Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
05-14-2010, 01:00 PM   #16
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
Site Supporter
enoeske's Avatar

Join Date: Jul 2009
Location: Surprise, Az
Photos: Gallery
Posts: 1,920
QuoteOriginally posted by Deni Quote
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.

QuoteOriginally posted by Deni Quote
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
Loyal Site Supporter
charliezap's Avatar

Join Date: Feb 2009
Location: saugus ma
Photos: Gallery | Albums
Posts: 8,239
Original Poster
QuoteOriginally posted by Deni Quote
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
Pentaxian
Class A's Avatar

Join Date: Aug 2008
Location: Wellington, New Zealand
Posts: 9,177
QuoteOriginally posted by blende8 Quote
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.

05-14-2010, 06:35 PM   #20
Veteran Member
mattdm's Avatar

Join Date: Feb 2007
Location: Boston, MA
Photos: Gallery
Posts: 2,964
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
05-15-2010, 01:32 AM   #21
Veteran Member




Join Date: May 2007
Location: Tirana, Albania, South Europe, Planet Earth
Photos: Gallery | Albums
Posts: 621
QuoteQuote:
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 .

QuoteQuote:
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
Veteran Member
alohadave's Avatar

Join Date: Jan 2008
Location: Quincy, MA
Photos: Gallery | Albums
Posts: 2,024
QuoteOriginally posted by mattdm Quote
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
Veteran Member
mattdm's Avatar

Join Date: Feb 2007
Location: Boston, MA
Photos: Gallery
Posts: 2,964
QuoteOriginally posted by alohadave Quote
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
Site Supporter
ChipB's Avatar

Join Date: Nov 2007
Location: Austin, TX area
Photos: Gallery | Albums
Posts: 1,637
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
Loyal Site Supporter
charliezap's Avatar

Join Date: Feb 2009
Location: saugus ma
Photos: Gallery | Albums
Posts: 8,239
Original Poster
QuoteOriginally posted by ChipB Quote
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
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
camera, dslr, duration, flash, k20d, photography, shot
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
high iso in-camera processing duration k7 vs k20d navicore Pentax DSLR Discussion 4 12-09-2009 12:03 PM
Fastest flash duration + flash power reknelb Flashes, Lighting, and Studio 7 10-14-2009 08:05 PM
* Best Flash for K20D? * PentaxForums-User Flashes, Lighting, and Studio 6 07-14-2009 04:15 PM
flash duration Lowell Goudge Flashes, Lighting, and Studio 2 07-03-2009 03:29 PM
Help with old flash on K20D winglik Pentax DSLR Discussion 5 02-28-2009 06:02 AM



All times are GMT -7. The time now is 09:26 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top