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Shutter, aperture and ISO are the three variables which control exposure.
Typically you set ISO as low as possible to reduce noise, aperture for the desired DOF, then select the shutter speed which produces the correct ambient light exposure. ISO and aperture affect flash exposure but not conventional flash because the shutter has two curtains. The first opens completely (which takes about 1/300th sec.), the flash goes off, then the second curtain closes ending the exposure.
Correct exposure in the technical sense is when there is detail record in the shadows and highlights at the same time. That is a function of the sensor design. Overlapping a key light over even "neutral" fill allows the scene to be fit to the sensor indoors. If we start in a dark room we'd want to first lay down a foundation of fill, just enough to record detail in the darkest areas...
Then all the other lights can be added....
Background light for separation....
Key light to create highlights and with it an illusion of 3D shape...
Accent lights from behind to outline and enhance the illusion of 3D shape.
When we light for detail on both ends of the tonal scale everything in the middle falls into place automatically because that's how the photographic process is engineered to work.
The dilemma outdoors is that most scenes exceed the range of the sensor with just the ambient light. The only way to match the scene to sensor is to:
1) Shoot into the shadows of the ambient. Shooting into the shadow side allows flash to lift it INDEPENDENTLY from the ambient. Flash can't overlap the sunny highlights and lift shadows at the same time; both will get brighter and the highlights will be blown before the shadows are filled. You can't fix dark eye sockets with flash; the problem isn't lack of fill, but shaded key light. The solution for dark eyes is raise them into the light. That's not practical facing the sun so we turn the back to the sun and keep the front side ENTIRELY IN THE SHADE.
2) Expose the ambient highlights (which will wind up being brightest) below clipping,
3) Add flash to the shadow side without it overlapping significantly and blowing the ambient highlights.
In a portrait lighting situation what the flash does if raised overhead is to create a highlight pattern over the fill light from the sky the person is facing when their back is to the sun..
In that situation the flash is actually a KEY light not fill. The shadows not hit by the flash will still be illuminated by the sky. Blurring the photo makes it easier to visualize the "mask" pattern of highlights the flash creates and how the contrast of the shadows filled by the skylight creates the illusion of 3D.
When lighter shadows are desired use two lights in a key and fill configuration as explained above. The only difference conceptually outdoors is that the soft indirect light from the sky helps lift both key and fill and lights the background , while the direct rays of the sun act as accent light. The net result is the same as the four-light indoor example, but doable with only two lights
Those are the basics. For more detail read my tutorials which you can find by clicking the WWW button below.
Chuck
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