I dont use a flash, I only use available light I have from the grow lights. The Cobs I run are super bright, I shoot at the lowest ISO and adjust from there, typically I shoot manual mode and shoot around 1000th-1200th of a sec in the bright light (basically shooting in full sun) It can depends on what lens you are using also, a faster lens requires that you shoot at a faster shutter speed to let less light in , as not to blow your exposure . To help capture the frost, I play with shooting angles to let the light reflect off the trichs , or use shadows to help achieve depth to help with the affect.