Take with a grain of salt any advertisement or specification on a light modifier that tells you how many "stops" of light is lost. They never answer the question, "Compared to what?"
The meter reading from any light source will depend on the modifier in place. Hotshoe flash units are extremely efficient when used direct, because of the internal reflector and fresnel lens. Diffuse this light in any way, and that inherent efficiency is gone.
What's more, the reading you get from a direct flash unit will depend on its angle of coverage. The GN from a 580EX varies from 15 meters to 58 meters, depending on the zoom setting. But this won't matter much if you blast it through some translucent material or bounce off a diffuse reflective surface. Measuring the "light loss" from a modifier is an enigma since it depends what zoom setting you're comparing it to.




