The last flashbulbs to be manufactured were Focal Plane type bulbs, from a factory in Northern Ireland (as I recall); they ceased production about fifteen years ago, I think.
I never had a flash unit that would make good use of those, never bought any.
I do, however, have a Speed Graphic with a good focal plane shutter, and I have an idea for building an LED-based flash unit that would support the 1/8 second or so (I think I can get enough light out of LED units made to replace halogen bulbs in headlamps). As I was kicking this idea around in my head, it occurred to me to wonder how the old timers arrived at correct exposure with focal plane bulbs?
With conventional flash, you have a Guide Number -- for a particular film speed, a given combination of bulb and reflector has a characteristic light level. Divide the Guide Number for the film speed you're using by the distance from flash to subject (in either feet or meters, depending on which was used to calculate the Guide Number) and out drops the aperture -- GN 100 at 20 feet gives f/5 (for which f/5.6 is probably close enough, though f/4.5 is another option for lenses with partial stop increments). This is based on the flash and shutter time being close enough to the same length that the flash intensity can be treated as constant during the exposure. With fast leaf shutters, there are corrections, of course -- a standard M type bulb burns for about 1/40 of a second, so if you're using a shutter faster than about 1/60 you have to compensate.
With a focal plane shutter and FP type bulbs, however, it's a whole different game. The bulb burns for the entire time it takes a Speed Graphic shutter to travel -- a bit over 1/8 second, or less depending on the tension setting -- with a nearly constant brightness (there's some tail-off, so it looks longer to the eye). Since you're using what amounts to constant light over the time of the exposure, both aperture and shutter speed enter into it.
I started to think you could have a sort of "EV Guide Number" -- divide by distance and get an EV out, and use the EV to get a combination of aperture and shutter speed. The problem is, EV is linear with scene brightness, while Guide Number as I learned it factors in the inverse square law that describes how light intensity drops off with distance. So, how do you factor in distance, aperture, and shutter speed for a focal plane bulb (equivalent) flash exposure?
I never had a flash unit that would make good use of those, never bought any.
I do, however, have a Speed Graphic with a good focal plane shutter, and I have an idea for building an LED-based flash unit that would support the 1/8 second or so (I think I can get enough light out of LED units made to replace halogen bulbs in headlamps). As I was kicking this idea around in my head, it occurred to me to wonder how the old timers arrived at correct exposure with focal plane bulbs?
With conventional flash, you have a Guide Number -- for a particular film speed, a given combination of bulb and reflector has a characteristic light level. Divide the Guide Number for the film speed you're using by the distance from flash to subject (in either feet or meters, depending on which was used to calculate the Guide Number) and out drops the aperture -- GN 100 at 20 feet gives f/5 (for which f/5.6 is probably close enough, though f/4.5 is another option for lenses with partial stop increments). This is based on the flash and shutter time being close enough to the same length that the flash intensity can be treated as constant during the exposure. With fast leaf shutters, there are corrections, of course -- a standard M type bulb burns for about 1/40 of a second, so if you're using a shutter faster than about 1/60 you have to compensate.
With a focal plane shutter and FP type bulbs, however, it's a whole different game. The bulb burns for the entire time it takes a Speed Graphic shutter to travel -- a bit over 1/8 second, or less depending on the tension setting -- with a nearly constant brightness (there's some tail-off, so it looks longer to the eye). Since you're using what amounts to constant light over the time of the exposure, both aperture and shutter speed enter into it.
I started to think you could have a sort of "EV Guide Number" -- divide by distance and get an EV out, and use the EV to get a combination of aperture and shutter speed. The problem is, EV is linear with scene brightness, while Guide Number as I learned it factors in the inverse square law that describes how light intensity drops off with distance. So, how do you factor in distance, aperture, and shutter speed for a focal plane bulb (equivalent) flash exposure?
