bulb function). No matter how the shutter speed is set, whether manually dialed in or automatically derived via the camera's computer, the shutter speed determines the length of time the film is exposed to the ambient light. The shutter speed also determines how long the film sees the subject. If the subject is not moving, the length of time the shutter is open and the film sees the subject does not matter. But if the subject is moving, long shutter speeds can result in out-of-focus subjects because movement is seen by the film.

 ~shutter speed~

The aperture regulates the amount of light that strikes the film. The aperture is a large hole normally at the rear of the lens through which light must travel to reach the film. It has blades that can be closed down, making the hole smaller, or opened up, making the hole larger. The larger the hole, the more light that can strike the film. The smaller the hole, the less amount of light that can strike the film. The size of this hole is described most commonly in f/stops (though not technically correct, but who cares as long as we all know what we're talking about). The larger the hole, the smaller the f/stop number is. The smaller the hole, the larger the f/stop number is. For example, a large hole would have an f/stop of f/1.4 and a small hole an f/stop of f/32. The size of this hole also determines the depth-of-field in the photograph, that is, the depth of focus in front of and behind the subject.

 ~F/Stop~

These are our only two means of regulating the light that strikes the film. In flash work, it's important to remember that shutter speed affects how long the light strikes the film and that aperture affects how much light strikes the film.

source:http://www.moosepeterson.com/techtips