Well, I have to admit I was maybe wrong saying that the 1/180 limit was physical (shutter) and faster x-sync was impossible, and those waiting for a miraculous firmware update were daydreamers...
After careful examination of the P-TTL patents (Patent #6,718,135 for those interested), I think I've found out why P-TTL is limited to 1/180...
EDIT: in fact, this patent describes an early implementation of P-TTL, and it was slightly modified since, so what you're about to read is false!
In fact, in wireless mode, the main flash (built-in or external) issues
during the exposure a preliminary low-power flash burst prior to its main burst to sync the slave flashes. There is then a 3ms wait between this low-power synch burst and the main flash...
And in all modes, there is at least a 1ms wait between the main flash and trailing curtain to let the flash reach its max power.
Sequence is :- Leading curtain
- wait for half the Tv (this one I'm not sure about, maybe they mean Tv to be the X-sync duration.)
- if wireless: (EDIT: This is the part that was modified)
- low-power synch burst
- 3ms wait
- Main burst
- 1ms wait (to allow the main flash burst to fully occur)
- Trailing curtain (to occur when Tv is reached)
So in wireless, there is already 4ms spent in waiting... It translates to 1/250 (+ the Curtain travel time) being the fastest speed at which we can do the waiting!!!
From this we can conclude that
Curtain travel time is less than 1.5ms, or 1/640, by the way : as time difference between 1/250 and the 1/180 x-sync is 1.5ms, and we can hope they haven't squandered precious time elsewhere...
So, theoretically, the fastest x-synch that you could attain with an
instantaneous flash burst should be 1/640 with the current shutters... Allow for some time for longer (and realistic) flash burst (Canon 550ex has a max flash duration of 1.2ms, for instance), and you end up with roughly 1/360 being the fastest x-sync you can have with modern flashes... If you keep the 1ms wait mentioned in the patent, you end up with a 1/400 shutter speed.
EDIT: the following sentence was proved false in the subsequent dialog... (The error was coming from the wireless mode needing those 3ms, as it was proven to be false and the sync burst is done right before the exposure)
SO IT SEEMS IN FACT TRUE THAT 1/250 (and even 1/360) CAN BE OBTAINED THOUGH A FIRMWARE UPDATE! (as long as you don't use wireless, though).
And it should be compatible with all existing Pentax DSLRs and Flashes!!!
All this is caused mainly by the wireless waiting time...
It's a pity they limited the whole flash system this way.
I guess they set the x-sync the same for wireless and standard because:
1) it would have confused people otherwise, or
2) wireless would have got flamed for being slower than standard flash...
Could someone verify all this?
(and next time I'll do the math before opening my big mouth!!!)
So maybe we will see two x-sync speed from now on : standard (1/250 or maybe even 1/360) and wireless (1/180)...
Last edited by dlacouture; 10-15-2010 at 07:11 PM.