Whats the difference between true watt seconds and effective watt seconds?
True watt seconds measure the actual number of watt seconds a flash unit is capable of storing. Effective watt seconds is a term that is used in an attempt to make you believe a flash unit is more powerful than it actually is. The seller of a flash unit advertises that the light is rated at 800 effective watt seconds and 300 true watt seconds. The implication is that the light is three times more powerful than other 300 ws flash units. In fact, the unit is only 300 watt seconds, but by using a highly polished reflector the light output can be increased by about one f stop which would be equivalent to a 600 watt second unit. By inflating the guide number by 1/3 f stop, the output is claimed to be 800 ws. Since there is no direct relationship between watt seconds and light output, effective watt seconds is not an accurate method of defining actual light output or a flash units ability to store electrical energy.
