The Sea & Sea YS-D1 is a compact slave strobe that can be used for wide-angle or macro photography above and under that water. An external flash is your most important accessory for UW photography. The ocean is one big, blue filter. As you descend, the reds and oranges quickly become muted shades of blue and gray. This strobe light will bring back your subjects' natural colors. The YS-D1 is a breakthrough in the market featuring a maximum guide number of 32 for 80x80 degrees, 24 for 100x100 degrees or 20 for 120x120 degrees.