Andreas Pirner <AndreasPirner@xxxxxx> wrote: 1/40,000 sec. is pretty much the
shortest flash duration that occurs when the flash is very close, lens wide
open and not diffusion of any kind.
I guess it is determined by regulating/measuring electronics, the behaviour of
the flash tube itself (it 'burns' in a curve too, pretty steep flanks,
but still a curve, kinda tailing off). The longest flash duration is full power
of a
single head high power studio flash. That may comes close to 1/200 sec. (as
already stated
in another post).
Here is a little bit of Engineering stuff on how these work/are designed:
There are two common types of flash circuits used in consumer flashes. The
simplest just discharges the flash capacitor directly into the flash tube. This
is only used nowdays on disposable cameras or low power fixed flashes. This is
termed "resistive circuit". Flash duration is quite short well under 1mS and
depends quite a bit on circuit component parasitic values. This is tough on
the discharge tube and efficiency is often low.
Almost all other flashes and in particular high power, auto and ttl flashes
use an air core inductor (coil) in series with the tube. Current passes through
the coil into the tube through an electronic switch. The coil greatly slows the
rate of rise of current (light) and reduces peak current and greatly extends
the duration of the flash. This extends tube life as peak currents are lower
and sputtering of the electrodes is reduced. It is also essential for variable
power/Auto/ttl as the light is integrated by a photo-transistor/capacitor
circuit (photodiode/opamp for OM2) and there needs to be time for the swich off
of current to occur, when the electronic (thyristor or igbt) switch operates.
At short autoflash durations the switch, switch off time, is usually a limiting
factor for minimum flash duration and flash output.
The Inductor is chosen so the current through the tube does not "ring" much
and reverse direction, as this is very damaging to the electrodes. To choose
the inductor an approximation is made as to "resistance" of the tube while
discharging. It actually follows a non-linear law but can be approximated at
average design current by a fixed resistance which is then used to choose an
inductor to make the LRC circuit a little "overdamped". This all tends to mean
that higher power flashes have longer duration for a full dump, but it depends
on the tube resistance, which depends on tube gas fill pressure,diameter length
etc. That is why the high power Metz CT60 flash say, has a longer maximum
duration than say a T20.
Variable power flashes just have a variable timer to cut off the current at
different times after flash start. This results in a rather weird time-light
output characteristic, because the discharge curve is very assymetrical with a
short upswing and long tail. The peak rate at which energy is released is
very impressive, something of the order of 10-100kW for a largish flash.
Tim Hughes
==============================================
List usage info: http://www.zuikoholic.com
List nannies: olympusadmin@xxxxxxxxxx
==============================================
|