Rainbowimaging 3 in 1 Stacking Grid Light Spot modifier System for Nikon SB900, SB910 This Light Modifier was designed to produce 3 different size of light spread (Small, Medium and Large) When aligned and stacked two grids together, produce the smallest spots of light Stack the small grid into the bezel to produce large size spots of light, Stack the larger size grid into the bezel to produce Medium size spots of light To insert a grid, align the grooves on the side of the grid with the corresponding alignment ridges on the inside of the grid bezel. Firmly push the grid into the bezel until it comes to a complete stop.