The concept is a picture of someone taking the last step down a dimly lit spiral staircase, so that there is motion but the figure is sharp at the bottom step. I have set my D90 to Manual mode and Rear curtain sync and exposure based on ambient light. I'm using an off camera SB700 in commander mode (it fires just fine). My problem is that the figure remains ghostly. I've tried overexposing--really ghostly!--and underexposing--a bit more evident--but I haven't found a setting that makes the figure sharp at the end of the step.