Fits over the outer rim of your softbox up to 48"(120cm) in diameter. Increase the softness of your light. factor = approx 1 F stop Will fit any brand of softbox Chimera, Photoflex, Broncolor, interfit, Calumet etc. Fits 120cm octagonal, and 60cm x 90cm, 90cm x 90cm and up to 40cm x 120cm rectangular boxes.