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).
great explanation clive... but i a bit blur now..
when u get too technical about rear and front entering and exiting?
What's the normal process? who enters first and exits? Thanks.
I have a very little bit about what u are talking about so i can slowly catch but for totally new newbie, sure catch no ball...