Why is the flash sychro speed of some low-end slr so slow,below 1/125s? why should the flash in FP (flat pulse)mode when a high shutter speed is used?

-- legnum (legnum212@email.com), October 30, 2001

Answers

It's not a matter of low-end, it's curtain travel. shutters with horizontal-travel shutters take longer to travel the greater distance across the film. the electronic flash can't fire until the first curtain is fully open, and it has to fire before the second curtain begins to travel; therefore, the time it takes for the curtain to travel across the film determines the synch speed: typically 1/60 for horizontal-travel (usually cloth curtain) shutters, 1/125 for vertical blade-type shutters, 1/200 and faster for high-velocity versions of the latter type.

in the horizontal-curtain, slower synch shutter category are such cameras as the nikon F, canon F1, all leica (and nikon and canon) rangefinders.... not necessarily a 'low-end' group. the main benefit of this design is much greater ruggedness, and generally quieter operation, than the blade shutter. the main disadvantage is the slower synch speed.

the FP mode (the name originally came from slow-burning Type FP "Focal Plane" flash bulbs) provides a rapid sequence of flashes over a relatively long period of time: about 1/60 second, long enough to allow the shutter to travel. with a continuous source of light coming from the flash, the shutter doesn't have to be fully open at any one instant for the flash to illuminate the whole frame - so all speeds can be used, just like shooting in ambient light.

rick :)=

rick_oleson.tripod.com

-- Rick Oleson (rick_oleson@yahoo.com), October 31, 2001.

Rick already gave a very good answer. I'll point you toward http://www.lanset.com/rcochran/flash/hss.html

I posted some pictures and explanation there to illustrate the difference between FP sync and standard sync. Briefly, with standard sync, the first curtain opens, the entire frame is exposed quickly at one time, and the second curtain closes. With FP sync, flash turns on, and the shutter curtains "wipe" a narrow slit across the film.

If you try to use a flash set to standard sync with the camera at too high a shutter speed, part of your picture will not see the flash, because one of the shutter curtains will have been covering part of the film at the instant when the flash fired.

-- Richard Cochran (rcochran@lanset.com), November 01, 2001.