Why not hss?

Started Jan 2, 2013 | Discussions thread
tom
tom
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Re: Why not hss?
In reply to howmanyds, Jan 2, 2013

howmanyds wrote:

oh, that makes sense then.

A few things to remember:

1. The guide number is used to determine the camera settings (aperture) assuming they are providing all the exposure.  The quoted value is the guide number at ISO 100.  The formula for aperture is:

f = GN/subject distance

distance is in the same units as the GN (in this case meters), and flash is pointed directly at the subject.

2.  Guide number are determined for indoor shooting.  The actual guide number out of doors is somewhat lower (no contribution from light reflected off walls.

3.  For some flashes (those that zoom to cover different focal length lenses) the GN will change depending on the focal length.  For example, my 5600 only has a guide number of 56 meters for focal lengths of 85mm or greater.  With a 24mm lens, the normal (not HSS) guide number drops to 30m (18m with the 17mm diffuser in place).  Note that a-mount flashes automatically zoom to the widest setting when used in wireless off-camera mode.

Under HSS (1/250 shutter speed) the GN at 24mm is 12m.  At 1/8000 it is 2.1m)

I am not sure whether the flash zoom is changed to compensate for the reduced field of view with an APS-c size sensor (sets a higher FLl position on the flash (more power) than the FL of the lens. )

4. When using HSS for fill in, the flash will most likely not be the primary source of light so the low guide numbers may not be too important - again, experiment.

Hope this helps

tom

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