If your camera has a built-in flash, chances are, it’s a pretty good one, but as you probably already know—and as any working pro will tell you—on-camera flash is limited in its usefulness. Straight-on flash is harsh and generally unattractive, especially for portraiture. Flash modifiers can help, but for sophisticated lighting that flatters your sub-ject, off-camera flash is a big advantage. (See Tom Bol’s "Lighting For Success" for some basic portrait lighting setups that work every time.)
In addition to the direction of the light, off-camera flash lets you control the distance of the flash output from your subject independently of your camera position. More sophisticated units, like those from Canon, Nikon and third-party manufacturers such as Metz and Nissin, can be programmed to work in groups for multiple-lighting setups that light not only your subject, but provide fill and illuminate the background, too.
In addition to your speedlights, there are a few key flash modifiers you’ll want to consider. First is a diffuser. Direct flash output can be contrasty and unflattering, but a diffuser will spread and soften the light for even skin tones without hot spots. Next is a snoot or a grid. These are used to channel light, creating a spotlight effect. With a snoot, light falloff is steep, meaning it creates a high-contrast spotlight; a grid provides a gentler gradient. Either can be used for dramatic light in a portrait. Another important flash accessory to have on hand is the gel filter, used to color the flash output—either to blend it more seamlessly with ambient light or to make a statement with bold color.
As camera metering technology has advanced with sophisticated multipoint detection systems, the perceived need for a handheld light meter has diminished. While it’s true that modern camera metering systems and LCD previews with histograms have made it much easier to get an acceptable exposure automatically, for truly inspired lighting, a handheld meter not only can help you achieve ultraprecise exposures, but can also help you gain a deeper understanding of light and how to use it creatively. While camera meters rely solely on the light coming through the lens and can therefore be thrown off by sidelighting, strong backlighting or extremely contrasty scenes, you can use a handheld meter for much more accurate readings.