> Well, at full power, a flash strobe is usually quite slow. Somewhere
> around 1/150s. Flash duration rapidly decreases with power (around
> 1/500s at 1/2 power, shorter than 1/10.000s for very slight flashes).
> This effectively very slightly decreases the available guide number
> when used with a high-speed sync cameras (1/250s or 1/300s), which is
> not a big deal.
>
> Consequently, a standard subject should be frozen by the flash if not
> too rapid, but a common speedlight cannot be used at full power to
> freeze fast action like water drops...
Very interesting. After I read your post, I took back the SB-28 user manual
and gathered the following informations:
At 50mm, 100 asa:
Speed Guide number
1/60s 42
1/250s 20
1/300s 16
1/500s 14
1/1000s 10
1/2000s 7
1/4000s 5
Flash Intensity Guide number
1 42
1/2 30
1/4 21
1/8 15
1/16 10.5
1/32 7.5
1/64 5.3
>From that I'm tempted to conclude the following:
Flash Intensity Flash duration
1 1/60s
1/2 1/125s
1/4 1/250s
1/8 1/500s
1/16 1/1000s
1/32 1/2000s
1/64 1/4000s
Anything wrong?
Is there a way to force a SB28 not to fire at full power when mounted on a
F100 without using it in manual mode?
Let's say that I want to be sure to freeze a basketball player in its quick
movement. Let's suppose that 1/500s is needed.
My guess is that since the flash intensity is automaticaly adjusted
regarding the required exposure, what I have to do is to take be close
enough to the main subject so that the flash will only have to fire at 1/8
of its full power.
Since guide number is proportional to range I have to be maximum 15/42 of
the full range far away from the main subject.
Of course this is an approximation since the main subject and background
brightnesses have an influence on the power the flash is fired but I cannot
figure out how to do else.
********
Yet another question in the same vein about the F100: How is it possible to
adjust flash intensity without adjusting background's one? I read that
bracketing could do so but is it possible to do so without using it?
Custom function #11 allows to adjust only flash intensity in flash
bracketing. Does anybody know if custom function #11 has an influence over
the exposition correction when using it with flash on (The user's guide is
completely mute about exposition correction when shooting with flash on!)
Thierry
