K20d flash duration

Deni · 05-14-2010, 01:00 PM

The Vivitar 285 HV with the yellow setting might have a 1/30,000 sec shutter but that doesn't mean that 1/30,000 sec is needed to stop the action of that subject.

The onboard flash, being less powerful, yet giving similar results in stopping movements, proves that a shutter speed much slower than 1/30,000 sec is needed to stop the action of the bird.

enoeske · 05-14-2010, 01:43 PM

Originally posted by Deni

The Vivitar 285 HV with the yellow setting might have a 1/30,000 sec shutter but that doesn't mean that 1/30,000 sec is needed to stop the action of that subject.

huh? Flashes don't have shutters.

Originally posted by Deni

The onboard flash, being less powerful, yet giving similar results in stopping movements, proves that a shutter speed much slower than 1/30,000 sec is needed to stop the action of the bird.

They give the same results because the flashes are firing at roughly 1/30,000 or whatever we figured out. The shutter speed is likely 1/180 to allow a flash to fire and kill maximum ambient light.

charliezap · 05-14-2010, 02:00 PM

Originally posted by Deni

The Vivitar 285 HV with the yellow setting might have a 1/30,000 sec shutter but that doesn't mean that 1/30,000 sec is needed to stop the action of that subject.

The onboard flash, being less powerful, yet giving similar results in stopping movements, proves that a shutter speed much slower than 1/30,000 sec is needed to stop the action of the bird.

A little experiment is in order--I'm going to take several shots of a fan spinning at 1725 rpm with stickers on the blades and compare the onboard K20d flash with the Vivitar 285HV at different power levels.If all goes well I will have them posted in a few hours--charliezap

Class A · 05-14-2010, 06:04 PM

Originally posted by blende8

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.

Yes, it fits, but so does any other flash duration.

The x-sync shutter speed only has relevance in that any quicker shutter speed means that the flash duration must be longer than the shutter speed as it needs to not only to illuminate for the time period any part of the sensor is exposed but for the whole time the slit travels over the sensor.

So the only the only thing we know from the x-sync shutter speed is that the flash duration isn't longer than 1/180, otherwise it could be used for shorter shutter speeds.

mattdm · 05-14-2010, 06:35 PM

I researched this a bit for my flash guide. A flash pulse is a curve, not binary on-off, so exactly when to start and stop measuring is difficult. There's two standard ways -- "T.5" and "T.1". This is the time for which the flash brightness is above 50% of the maximum and 10% of the maximum respectively. T.1 is more useful in terms of measuring actual effect the exposure. T.5 is easier to measure, and sounds more impressive, so unless someone says otherwise, that's what they're quoting.

Metz gets huge points in my mind for giving a full table of T.1 values in their manual.

More detail here:

Paul C. Buff FORUM Archive • View topic - Flash Duration Explained

Deni · 05-15-2010, 01:32 AM

Quote:

huh? Flashes don't have shutters.

I said it for the sake of the discussion since in limited light, the "flash duration" mimics the shutter speed

.

Quote:

A little experiment is in order--I'm going to take several shots of a fan spinning at 1725 rpm with stickers on the blades and compare the onboard K20d flash with the Vivitar 285HV at different power levels.If all goes well I will have them posted in a few hours--charliezap

The flash duration is always going to change based on available light, aperture, ISO, pre flash (with non A lenses the flash is more powerful than with A lenses), whether red eye reduction is on/off etc.

One experiment you could do with the spinning fan is taking a series of images without flash, and checking at which shutter speed the fan looks static.

Then duplicate it with the onboard flash in a dark area to get a ballpark idea of the onboard flash duration.

alohadave · 05-16-2010, 01:22 PM

Originally posted by mattdm

I researched this a bit for my flash guide. A flash pulse is a curve, not binary on-off, so exactly when to start and stop measuring is difficult. There's two standard ways -- "T.5" and "T.1". This is the time for which the flash brightness is above 50% of the maximum and 10% of the maximum respectively. T.1 is more useful in terms of measuring actual effect the exposure. T.5 is easier to measure, and sounds more impressive, so unless someone says otherwise, that's what they're quoting.

Metz gets huge points in my mind for giving a full table of T.1 values in their manual.

More detail here:

Paul C. Buff FORUM Archive • View topic - Flash Duration Explained

That is true for studio strobes. Hot shoes flashes work differently and the flashes durations are opposite of studio strobes.

In a studio strobe, the highest power has the lower duration, and lower power has a longer duration.

In a hot shoe flash, the flash duration is controlled by a power cutoff. To make the flash duration shorter, the circuitry cuts power to the flash bulb at the designated time.

mattdm · 05-16-2010, 04:25 PM

Originally posted by alohadave

That is true for studio strobes. Hot shoes flashes work differently and the flashes durations are opposite of studio strobes.

In a studio strobe, the highest power has the lower duration, and lower power has a longer duration.

In a hot shoe flash, the flash duration is controlled by a power cutoff. To make the flash duration shorter, the circuitry cuts power to the flash bulb at the designated time.

Sorry; poor choice of links in my previous post, which may have confused things. The important point is that even with the cutoff, it's still a curve, not a binary on-off. Here's a chart of actual hotshoe flash output (in this case, from canon): Discharge Curves of Electronic Flash at Different Power Settings

Where actually, I see that the cutoff for shorter duration really is almost instantaneous. Nonetheless, the point about T.1 vs. T.5 (particularly for full-power flash) still stands.

**ChipB** · 05-17-2010, 06:37 PM

I'm kinda curious as to why it matters what the duration of the on-board flash is????

AFAIK there's no way to control it - the camera computer reads the pre-flash data and then controls the amount of light it emits (unless you're using a manual lens then it's 100%) - hopefully you end up with a properly exposed photo.

charliezap · 05-17-2010, 08:17 PM

Originally posted by ChipB

I'm kinda curious as to why it matters what the duration of the on-board flash is????

AFAIK there's no way to control it - the camera computer reads the pre-flash data and then controls the amount of light it emits (unless you're using a manual lens then it's 100%) - hopefully you end up with a properly exposed photo.

I started this thread to find out if the onboard flash is "fast' enough to capture a hummingbird's wings in motion.I know now that it is really pretty quick up close as my test results (half arsed) can attest to--------https://www.pentaxforums.com/forums/pentax-dslr-discussion/101578-test-result...durations.html
--charliezap

05-14-2010, 06:35 PM	#20
mattdm Veteran Member Join Date: Feb 2007 Location: Boston, MA Photos: Gallery Posts: 2,948	Measuring flash time I researched this a bit for my flash guide. A flash pulse is a curve, not binary on-off, so exactly when to start and stop measuring is difficult. There's two standard ways -- "T.5" and "T.1". This is the time for which the flash brightness is above 50% of the maximum and 10% of the maximum respectively. T.1 is more useful in terms of measuring actual effect the exposure. T.5 is easier to measure, and sounds more impressive, so unless someone says otherwise, that's what they're quoting. Metz gets huge points in my mind for giving a full table of T.1 values in their manual. More detail here: Paul C. Buff FORUM Archive • View topic - Flash Duration Explained

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05-14-2010, 01:00 PM	#16
Deni Veteran Member Join Date: May 2007 Location: Tirana, Albania, South Europe, Planet Earth Photos: Gallery \| Albums Posts: 621	The Vivitar 285 HV with the yellow setting might have a 1/30,000 sec shutter but that doesn't mean that 1/30,000 sec is needed to stop the action of that subject. The onboard flash, being less powerful, yet giving similar results in stopping movements, proves that a shutter speed much slower than 1/30,000 sec is needed to stop the action of the bird.

05-14-2010, 01:43 PM	#17
enoeske Veteran Member Join Date: Jul 2009 Location: Surprise, Az Photos: Gallery Posts: 2,136	Originally posted by Deni The Vivitar 285 HV with the yellow setting might have a 1/30,000 sec shutter but that doesn't mean that 1/30,000 sec is needed to stop the action of that subject. huh? Flashes don't have shutters. Originally posted by Deni The onboard flash, being less powerful, yet giving similar results in stopping movements, proves that a shutter speed much slower than 1/30,000 sec is needed to stop the action of the bird. They give the same results because the flashes are firing at roughly 1/30,000 or whatever we figured out. The shutter speed is likely 1/180 to allow a flash to fire and kill maximum ambient light.

05-14-2010, 02:00 PM	#18
charliezap Pentaxian Join Date: Feb 2009 Location: saugus ma Photos: Gallery \| Albums Posts: 8,686 Original Poster	Originally posted by Deni The Vivitar 285 HV with the yellow setting might have a 1/30,000 sec shutter but that doesn't mean that 1/30,000 sec is needed to stop the action of that subject. The onboard flash, being less powerful, yet giving similar results in stopping movements, proves that a shutter speed much slower than 1/30,000 sec is needed to stop the action of the bird. A little experiment is in order--I'm going to take several shots of a fan spinning at 1725 rpm with stickers on the blades and compare the onboard K20d flash with the Vivitar 285HV at different power levels.If all goes well I will have them posted in a few hours--charliezap

05-14-2010, 06:04 PM	#19
Class A Pentaxian Join Date: Aug 2008 Location: Wellington, New Zealand Posts: 11,251	Originally posted by blende8 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. Yes, it fits, but so does any other flash duration. The x-sync shutter speed only has relevance in that any quicker shutter speed means that the flash duration must be longer than the shutter speed as it needs to not only to illuminate for the time period any part of the sensor is exposed but for the whole time the slit travels over the sensor. So the only the only thing we know from the x-sync shutter speed is that the flash duration isn't longer than 1/180, otherwise it could be used for shorter shutter speeds.

05-15-2010, 01:32 AM	#21
Deni Veteran Member Join Date: May 2007 Location: Tirana, Albania, South Europe, Planet Earth Photos: Gallery \| Albums Posts: 621	Quote: huh? Flashes don't have shutters. I said it for the sake of the discussion since in limited light, the "flash duration" mimics the shutter speed . Quote: A little experiment is in order--I'm going to take several shots of a fan spinning at 1725 rpm with stickers on the blades and compare the onboard K20d flash with the Vivitar 285HV at different power levels.If all goes well I will have them posted in a few hours--charliezap The flash duration is always going to change based on available light, aperture, ISO, pre flash (with non A lenses the flash is more powerful than with A lenses), whether red eye reduction is on/off etc. One experiment you could do with the spinning fan is taking a series of images without flash, and checking at which shutter speed the fan looks static. Then duplicate it with the onboard flash in a dark area to get a ballpark idea of the onboard flash duration.

05-16-2010, 01:22 PM	#22
alohadave Veteran Member Join Date: Jan 2008 Location: Quincy, MA Photos: Gallery \| Albums Posts: 2,024	Originally posted by mattdm I researched this a bit for my flash guide. A flash pulse is a curve, not binary on-off, so exactly when to start and stop measuring is difficult. There's two standard ways -- "T.5" and "T.1". This is the time for which the flash brightness is above 50% of the maximum and 10% of the maximum respectively. T.1 is more useful in terms of measuring actual effect the exposure. T.5 is easier to measure, and sounds more impressive, so unless someone says otherwise, that's what they're quoting. Metz gets huge points in my mind for giving a full table of T.1 values in their manual. More detail here: Paul C. Buff FORUM Archive • View topic - Flash Duration Explained That is true for studio strobes. Hot shoes flashes work differently and the flashes durations are opposite of studio strobes. In a studio strobe, the highest power has the lower duration, and lower power has a longer duration. In a hot shoe flash, the flash duration is controlled by a power cutoff. To make the flash duration shorter, the circuitry cuts power to the flash bulb at the designated time.

05-16-2010, 04:25 PM	#23
mattdm Veteran Member Join Date: Feb 2007 Location: Boston, MA Photos: Gallery Posts: 2,948	Originally posted by alohadave That is true for studio strobes. Hot shoes flashes work differently and the flashes durations are opposite of studio strobes. In a studio strobe, the highest power has the lower duration, and lower power has a longer duration. In a hot shoe flash, the flash duration is controlled by a power cutoff. To make the flash duration shorter, the circuitry cuts power to the flash bulb at the designated time. Sorry; poor choice of links in my previous post, which may have confused things. The important point is that even with the cutoff, it's still a curve, not a binary on-off. Here's a chart of actual hotshoe flash output (in this case, from canon): Discharge Curves of Electronic Flash at Different Power Settings Where actually, I see that the cutoff for shorter duration really is almost instantaneous. Nonetheless, the point about T.1 vs. T.5 (particularly for full-power flash) still stands.

05-17-2010, 06:37 PM	#24
ChipB Site Supporter Join Date: Nov 2007 Location: Eau Claire, WI Photos: Gallery \| Albums Posts: 2,701	I'm kinda curious as to why it matters what the duration of the on-board flash is???? AFAIK there's no way to control it - the camera computer reads the pre-flash data and then controls the amount of light it emits (unless you're using a manual lens then it's 100%) - hopefully you end up with a properly exposed photo.

05-17-2010, 08:17 PM	#25
charliezap Pentaxian Join Date: Feb 2009 Location: saugus ma Photos: Gallery \| Albums Posts: 8,686 Original Poster	Originally posted by ChipB I'm kinda curious as to why it matters what the duration of the on-board flash is???? AFAIK there's no way to control it - the camera computer reads the pre-flash data and then controls the amount of light it emits (unless you're using a manual lens then it's 100%) - hopefully you end up with a properly exposed photo. I started this thread to find out if the onboard flash is "fast' enough to capture a hummingbird's wings in motion.I know now that it is really pretty quick up close as my test results (half arsed) can attest to--------https://www.pentaxforums.com/forums/pentax-dslr-discussion/101578-test-result...durations.html --charliezap