okay so my dad gave me his old external flash (vivitar 550 FD M/P/O) that he used his old pentex SLR and i was wondering if i can use it on my d40. i have heard that they might like overload the voltage but i am not sure where to check this out... i googled but couldnt find much on this external flash, i suppose its real old.. also it is a "M/P/O" model, which means its not for nikon, however i loaded it on my mount and it does work with the shutter.. however i do not know how to work it, i just pressed the shutter release and the flash worked with that... help!! thanks!!