High Speed Sync: auto fp and cls

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 


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