D300 built-in flash, Part 1: Blinding speed

Started Dec 25, 2008 | Discussions thread
David314 Veteran Member • Posts: 4,790
ok, lets see your 1/2 power trace

Kluso wrote:

notice the OP's measurements agree with mine for full duration

No, they do not, the OP's traces are as expected, yours are clipped.

the OP's traces clearly show that for full power there is a decaying exponential function that exceeds 1/1000

here is the OP's view of a clipped trace and not

notice - in spite of the clipping the length of the pulse is the same

btw this is a 1/2 power pusle -

Here are my traces, 2 versions clipped and not clipped

and here is your full power curve

you attribute the slow decay to lack of active circuitry to pull down the signal

that maybe true - to verify that we would have to see a 1/2 power or less pulse

but note the OP's circuit appears to be an active circuit - you can see this from the 1/2 power pulse

so just forget my scope traces - notice in the OP's traces the difference between 1/2 power and full power - if the full power is affected by the capacitance of the circuit and is wrongly showing a decaying exponential, how is it that the 1/2 power and less pulses decay so fast?

i suggest you go back into the lab and capture 1/2 power and less pulses along with a full power pulse

but please read this first from the OP

"Triggering and Power Control

Flashes work by discharging a storage capacitor into the flash tube. When full power is used, the discharge is allowed to run its full course. To obtain lower powers, the pulse is shut off mid-way. Thus whatever "power level" is used, the pulse profile follows the same curve, with lower-power pulses simply being shorter.

Once the storage capacitor is charged and ready, the process begins with the application of a high-voltage trigger pulse to a trigger electrode on the flash tube. After this, it takes about 10-12usec for the flash tube to start producing light. The initial build-up is exponential until about 15usec after trigger (light level is still very low here) and the light output then ramps up roughly linearly until about 30us, where the light level is at half peak brightness. After that, it rounds off slowly to its peak value at 120-140us, then begins an exponential decay as the storage capacitor loses voltage. "

or this from the OP in regards to full power flash at 1/250 shutter speedr

"Note that although the flash has been set for full power, it is not actually producing full power, as it has been limited in duration by the camera. A full power discharge continues to decay exponentially for many milliseconds and exhibits no cutoff edge."

so now you better work hard on discrediting Marianne Oelund's results

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