Monday, November 15, 2010

Fill Flash – Auto FP mode

Auto FP is a hand-held flash technique that enables compatible speed lights and DSLR bodies to fire flash at shutter speeds far above the “normal” sync speed.  Auto FP, AKA High Speed Sync, is useful in brightly lit scenes that need fill flash and small f stops (wide apertures).  Outdoor wedding and sports assignments are perfect examples:  the photographer can use a narrow depth of field to isolate a back-lit subject while balancing the different light values of foreground and back ground. In the example below a beautiful model, me, is illuminated by fill flash while standing against a very bright sky.  I used a Nikon SB600 speed light set at manual full power, a D90 programmed for FP (custom menu e5), exposed at f/5.6 and 1/2,500 second.  Normally the highest sync speed is 1/200 second.   

If you want to try this at home, I suggest finding a scene with a downward slope, a clear view of mostly sky, and a cute model.  Mary was busy....

To use high speed sync, first determine if your gear has a FP setting.  Then practice setting the menu and shooting before you get a real assignment.

My work flow is to first pick an ISO and f stop, and then set the shutter for a good exposure of the highlights.  In other words, you compensate for the small f stop by increasing the shutter speed way beyond the sync speed.  Recall that flash is controlled by f stop and subject distance.  A narrow aperture and/or large distance can kill the fill flash.  The SB600 is a relatively weak speed light, and that means you will be close to the subject and you will be using a small f stop (wide aperture).  My lenses are slow (f/3.5 at widest zoom), so I tend to get a wide angle perspective using this technique.  A fast lens and a powerful speed light are much more versatile.

At speeds faster than sync, the curtain shutter in a DSLR is never fully open.  Instead, it travels as a narrow slit of exposure down (or up) the sensor chip.  In this situation, a single flash pop would yield a narrow bar of exposure somewhere in the frame.  The FP mode works by firing very fast multiple bursts of flash for the duration of the shutter curtain's travel.  These multi-bursts mimic the normal illumination of the sun in a fast shutter setting.

Another way to think about exposure in FP mode is this: the bright sunlit areas of the middle and background are controlled by the combination of shutter and aperture settings, and the dark areas of the foreground are controlled by the flash.  The background doesn’t care about the flash because flash only works over a short range: distant areas will never feel the flash.  The flash can illuminate the nearby dark areas if the aperture is wide enough.  The dark areas don’t care about the shutter speed because FP mode will fire often enough to light them up. 

Confused?  Get your gear hooked up and practice.  Most speed lights allow you to adjust the power output manually (1/2, 1/4.....1/64).  Eight pictures with exposure setting notes are displayed below.

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