Good flashes are taken in borderline areas
In automatic as well as in TTL flash mode it is known that the flow of power to the flash bulb in shut off in the instant, in which the amount of light is sufficient for the exposure. The portion of the electronic that cuts off the power is known as the threshold limit stage.Every time when this becomes active, the excess energy for the exposure is cut off. The exposure control shows “o.k.” to signalise a sufficient amount of light.If, however, the total energy is just enough for a correct exposure, then the exposure control can’t give an “o.k.” reading, because nothing has been cut off. The display shows “underexposed” for this reason, even though the amount of light is sufficient.For this reason we recommend determining the threshold values for your flash unit and the type of film used by making test exposures. You extend the range of use of your flash unit in this manner and you will be able to make exposures that, according to the display, should not be possible to make.