Review / Summary of High-Speed Synch in Flash Photography

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

C J Southern wrote:

Have I got it right now?

To a large degree. Don't discount HSS (FP) flash however. I'm not a
great fan of it, but the specific situation of outdoor in bright light,
yet wanting a shallow DOF and fill flash is quite valid. ND's are good,
but they do slow down your shooting pace.

It's not so much about "competing with the sun" as the fact that it is
hard to get large apertures. One would be happy with an ISO 25 print
film rated at ISO 15 if everything were outdoor in bright light all of
the time, esp. with todays fast glass.

Cheers,
Alan

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