Flash photography is probably the medium’s least understood aspect. Flash photography is considerably more complex than working with natural light alone, as you are simultaneously controlling two exposures–the flash exposure and the ambient-light exposure. With a modest investment in equipment and an understanding of basic principles, you can achieve professional, natural-looking results easily and consistently. Using flash adds highlights to a subject’s eyes and can even out harsh, contrasting shadows.
Basic equipment includes an external flash unit, a flash bracket for mounting the flash off the camera, and a flash diffuser for softening the light. A second (and even a third) flash unit allows for more sophisticated lighting techniques. An external flash is many times more powerful than a built-in flash and the flash head can be tilted for bounce flash. If you are serious about portraiture, you will want to invest in a quality flash.
Most cameras have a flash sync speed of 1/200 or 1/250 second. This is the fastest shutter speed you can use with a flash. Any faster will result in a partially exposed image, though many of the new digital SLRs have an automatic override which resets the shutter to its proper sync speed. Slower shutter speeds are fine and do not affect flash exposure, but will increase the amount of ambient exposure. Be sure to check your camera manual for the proper sync speed setting.
Flash illumination is strongly affected by distance. When you double the distance from flash to subject, four times the light is required to provide the same illumination. Double that distance again and sixteen times the amount of light is needed to provide the same amount of illumination. This is known as the Inverse Square Law. To further illustrate this point, if your subject is properly exposed at 2.8 feet, you will be 1 stop underexposed at 4.0 feet, 2 stops underexposed at 5.6 feet, three stops underexposed at 8.0 feet, etc.. You can see that if you have two subjects which are placed at varying distances, you will need a second light source if both subjects are to be properly exposed.
We are all familiar with the harsh shadows commonly associated with flash photography. Bounce flash provides a softer, more uniform light to a scene. Simply point the flash at a 90 degree angle to a white wall or ceiling. Avoid colored walls, as they alter color balance. Bounce flash usually requires about a 2-stop increase in exposure to compensate for light dispersal. Bracket (change your aperture setting, not your shutter speed) to insure an optimum image. You can use a piece of white foam board as a reflector in outdoor portraiture, or to fill shadow areas. A flash diffuser fits over the flash head to soften the light and provide a more even illumination. Red-eye is another common occurrence with flash photography. To remedy this problem, move the flash unit further away from the lens. You can hold the flash to the side and at arm’s length, or mount it on a flash bracket or tripod. You can also have the subject look slightly away from the camera. Avoid backgrounds such as glass, mirrors, and other highly reflective surfaces which throw light back into the camera.
If you have any questions, comments, or suggestions, please contact me.
Until next time, happy image-making…