Most of
the modern DSLRs can work in high speed sync mode. That is the flash works at
shutter speed higher than that indicated as the sync speed.
As we all
know how the front and rear curtain of the shutter work to achieve those high
shutter speed. First curtain travels and before the first curtain covers the
entire sensor frame the second frame starts to close leaving just a small slit
being exposed at that time. if the flash fires at this time only that part of
the frame which was exposed at that point gets the flash light leaving other
parts dark.
This is
the reason why there is a sync speed for focal plane shutters which is much
slower than the the leaf shutters which is built on to the lens unit making
every lens even costlier.
But in
modern day cameras there is an option to bypass this sync speed and use flash
at much higher speeds. The AUTO-FP option in the Flash menu in Nikon is the one
you have to use for this. Canon users can use the High Speed SYNC option on
Canon cameras. This is built in to most of the cameras but we need dedicated
flashes which can work in this mode. Nikon SB600, Nikon SB700, Nikon SB800,
SB900, SB910 support his mode. Built in popup flash do not work in this mode. Its called AUTO-FP in Nikon because it works in high speed sync mode as long as the shutter speed is higher than the sync speed. But when the shutter speed is equal or less than the the sync speed of the camera it automatically switches the flash to normal sync mode giving a single pulse of light
How the
flash works is instead of putting out light in one single pulse, the flash
actually flickers for longer amount of time throwing out light
for longer duration as long as the 2 shutters roll over the
sensor completely This can be used only as a fill in light as the
flash looses around 2 stops of light in this mode. This is highly useful for
nature photographers or out door photographers when we need
high shutters and open aperture to blur background fill in flash in
this mode can help to put some fill light and give catch light to bird's eye or
the subject's eye. To improve the throw of lights from flash there are custom
made Fresnel lens called the Better Beamers from www.lenscoat.com
which allows the light to be magnified there by improving the throw and hence
covering more distance when using longer telephoto
lens.
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