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01-26-2011, 10:52 AM   #1
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Please describe Flash Sync @ 1/160th and Freezing Motion

I understand the process of the shutter curtains and the burst of light across the sensor and what happens when you set your shutter faster then the sync speed.
I also understand the relationship to aperture and iso used to control the ambient light in a strobist environment.

I don't comprehend how we freeze motion at less then 1/160th WITH the flash, when WITHOUT the flash we need to be greater then 1/250th.

01-26-2011, 11:30 AM   #2
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The flash duration is many times shorter than the shutter so that becomes the pseudo shutter speed - if you have too much ambient your flash will freeze the motion but there may be some blurring because of the ambient light / shutter speed combination, using trailing curtain in this scenario ensures any bluring appears behind the frozen subject.
Hope this helps
Darren
01-26-2011, 11:31 AM   #3
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The flash burst is really short. Exactly how short, it depends on the flash model and the power level used. There's some information on Canon flashes I founds in a google search which might be accurate. But as you probably suspect, Pentax info is a little thin.
01-26-2011, 11:33 AM   #4
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Not sure on the exact duration of the flash's output, but essentially the shutter is providing ambient light and the flash is lighting the subject for a duration much shorter than the shutter speed.

Here's a link that came up when I googled it.

Rear-Curtain Sync - Pentax P-TTL Flash Comparison

edit: Wow, googled a link and we have three responses already.

01-26-2011, 11:34 AM   #5
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Thanks. I understand that the burst is approx 1/1000th of a second and that with those numbers that 1/160th is sufficient.

But if my shutter is open for 1/160th, how am I freezing anything? Should I assume that the foreground (what my flash is focused on) will freeze and that my background (collecting the ambient) may blur?
01-26-2011, 11:43 AM   #6
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I'm very close to getting this, just need someone to explain it the right way using an example and numbers...if possible.

I'll suggest a scenario in which a biker (my avatar) is riding in the middle of the road with 2 speedlights in the middle of the road. The cyclist must ride between the 2 speed lights and when he does the strobes will fire. The shutter is set to 1/160th and the aperture is opened up to collect the ambient around the shot.
Are you suggesting that the rider will be frozen with no blur, riding at 30 mph using a shutter of 1/160th?

The only reason I ask is because at 30mph without the flash, the rider would be a complete blur. This is where I'm lost.

If I had to guess, I'd say that when the flash is in use the exposure is only captured for a fraction of the time while the rest of the 1/160th of a second is just the motion of the curtains moving.
But when the flash is not fired, the exposure is taking place over the duration of 1/160th.

But my gut tells me I'm way off
01-26-2011, 11:48 AM   #7
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QuoteQuote:
But if my shutter is open for 1/160th, how am I freezing anything?
You assume that ambient light plays a role. Let's leave ambient light out of the equation.

Take a moving subject in a pitch black environment. At the moment that the flash fires, you will have a burst of light for a short duration (e.g. 1/1000s); after that it's pitch black again and the camera will not record anything. And there is your frozen subject.

PS
If ambient light plays a role, I'm not sure if you can really freeze the motion with the 1/160s but I doubt it; that is why there are calls for faster X-syncs. My thought is that closing the aperture will help to make it less obvious as less ambient light reaches the sensor during that time but that will also influence the 'reach' of the flash

Last edited by sterretje; 01-26-2011 at 12:00 PM. Reason: added ps
01-26-2011, 11:50 AM   #8
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QuoteOriginally posted by grainbelt Quote
Not sure on the exact duration of the flash's output
Funny, I googled that one and found it.

01-26-2011, 12:02 PM   #9
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QuoteOriginally posted by sterretje Quote
You assume that ambient light plays a role. Let's leave ambient light out of the equation.

Take a moving subject in a pitch black environment. At the moment that the flash fires, you will have a burst of light for a short duration (e.g. 1/1000s); after that it's pitch black again and the camera will not record anything. And there is your frozen subject.
I'm sorry. This is the part I really have a tough time with. The way this is worded, to me it sounds like I would need 1/1000 to capture.

I'm going to set this scenario up myself and see what happens. I guess that's the best way to learn. The proposed set up wouldn't matter if the shot was at high noon or at midnight?
01-26-2011, 12:13 PM   #10
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The flash duration becomes the shutter speed.
While the shutter may be open for 1/160th, the flash duration is much shorter.
Note that in some scenarios, if the ambient light is within a couple of stops of the flash light, you will see some ghosting from the ambient light. This would potentially correspond to your high noon scenario.
Generally, if we are using flash to freeze action, we want the ambient light low enough that it isn't a significant part of the exposure.
I was able to find (quite easily) published numbers for Nikon's flash duration at full power (~1/800th second). As the output of the flash drops, the flash duration gets shorter, so that at minimum output, the flash may be firing in the 1/20000 second range.
I've had a hard time finding published flash duration times for Pentax flash units, so I can't say what their maximum burn time is.
If you've found that info, I would appreciate being pointed in the right direction.
One of the ongoing debates is why Pentax is staying with the relatively non state of the art 1/180 shutter speed for maximum flash sync.
I know my Metz flash at maximum output is ~1/200 second, meaning that I couldn't use 1/250 second if I was needing close to the maximum power as the flash output would be clipped by the shutter closing.
01-26-2011, 12:13 PM   #11
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if you consider the composite image of a shot at 1/160 of the biker passing your camera with the lens wide open, and 1/160th of a second, this shot will be blurry.

If you consider a flash where the flash duration is in the order of 1/1000 of a second (i.e. the time the shutter is actually wide open), this image will be sharp and frozen in time. due to the short duration of the flash burst.

What you will actually see, assuming the shutter starts moving at the bottom of the framewould be the following as the sequence of the frame evolves.

a blurred imageleaning in the direction of motion across the frame, as the leading curtain rises, in somewhere like 1/160th of a second, then a bright sharply defined outline of the bike, when the foash fires in 1/1000 of a second, followed by additional motion blurr moving to the right, from the bottom as the trailing curtain moves following the end of the flash and wide open period of the frame (about another 1/160 of a second)

the reason that the shutter speed is 1/160th of a second is that no portion of the frame is exposed for more than that period. As the shutter speed increases, the speed fo the leading curtain does not change, the only change is the delay between (or gap distance between) the departure of the leading curtain and the trailing curtain. Above sync speed what actually happens is that there is a moving slit across the image, as opposed to the shutter being open for the duration.

In the bad old days, people with speed graphics, and other larg format cameras with focal plane shutters had images of cars taken at speed where the entire car was tilted or leaning forward. the tilt was a function of subject speed and the speed of the movement of the slit of the shutter across the film at speeds above sync.
01-26-2011, 12:27 PM   #12
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I did edit my post while you were replying to make clear to which post I was replying and I added the PS.

My scenario was based on a pitch black environment.
  1. While the shutter opens, the camera does not see anything because it's pitch black.
  2. At the very moment that both curtains are open the flash fires with a burst of light (let's say 1/1000s or 1ms), This will make the subject and environment visible (for 1ms) and the camera will see subject and environment.
  3. After this it's pitch black again and for the remaining time that the shutter is open and while the curtain closes the camera will not see anything. With a shutter speed of 1/180s the time that the shutter is open is approx 5ms (1/180). Subtract the 1ms of the flash burst so for the remaining 4 ms the camera does not see anything.
01-26-2011, 12:32 PM   #13
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So then will 1/250th freeze "more motion" then 1/160th? All the reading I've been doing is claiming to set the sync to a minimum of 1/250 using Nikon and Canon gear. Albiet, I'm trying to learn.
In this vid, will the water have been frozen at 1/160 or is it imperative to be faster
01-26-2011, 12:35 PM   #14
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@Wheatfield

Manual for the AF540FGZ gives the longest flash duration on page 89 (slightly hidden between the tables); 1/1200s; it's the only manual that I have at hand to check.

For Metz flashes it depends on the model; from the manuals that I've researched it looks like hammerheads have 1/200s for full power while others have 1/125s
01-26-2011, 12:44 PM   #15
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QuoteOriginally posted by Deiberson Quote
So then will 1/250th freeze "more motion" then 1/160th? All the reading I've been doing is claiming to set the sync to a minimum of 1/250 using Nikon and Canon gear. Albiet, I'm trying to learn.
In this vid, will the water have been frozen at 1/160 or is it imperative to be faster
http://www.youtube.com/watch?v=f2krz58XfUw
the reason people bitch about the performance of pentax and 1/180th of a second is that depending on the shot, you can exclude another 1/2 stop of ambient light or have the lens open a little more (for DOF effects) with 1/250th. Additionally canon and I believe nikon allow max sync speed for both leading and trailing curtain sync, where as pentax cuts shutter speed in half for trailing curtain sync.

But the half stop is not a real limiting factor, it is just different.

one thing a higher sync speed may do, is to force the flash duration to be shorter, and hence the flash actually brighter to get the same guide number. As a result, the improved (reduced flash duration may be a little better at freezing the image. Note people who shoot hummingbirds, usually move the flash as close to the subject as possible, and use short lenses, so that the flash is at min power and 20-50micro secondis long. this can actually freeze the wings, which are at something like several thousand beets per minute
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