>Some Nikon cameras (eg. D300 and D700) can be set to either >1/250th or 1/320th of a second for auto high speed synch. >Does anyone know why Nikon has provided two options and when >you would use one or the other? > >The question was posed recently by another Nikonian on the >D700 forum but no one was able to provide a definative answer. > My curiousity was aroused so I have decided to ask here.
I can't seem to come up with a definitive answer, but I do know that in the 1/320th mode, the maximum effective flash power is about a half stop less power than the 1/250th mode.
I also know that one problem when developing the FP Flash mode was to get the flash to turn on fast enough at the beginning of the shutter sequence to be fully on by the time the shutter opened.
This indicates that the turn-on time is just fast enough for the 1/320th mode.
Another key problem during development was finding an alternate ionization level that would support an efficient flash pulse of about 5ms long. The key here is 'efficient'. For FP Sync mode to have anywhere near the effective power of the regular sync pulse, the ionization level had to be very carefully chosen.
Also, when the flash fires in an alternate ionization level, there is a risk of color shift.
Since the effective power is less with 1/320th, I believe the flash is being kept on for the same period of time for both 1/320th and 1/250th, so in the 1/320th mode, more of the pulse is thrown away at the end of the cycle.
I personally always use the 1/250th mode in order to get the highest effective maximum power possible.
BTW, a lot of people say the flash fires multiple pulses in the FP Sync mode. This is not true. It fires continuously at a reduced ionization level as I already discussed, just like the flash beacons do on towers at night.