Review / Summary of High-Speed Synch in Flash Photography

Discussion in 'Digital SLR' started by C J Southern, Dec 27, 2005.

  1. C J Southern

    C J Southern Guest

    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,


    C J Southern, Dec 27, 2005
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  2. C J Southern

    Alan Browne Guest

    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.

    Alan Browne, Dec 31, 2005
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