There's more to light than how it falls on film. There's the aesthetic side which, once understood by the photographer, improves the overall quality of shots.
Start at the beginning and the end, with the spectacular colors of sunrise or sunset. Certain atmospheric conditions and cloud formations make for breathtaking scenes, or provide dramatic backgrounds for a forlorn subject such as the silhouette of a weeping willow.
As a rule, to silhouette people against a brilliantly lit sky is effective. But to use the light from such an early morning or early evening shot to illuminate a model's face spells disaster. The predominantly red-yellow-orange light casts an unnatural look to the skin, and the resulting photo gives the feeling of improper exposure. You can make an exception by including a small piece of the sky in the shot to give a clear idea of why the model's skin is red.
If you're intent upon shooting a model "naturally" before a blazing sky, use a fill flash to more adequately color the model's face and make him or her look more natural.
Also, whenever possible, you can heighten the effect of a sunrise or sunset by including in the photo any reflecting pools of water. This works best on a still day, when the glassy smooth surface reflects nearby trees, buildings, and low-hanging clouds in dramatic shades of orange.
Of course, you can't always be out shooting when the sky is a brilliant hue. Sometimes it's foggy or hazy, overcast or raining. But you shouldn't leave your camera at home: some of the most beautiful and poignant shots have been taken on dreary, overcast days.
One of my favorite "bad light" shots is mist hanging over a cool mountain stream deep down in a valley. Especially on the heels of a sultry, hot day, the moisture seems to rise from the water and hang there begging to be photographed. And if youhappen to have your camera pointed in the right direction when the sun burns through the haze and its rays begin dancing off the low-hanging fog, lucky you.
Shooting indoors can offer creative use of available light, too. You needn't automatically reach for the flash -- especially not if your camera is loaded with fast (ASA 400) film. During the day, check out the light coming through a window facing south or west. Add a cat or a small toddler peering through the pane, and you have the makings of a very effective shot. If you have an adjustable camera, overexpose by one or two f/stops to compensate for the bright background (the glass) unless you're satisfied with a silhouette.
Also, try some people-shots illuminated by light from a fireplace or candle. While the resulting photos will have a marked orange cast, if you include the source of light in the scene, the viewer will accept the cast more easily. Again, use a fast film and, in dim situations, mount your camera on a tripod to eliminate the blur that comes from camera movement during long exposures.