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Thread: How does slow syncro flash work?

  1. #1
    qhelix
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    Default How does slow syncro flash work?

    i know it's best to use the slow syncro flash when we wanna take photos against a brightly lit background at night, but how does it really work? is the main subject (ie. my friends) taken with a quicker shutter speed while the background is captured on the 2nd flash with a slower shutter speed? If that's the case then it's still possible to handhold the camera and take the picture right? The main subject will be sharp, but the background may be slightly blurred right? Or am i wrong?

  2. #2

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    Slow sync is used whenever you want to capture the background/ambience of any low light condition.

    What the camera does is to set the exposure and shutter speed for a proper exposure of the entire scene.(slow shutter speed)

    However, the flash will also fire to properly expose the foreground subject.(Duration of flash is typically 1/1000s or less)

    How clear/sharp the subject and background is will depend on how static it is, how steady the camera and the amount of ambient/background lighting. In some cases, you may even see the background "eat" into the subject if its not totally still.

  3. #3
    qhelix
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    umm...ok, so under normal conditions do you think it's still ok to use it to take pictures of friends handheld?

  4. #4

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    Originally posted by qhelix
    umm...ok, so under normal conditions do you think it's still ok to use it to take pictures of friends handheld?
    What do you mean "normal" conditions? Any condition that requires it is usually low light condition.

    It all depends on how still they can stay and how still you can hold. You have to find your own threshold for handhold shots.

  5. #5
    qhelix
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    Originally posted by Zerstorer
    It all depends on how still they can stay and how still you can hold. You have to find your own threshold for handhold shots. [/B]
    ok thanks. guess i'll just have to experiment when i get the opportunity.

  6. #6

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    just remember that there are 2 exposures going on at the same time

    (1)background=ambient exposure which is the usual shutter speed x aperture to expose for the background
    (2)flash exposure which is where the flash fires to expose for ur friend=main subject

    bacause some cameras (low-end ones)are made n programmed to fire flash at the default x-sync speed, so somewhere along the line the term "slow sync" came along to differentiate from this behaviour ie slowsync just means dont have to strictly fire at x-speed ie like as what zero said, both types of exposures(1) n (2) occur at the same time. so there's really nothing special extra about "slowsync"

    the actual more special extra stuff are "rearsync", "frontsync" and "highspeedsync" , "x-sync"

  7. #7
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    highspeed sync seems like a gimick that some cameras have...
    what is x-sync? please educate me... thanks

  8. #8

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    Originally posted by showtime
    highspeed sync seems like a gimick that some cameras have...
    what is x-sync? please educate me... thanks
    highspeed sync is not just a nice name for a gimmick. it does actually work. it's particularly (to me) useful as fill-in flash in bright lighting, or for macro work where you want only enough light to expose the subject and leave the background dark.

    if i'm not wrong, x-sync is a term originally used for manual cameras. those cameras have only one usable shutter speed for flash photography, the x-sync speed.

  9. #9

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    If I'm not wrong X-sync is simply the generic term for the external flash sync speed. Front/Rear curtain sync are derivations of it.

    High-speed sync attempts to overcome the limitations of the max X-sync speed by having the flash fire multiple pulses as the shutter curtains move to avoid screening effects. This allows extremely high shutter speeds to be used with flash, but vastly lowers the working distance of the flash.

  10. #10

    Default the story of sync-ing

    sync-ing is the term to describe how 2 active components -- (1)flash firing (2) moving shutter-- cooperate during exposure

    long ago, cam body has leaf-type shutter: flash firing can sync at all shutter speeds

    modern day, cam body has upwards moving vertical FP (focal plane) shutter made of 2 curtains (front n rear)which presents a slight problem: at a particularly high shutter speed, the front curtain exits the frame just as the rear curtain enters to cover up the frame=>frame is TOTALLY exposed for just a split second

    below that particular shutter speed, the rear curtain enters the frame a while after the front curtain exits=>frame is TOTALLY exposed for a while longer

    above that particular shutter speed, the rear curtain enters b4 the front curtain exits => the frame is NEVER TOTALLY EXPOSED. rather is now exposed by a effective "moving slit" between the 2 curtains. this "moving slit"gets narrower at higher speeds

    that particular shutter speed is the x-speed = x-sync speed. ie its the fastest limit for the frame 2b TOTALLY exposed. different SLR has different x-sync speed eg. EOS3 x-speed is 1/200, F100 x-speed is 1/250

    since normal flash duration is very short (eg 1/10000 s) so if the flash fires NORMALLY at strictly higher that x-sync speed, (eg 1/2000 s) , the result is: as the "moving slit" starts to move upwards, flash fires. fine. but the moving slit still moves, whereas the flash has already stopped firing (since for eg 1/10000 s is faster than 1/2000 s)
    result: picture looks strange coz lower portion is brigthly lit by flash but upper portion is dark. ie yucks

    solution? for shutter speeds faster than x-speed,flash will not fire NORMALLY, but it will fire SPECIALLY. exactly how specially? main idea is to let it fire in repeated bursts so that every exposed position during the upward movement of the moving slit is covered (by flash fire) this is then called HIGH SPEED SYNC

    then for rear sync and front sync, both these choices happen during slow-sync . so now,slow sync is simply flash firing at any shutter speed strictly slower than x-speed. recall that for vertical FP shutter at really slow speeds (eg 2 seconds) the rear curtain waits a while then follows after the front curtain, long after the front curtain has exposed the frame. during this significant waiting gap, the flash can either fire NORMALLY(1)just as the front curtain completely exits to totally expose the frame OR (2)just before the rear curtain covers up the entire frame. so, (1) is front sync, (2) is rear sync
    result? if u shoot a moving object in dark surroundings with front sync, object looks as if it were moving backwards(ghostly trail with brightly lit initial moving postion). if with rear sync, object looks as if moving forwards(ghostly trail with brigthly lit final moving position).
    Last edited by clive; 27th May 2003 at 11:36 AM.

  11. #11
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    x-sync speed is the fastest shutter speed where the flash could fire normally and expose the subject properly. it's related more to shutter design than the camera type (if I'm not wrong ).

    for non-slr digital cameras, generally the limitation for flash sync shutter speed is the actual flash duration (which generally is shorter than 1/1000s. it could be longer with a flash firing at full power). i.e., the flash could fire normally and expose the subject properly with a fast shutter speed of 1/1000s.

    For close-up shots with flash (where the flash duration could be as fast as 1/10000s), I could achieve proper flash sync and exposure using 1/10000s shutter speed on my S602Z.
    Check out my wildlife pics at www.instagram.com/conrad_nature

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