This is an Electronics video tutorial where you will learn how to manually adjust the zoom position on the Nikon SB-900 with the diffuse dome attached. This is called the dome trick. It is not documented in the SB-900 user's manual. The manual states when the diffuse dome is attached to the flash head, it gives a fixed zoom position of 14mm. However, if you attach the diffuse dome upside down, you can adjust the zoom position manually. The video then demonstrates changing of zoom position after fixing the dome upside down. See it to believe it and then do it.