I learned a lot from a few of you with that last post - I obviously had a
few gaps to be filled in this area. I put all that I've learned in a
summary - having done that I thought some of you might be so good as to
review it to make sure I've got the gist of it now ...

"You can shoot above 1/250th by putting the flash into FP (High-speed synch)
mode ...

But ...

Shutters consist of 2 curtains. Below X-sync speed the 2 curtains fully open
at the same time - fully exposing the sensor.  Above  X-Sync speed the
curtains can't open fast enough so their openings are staggered -
effectively creating a moving slit. In summary, X-Sync speed has NOTHING to
do with the speed at which flashes can sync with the shutter - it's purely a
mechanical shutter limitation.

With shots below the X-sync speed the curtains open - flash fires - and some
time later the shutter closes - it's important to note that the flash
duration is usually only a small fraction of the time the shutter is open.
So - if the sole source of lighting is from the flash (as with strobes that
kill the modeling light when the flash fires) then shutter speed becomes
irrelivant - doesn't matter if you set it for 1/250th or (taken to the
extreme, so long as the modeling light remains off) 30 seconds - Exposure is
only occuring while the flash is lit up - not while the shutter is open.

Above x-sync speed (in FP mode) everything changes ... because only a
fraction of the sensor is exposed at any given instant the flash has to
illuminate the scene for the entire time the sensor is exposed - the flash
can't stay on for this long, so it switches on and off at 50,000 times a
second (so even at 1/8000th it would still flash around 6 or 7 times). This
works well if the flashes are powerful and the subjects are fairly close and
the flashes are doing most of the work (eg a group of people at 2 to 3
metres indoors with poor ambient light) - but outdoors on a bright day (eg
wedding) then they can have a lot of trouble competing with the sun (it also
drains the batteries a lot quicker as the flash may be flashing a 50,000
time a second for up to 1/300 of a sec verses a single flash that typically
lasts 1/30,000th of a second.

So what's the answer?

The answer is to get your shutter speed below X-Sync speed - and assuming
that we can't do it by changing F-Stop (because we want a shallow DOF) then
what we need are ND (Neutral Density) filters!"

Have I got it right now?

Many thanks to all,

Cheers,

Colin