[charliezap]

Just a quick question--Does anyone know for sure what the flash duration is for the k20d pop up flash?This shot was taken with the Vivitar 285 HV with the yellow setting 1/30,000 sec. A similiar shot was taken with the onboard k20d flash and the stopping action was pretty darn close.---charliezap

[blende8]

There is not the flash duration for a flash.
The duration depends on the power it is emitting.
E.g. the AF540 needs 1/200 s to give its full power, but has the shortest possible time as 1/20.000 s.

[enoeske]

There is not the flash duration for a flash.
The duration depends on the power it is emitting.
E.g. the AF540 needs 1/200 s to give its full power, but has the shortest possible time as 1/20.000 s.

The power controls the length of the flash. The longer the flash, the brighter it appears. Low power (1/64 - 1/32 power and such) are very very short durations of the flash while high power (1/2 - 1/1 power) are longer. But the flash itself is not different.

So you can measure the flash duration if you can get a consistent power setting. This likely means using a manual lens to avoid ttl. Then you will have full power flashes which will mean a long duration. I don't know of a way to set the onboard flash to low consistent power.

[blende8]

So you can measure the flash duration if you can get a consistent power setting.

Of course, yes.
IIRC the built-in flash gives full power with M-lenses always.

[charliezap]

My question still is "how fast is the flash" in parts of a second. 1/???

[Lowell Goudge]

There is not the flash duration for a flash.
The duration depends on the power it is emitting.
E.g. the AF540 needs 1/200 s to give its full power, but has the shortest possible time as 1/20.000 s.

no, in fact it needs a lot less than 1/200th

the 1/180 flash sync (5.4 mS) is the whole exposure time. this time is split into 3 segments. Leading curtain opening time, full open time, and trailing curtain closing time.

generally the two curtains move at the same speed, and each could move as slow as 1/180 of a second with the trailing curtain beginning to close the instant that the leading curtain gets fully open. therefore no part of the frame is exposed for more than 1/180 of a second.,

in reality the curtain opening and closing times will be shorter than 1/180, because they do need to leave time for the flash to fire in between.

I would suspect that they actually leave about 1-2 mS of full frame open time, and full power of the flash will be tuned to that time.

I tried to get this data once from pentax but they wouldn't give it.

The best way woud be to measure a very fast spinning object of known speed, and triggering the flash at each power, 1/64th etc and measure the angular rotation of the object to calibrate the flash. Full power is a little dangerous because duration may also depend on the state of charge of the capacitor and therefore be a little inconsistent.

some flashes have minimum power settings in the 1/20,000 to 1/50,000 range, but note that flash duration doubles for each doubling of power

[charliezap]

As a guess I would say that the K20's flash duration is around 1/10,000 sec.I will take a shot later of the h'bird with the onboard flash and try to compare it to the above photo taken at 1/30,000sec--charliezap

[blende8]

I took the 1/200 s from the manual of the AF540.

[Lowell Goudge]

I took the 1/200 s from the manual of the AF540.

perhaps, but from the example I gave, you can see it must be faster than that.

[alohadave]

The best way woud be to measure a very fast spinning object of known speed, and triggering the flash at each power, 1/64th etc and measure the angular rotation of the object to calibrate the flash. Full power is a little dangerous because duration may also depend on the state of charge of the capacitor and therefore be a little inconsistent.

The very best way would be to use a thyristor or phototransister as a sensor in front of the flash connected to an oscilloscope. Alternatively, you could connect the o-scope lead directly to the contact of the flash tube.

[Lowell Goudge]

The very best way would be to use a thyristor or phototransister as a sensor in front of the flash connected to an oscilloscope. Alternatively, you could connect the o-scope lead directly to the contact of the flash tube.

when I get time and a storage scope

[Marc Sabatella]

My question still is "how fast is the flash" in parts of a second. 1/???

Yes, but the answer they gave you is correct: it depends. On a shot in which the flash happened to fire at full power, it's 1/200" or whatever; but if the ISO, aperture, and subject distance are such that the flash does *not* need to fire at full power (read: most of the time) then the flash fires for less time.

[sterretje]

My question still is "how fast is the flash" in parts of a second. 1/???

From the user manual, maximum flash duration is 1/1000 sec. This is when the flash has to release its full power. If you set it to 1/16 power, the flash duration therefore will be 1/16000 sec (see partial quote from Lowell below). In automode, the flash duration can go down to 1/30000 sec (spec in user manual); there is however no way to determine what the flash duration was at the time that the photo was taken.

You might be able to do an estimation (probably more of a guess instead of an estimation) based on the physics of the bird by calculating the speed of the wings of the bird. But I leave that to to you.

some flashes have minimum power settings in the 1/20,000 to 1/50,000 range, but note that flash duration doubles for each doubling of power

[blende8]

perhaps, but from the example I gave, you can see it must be faster than that.

Flash synchronization speed is the shortest time at which the curtains are fully open. This is 1/180 s for Pentax cameras.
1/200 s shortest flash duration (w. max. power) fits good here.

[jjdgti]

Thanks for the info. I understand a little bit more the functioning of the flash.