High contrast Photography
If quality and direction are two basic tenets of good photography, then contrast is where it gets a tiny bit more technical. Contrast is the difference between the bright, lit part of the scene, and the dark, unlit shadow areas. If there's a huge difference in brightness, it's high contrast; if not, it's low contrast. If you're armed with a flash meter you can take highlight and shadow readings, work out the difference in terms of f-stops, and then determine if that range falls within the dynamic range of your particular camera's sensor. Unsurprisingly, that's far too technical for most people, especially if they're trying to work reasonably fast, so a good way to judge contrast before you take a photo is to squint at the subject. I often use this tip when I am taking portrait photos or those special photographs at a wedding as the official photographer; good contrast can look really effective when taking those photographs of the Groom and his Groomsmen.
You may get some strange looks and have to explain yourself, but it works! Of course, the other alternative is to take a photo and check the LCD screen on the camera to see if you have managed to retain detail in both the shadows and highlights. You can use the histogram for this, but it's easiest just to look at the picture on the screen. It's not technically ideal, but it works for many people. If you discover you have too much contrast, you have two options The first is to adjust your exposure so that the most important part of the scene is captured properly. If the main area of interest is in shadow—someone's face that is backlit, for example—then by exposing for the shadows the background will be high-key, featureless, and bright. This may be the effect you're after. Alternatively, you can expose for the highlight areas and let the shadows go to black it's a creative decision.
The other option is to reduce the contrast, usually by filling the shadow areas with some light. You could use a reflector or an additional light source to do this. This is called a "fill light" as opposed to the "main light." A word of warning: if your main light source is, say, 45 degrees to the right of your camera, many people put their fill at the other side at a similar angle, usually set at a lower power setting. However, these really are two very different light sources and will produce two sets of opposing shadows that may partially cancel each other out. It's a very old-school approach
The new rule is that a fill light shouldn't go farther over than the axis of the camera lens. Actually, the axis of the camera is an ideal place for a fill light. An onboard flash, or a ringflash—a kind of flash that goes right around the outside of the lens—can be a great fill light. That way, any shadows go behind the main subject, so there are no conflicting second shadows. However, rules are meant to be broken, so often main and fill lights do come from different sides. That's the beauty of photography—there really is no right or wrong way to do it.
Its all about looking at the subject or scene in-front of your eyes and understanding the light and how you want this to impact on the photograph you are about to take.