What is Contrast?
Lighting contrast is the ratio between the brightness of the most strongly lit parts of your subject and the darkest (shadowed) areas. Photographic film and CCD sensors in digital cameras cannot accommodate as wide a range of brightness (luminance) in the same scene as can the eye. Often this means that when you expose to get detail in the lightest areas the shadows reproduce featureless black, even though you could see details there at the time. Alternatively, exposing to show detail in the shadows ‘burns out’ details in lighter areas.
The problem is greatest with hard side or top lighting: although the lit surfaces then show excellent form and texture there are often large very dark shadow areas. If you want to improve shadow detail you might be tempted to add an extra, direct, light source from the opposite direction, but this often forms an extra set of cross-shadows which can be confusing, ‘stagey’ and unnatural. Often a much better solution is to have matt reflector board on the shadow side to bounce back some of the spilt main light as soft, diffused illumination. This is known as shadow-filling. For relatively small subjects, portraits, etc., you can fill in using white card, cloth, newspaper, or a nearby pale-surfaced wall. With large subjects in direct sunlight, you may have to wait until there is cloud elsewhere in the sky to reflect back some soft light, or until the sun itself is diffused. With close subjects you can use flash on the camera, preferably diffused with care to add a little soft frontal light, without overwhelming the main light.