flash comes in 3 types:
* vintage versions where the duration of the flash is fixed,
aka Manual, you have to calculate the f-stop based on the
estimated distance from you and the subject.
* 2nd generation came with pre-set apertures as well as manual modes,
typically f2.8, f5.6 and f11 would be provided. you simply had to choose the iso on
the flash and the aperture, on your camera, you would set the same aperture as
the flash.
* 3rd generation flash featured TTL, this allowed you to choose the aperture on the lens
and the flash will automatically compensate for the iso and the aperture.
i find the 2nd generation most useful, if you can search for some of the cheap vivitar flash
as well as the leica CF (compact flash). most of these flash are cheap as they do not work
with digital cameras.
hope this helps.
raytoei.