There seems to be some confusion regarding flash
duration. It's generally measured 2 ways. First is at
t.05, or when a flash goes from 0.5 max brightness to
max brightness, then back down to 0.5 max brightness (
all at a given watt sec rating ). This is the method
used by most flash manufacturers, but isn't the best
method, as exposure on film is still taking place
outside these parameters. It also tends to overstate the
ability of a particular flash to freeze action. The
second, more accurate method is a reading at t0.1, or
when the flash goes from 0.1 max brightness to max
brightness, and backdown to 0.1 max brightness. This
measuring method more accurately simulates the freezing
effects of an equivalent shutter speed, as virtually no
exposure will affect the film outside these parameters.
Most studio strobes are relatively slow as measured via
t.0.1 .
A typical speedotron flash at 2400ws has a flash
duration at full power of approx 1/100th of a sec.
Dynalights at the same power setting have an even longer
flash duration ( due to the fact that they operate at
450v, instead of speedo's 900v ).
I'll check my SB-26 with my Broncolor flash duration
meter tomorrow to find out what it's full power duration
is at t.0.1. BTW, if a particular flash unit measures
1/600th sec at t.0.5, the flash duration at t.0.1 would
be around 1/200th sec. You can check a particular flash
unit simply by changing the shutter speed on a high
quality flash meter and seeing if the flash light
reading changes with it as you re-trigger the strobe. If
it does, you're clipping into the flash duration at
higher shutter speeds. Pete
