switching illumination pattern

The SB-900 and SB-700 Speedlights offers the choice of three light output patterns—standard, center-weighted and even—to provide flash illumination for almost every shooting situation. The standard pattern is basic illumination for usual flash shooting environments. The center-weighted pattern provides larger guide numbers at the center of the image than other illumination patterns at the same focal length, making it particularly suitable for portraits. With the even pattern, the light falloff at the edge of the image is less than with the standard illumination pattern, making the pattern ideal for shooting group photographs indoors.