11
power point is commonly used in engineering and serves the purpose of
reporting the bandwidth of the flash duration, but the graph below clearly shows
that a power point measurement does not truly report the total flash duration.
The clock doesn’t start running until the flash discharge reaches 50% of its peak,
and stops when it falls below 50%. Since the color temperature of the flash
varies over time (more blue in the beginning and more red towards the tail [pgs
48-60]), it is important that flash duration be reported at the 1/10 power levels to
express the total flash duration and not cut off any of the available flash power.
Flash Duration
0
0.5
1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time
Intensity
t=1/2 Power Duration
Total Flash Duration (t=1/10)
You can see that there is quite a bit of light which would be wasted, if we set our
shutter speed to the duration using the t = 1/2 technique. In effect, the flash
duration is longer than you think it is — up to three times longer. So t= 1/2 or t
= 1/3 are not really useful measurements, since we’re used to thinking in terms
of shutter speeds. We at Photogenic want you to get consistent and constant
color temperature and all the flash power you paid for. That’s why we report all
Solair flash durations using the t=1/10 technique. As you can see, setting your
shutter speed to the t=0.1 flash duration or greater will result in your camera
capturing virtually all of the light available.
