I have this Quantaray QTB7500 model flash and am using it on my Nikon D90. Though the only two settings that seem to produce any difference is the 1/16 and full power settings. I am used to using Nikon Speedlights so this Quantaray has me somewhat confused on how to adjust the settings as appropriate for the f stop and shutter speeds, like I normally would with A digital flash.... Any help would be GREATLY appreciated!!!