FP is for using flash bulbs with the focal plane shutter.
The FP flash bulb element starts burning shortly before the shutter opens and burns throughout the whole of the exposure, unlike an electronic flash pop that may be anywhere from 1/500s down to 1/40,000 or faster. So, with the long burn time of an FP bulb, the moving slit that comprises a focal plane shutter is not revealed to be a slit at speeds shorter in duration than the X-sync speed. (X sync is the highest speed that both curtains of a focal plane shutter are completely out of the frame). But the effective shutter speed with an FP bulb will often be much slower than a typical electronic flash pop, even used at 1/1000s or 1/2000s. Which may be worth considering before you try to source these rare bulbs-- especially if your intent was to stop action.
Most all pro 35mm cameras since the early 90's have horizontally-oriented, metal blade shutter curtains to enable a faster X-sync speed of 1/250 or 1/300s, much faster than the X sync speeds of the OM2 era cameras that used a vertical slit moving cloth curtain (typically 1/60s). This 2 stop improvement opened the door for daylight-balanced fill flash-- without having to stop down to f/22 with Kodachrome 64 or Fujichrome 100. This slower sync remains, however, a limitation with the older cameras. My Pentax LX with a 1/75s sync was mostly limited to using Kodachrome 25 and Velvia 50 for macro work with a flash on a butterfly bracket. As the main light, in dim conditions, the flash worked well at f/16 but in daylight there would sometimes be motion blur from the ambient light creating a double image.
Modern cameras with "FP" settings for electronic flash differ in that they strobe the flash throughout the duration of the moving slit to achieve a similar effect-- except for the strobing part (motion blur may result, with very fast motion like beating insect wings).
Hope this explanation is helpful.
Last edited by Pupfish; 05-09-2009 at 10:53 AM. Click to view previous post history.
Flash bulbs need time to reach full output.
So bulbs need to be ignited some time before the shutter starts opening. That's what the FP contact does.
FP type bulbs burn long enough, and with near constant output, for the slit of the "F"ocal "P"lane shutter to travel across the film gate. Hence they can be used at all shutterspeeds.
X is for electronic flash.
Electronic flash 'burns' for a very short time. So the shutter needs to be fully open when the flash is triggered, or else some parts are lit by the flash, while others are not. That limits the fastest usable shutter speed to the fastest speed that still fully uncovers the film.
Electronic flash also reaches full output level the moment it is triggered. So it needs to be triggered when the first curtain has cleared the film gate completely, but before the second shutter curtain starts covering the film gate again.
So both the moment the flash is triggered and the speeds at with you can use either type of flash (and flash contact) are different.
Olympus also made an electronic flash unit that, instead of releasing all of it's 'power' the moment the unit is triggered, fires in a series of pulses, that keep the flash's output at (or near enough) the same level for much longer.
With such a unit, you can use all shutterspeeds, not just the X-synch, just like you would using FP flashbulbs.
This flash unit could only be used with the OM 4 Ti (and 3 Ti?)
FP bulbs (and the FP electronic flash unit) should be regarded as continuous light sources (because they are). The aperture you use when using these depends on the shutterspeed you select (and vice versa). Not just on the distance and the flash's output.
Using regular electronic flash units, the flash duration is usually much shorter than the shutterspeed, and only the aperture has an effect on exposure.
Last edited by Q.G.; 05-09-2009 at 10:36 AM. Click to view previous post history.