I talk flash a lot. With a lot of people. Individuals. Classes. And each time I need to emphasize the nature of flash photography. Here it is, briefly: many people assume flash exists to take a dark subject or environment and make it brighter. This is untrue. Flash exists to control the amount of light contrast in a photograph.
Many times we are shooting a portrait and attempting to lower the contrast. This would oftentimes be called a high-key shot, or one with low contrast where the subject is defined by the shadows. A low-key shot has large amounts of light contrast and we define the subject using highlights. But today I want to explore light contrast without talking about portraiture.
A flash creates a pool of light. If this light, as it hits a subject is the same brightness as the ambient light, then all it can do is eliminate shadows and thus lower light contrast. However, if it is brighter than ambient light, and we set our camera’s exposure to match it, then any part of the frame the flash doesn’t touch will get darker. The greater the difference between the brightness (or luminosity, for those of you playing the home game) of the subject relative to the background then the darker the background will become. In this way a flash can be used to either lower or increase light contrast. These days, I utilize this concept whenever I shoot flower photography.
This is a totally normal picture of a flower. It's exposed properly, it's in focus the way I want it, but I like to add some punch to my photographs. Specifically, I’m thinking about light contrast. Don’t forget, if a subject is brighter than the background it appears to pop out of the frame. So let’s add some contrast.
The technique couldn’t be easier. Trigger your flash off camera. Hold it over the flower and pointed down and slightly towards the camera. If you point it away you’ll light more of the background, if you don’t then the background will fall into black faster. Your choice. I don’t always use this technique, but when I go back through my macro images these always jump out as my favorites.