Most cameras have a built-in flash that operates in several modes. In addition to automatic mode, in which the camera gauges the available light and fires the flash if needed, you typically get the following options: fill flash, no flash, red-eye reduction, and night-time flash (also called slow-sync flash). Higher-end cameras may allow you to add an external flash unit as well.
Fill flash (or force flash)
This mode triggers the flash regardless of the light in the scene. Fill-flash mode is especially helpful for outdoor shots, such as the one in Figure 1. The image on the left was captured using auto-flash mode. Because the picture was taken on a sunny day, the camera didn't see the need for a flash. Switching to fill-flash mode and forcing the flash to fire threw some additional light on the subject's face, bringing her eyes into visible range.
Figure 1: Adding flash light brings the eyes out from the shadows created by the hat.
No flash
Choose this setting when you don't want to use the flash. For example, you may want to use this mode when you're shooting highly reflective objects, such as glass or metal, because the flash can cause blown highlights (areas that are completely white, with no tonal detail). You may also want to turn off the flash simply because the quality of the existing light is part of what makes the scene compelling, such as a burning candle.
When you turn off the flash, the camera may reduce the shutter speed to compensate for the dim lighting. That means that you need to hold the camera steady for a longer period of time to avoid blurry images. Use a tripod or otherwise brace the camera for best results.
Flash with red-eye reduction
Anyone who's taken people pictures with a point-and-shoot camera - digital or film - is familiar with the so-called red-eye problem. The flash reflects in the subject's eyes, and the result is a red glint in the eye. Red-eye reduction mode aims to thwart this phenomenon by firing a low-power flash before the "real" flash goes off or by lighting a little lamp for a second or two prior to capturing the image. The idea is that the prelight causes the iris of the eye to shut down a little, thereby lessening the chances of a reflection when the final flash goes off.
Unfortunately, red-eye reduction on digital cameras doesn't work much better than it does on film cameras. Often, you still wind up with fire in the eyes - hey, the manufacturer only promised to reduce red eye, not eliminate it, right? Worse, your subjects sometimes think the preflash or light is the real flash and start walking away just when the picture is actually being captured. So if you shoot with red-eye mode turned on, be sure to explain to your subjects what's going to happen.
Slow-sync flash
Slow-sync flash, which sometimes also goes by the name nighttime flash, increases the exposure time beyond what the camera normally sets for flash pictures.
With a normal flash, your main subject is illuminated, but background elements beyond the reach of the flash are dark, as in the top example in Figure 2. The longer exposure time provided by slow-sync flash allows more ambient light to enter the camera, resulting in a lighter background (the bottom example in Figure 2).
Figure 2: Slow-sync flash produces a brighter background than normal flash mode.
Whether a brighter background is desirable depends upon the subject and your artistic mood. However, remember that the slower shutter speed required for slow-sync flash can easily result in a blurred image; both camera and subject must remain absolutely still during the entire exposure to avoid that problem. In addition, colors in slow-sync pictures may appear slightly warmer because of white-balance issues.
External flash
Higher-end digital models may enable you to use a separate flash unit with your digital camera. In this mode, the camera's on-board flash is disabled, and you must set the correct exposure to work with your flash. This option is great for professionals and advanced photo hobbyists who have the expertise and equipment to use it; check your camera manual to find out what type of external flash works with your camera and how to connect the flash.
If your camera doesn't offer an accessory off-camera flash connection, you can get the benefits of an external flash by using so-called "slave" flash units. These small, self-contained, battery-operated flash units have built-in photo eyes that trigger the supplemental flash when the camera's flash goes off. If you're trying to photograph an event in a room that's dimly lit, you can put several slave units in different places. All the units fire when you take a picture anywhere in the room.
