Flash sync speed = fastest speed that has a freeze motion effect?

Progenix Oj101

Progenix Oj101

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I'm just thinking here, I'm not sure if I'm way out or on the right track. If your flash sync speed is 1/60, or about 17 ms, the fastest speed that the shutter curtains can travel across the sensor will be 17 ms. Using a shutter speed of 1/4000th of a second will still take 17 ms for the exposure, just that each pixel is exposed for only 1/4000th of a second. My thought is if something which takes up a good part of the frame is moving horizontally past the camera at speed, it will be visibly skewed the same way you see rolling shutter in video. If the object can move from one side of the frame to the other in the 17 ms, it should be skewed diagonally across the frame and not be visibly frozen, so to speak. This is purely for my curiosity.
 
This looks like it might help answer a few qns.

[video=youtube;hvZ6VujbhjM]https://www.youtube.com/watch?v=hvZ6VujbhjM#t=84[/video]
 
I think either you're confused, or we're confused about what you are asking... Not sure which.
 
Let me put it this way. We're going to use shutter speed of 1/4000 (0.25 ms) on a camera which has a flash sync speed of 1/60 (17 ms). The fastest the shutter curtains can move gives you your flash sync speed, so even if you're using a shutter speed of 1/16000 it will still take 1/60 for the shutters to complete their movement. The increase in shutter speed above flash sync speed is simply the rear curtain beginning to close before the front curtain has finished opening.

If you have a vertical object at the right of your frame which can move all the way to the left of your frame in the 17 ms it takes your shutter curtains to move. When the shutter curtains expose the top of the frame, the object is on the right. When the curtains get to the bottom of the frame 17 ms later, the object is on the left of the frame.

What you end up with is the object as a diagonal line from the top right to the bottom left of the frame (well, reversed as the image projected from the shutter is upside down). Therefore going faster than the flash sync speed of 1/60 isn't going to help to make the object appear to stand still. Am I right? I hope you understand what I'm saying :o
 
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Oj0 said:
I'm just thinking here, I'm not sure if I'm way out or on the right track. If your flash sync speed is 1/60, or about 17 ms, the fastest speed that the shutter curtains can travel across the sensor will be 17 ms. Using a shutter speed of 1/4000th of a second will still take 17 ms for the exposure, just that each pixel is exposed for only 1/4000th of a second. My thought is if something which takes up a good part of the frame is moving horizontally past the camera at speed, it will be visibly skewed the same way you see rolling shutter in video. If the object can move from one side of the frame to the other in the 17 ms, it should be skewed diagonally across the frame and not be visibly frozen, so to speak. This is purely for my curiosity.
Click to expand...

You are on the right track, but you have not taken into account how the sensor reads out the data, and at what speed the readout occurs. If the sensor can read the data fast enough, rolling shutter becomes negligible.

I am not quite sure what you are trying to determine here though, since having a shutterspeed shorter than the flash sync will cause a shadow of the shutter to be thrown over the sensor, and black out parts of the exposure.

Let me put it this way. We're going to use shutter speed of 1/4000 (0.25 ms) on a camera which has a flash sync speed of 1/60 (17 ms). If you have a vertical object at the right of your frame which can move all the way to the left of your frame in the 17 ms it takes your shutter curtains to move. When the shutter curtains expose the top of the frame, the object is on the right. When the curtains get to the bottom of the frame 17 ms later, the object is on the left of the frame.

What you end up with is the object as a diagonal line from the top right to the bottom left of the frame (well, reversed as the image projected from the shutter is upside down). Therefore going faster than the flash sync speed of 1/60 isn't going to help to make the object appear to stand still. Am I right?
Click to expand...

I think think you are confused in how the exposure mechanism works. Flash sync speed is only relevant when you are using the flash. It has no relevancy to a normal exposure not using the flash. So using a shutter speed of 1/4000 (0.25ms) means that the shutter will open, stay open, and a close and the total duration that light is allowed through is 0.25 ms. Not 17ms.

If you are using flash, the sync speed is necessary from a timing point of view as the flash need to fire at the exact moment the shutter is fully open. If the shutter is partially open, or the flash fires too early or late, the flash light will cast a shadow on the sensor / film. So in a sense, a camera with a long flash sync of 1/60 means that its circuitry/mechanics are not accurate enough to sync with a short flash duration. Whereas a modern Pro DSLR can sync at 1/250 due to its modern mechanics/circuitry.

Wikipedia has a very nice explanation of it all http://en.wikipedia.org/wiki/Flash_synchronization
 
Guys, please ignore flash altogether. I'm not talking about flash here, I'm referring to flash sync speed as it will be the mechanical limitation of the speed of the shutter. I know how flash sync works when using flash, I'm asking a different question here.
 
Let me try again. A flash sync speed of only 1/60, or 17 ms, is a limitation due to the shutter curtains having a slow travel speed. In this case, a shutter speed faster than 1/60 will result in the rear curtain beginning to close before the front curtain is fully open. With a shutter speed of 1/4000 on a camera that has a sync speed of 1/160, the rear curtain will begin closing once the front curtain is only 4% of its way across the sensor. The total time that each pixel is exposed is only 1/4000, but the total time taken to expose the image is 1/60.

I think I'm answering my own question here :D
 
Oj0 said:
Let me try again. A flash sync speed of only 1/60, or 17 ms, is a limitation due to the shutter curtains having a slow travel speed. In this case, a shutter speed faster than 1/60 will result in the rear curtain beginning to close before the front curtain is fully open. With a shutter speed of 1/4000 on a camera that has a sync speed of 1/160, the rear curtain will begin closing once the front curtain is only 4% of its way across the sensor. The total time that each pixel is exposed is only 1/4000, but the total time taken to expose the image is 1/60.

I think I'm answering my own question here :D
Click to expand...

I am still confused, how can a camera have a shutter speed of 1/4000th and a limitation of 1/60? The shutter opens and closes in 1/4000th. If it was an electronic shutter it would make sense but you have not mentioned that.

If the shutter speed is 1/4000 then the shutter moves (opens and closes) in that time, not 1/60... :confused::confused::confused:
 
Edduck said:
I am still confused, how can a camera have a shutter speed of 1/4000th and a limitation of 1/60? The shutter opens and closes in 1/4000th. If it was an electronic shutter it would make sense but you have not mentioned that.

If the shutter speed is 1/4000 then the shutter moves (opens and closes) in that time, not 1/60... :confused::confused::confused:
Click to expand...
No - he's right. Below your flash sync speed both curtains are open for at least part of the exposure - anything faster than that and the second curtain is already closing while the first is still opening - the faster you you go the smaller the swath of exposed sensor is.
 
Oj0 said:
My thought is if something which takes up a good part of the frame is moving horizontally past the camera at speed, it will be visibly skewed the same way you see rolling shutter in video.
Click to expand...

Correct. Take something like a Dremel tool and paint a white stripe radially across the front of a cutting disc. Spin the Dremel up, and take a photo of the front of the spinning disc (do not use flash). Instead of getting a motion-blurred straight white line, you will instead get a motion blurred curved line, just like the rolling shutter effect in (most) video cams.

I agree with your reasoning: the max sync speed is in effect expressing the maximum shutter speed at which both curtains will be open, thus revealing the true speed at which the curtain travels.

To freeze motion faster than what the max sync speed allows, you have to bring down the ambient light level so that the flash is the only source of light. This way the slow shutter open time is irrelevant, and only the duration of the flash pulse controls the "true" exposure time, hence the motion freezing ability. Take a wild guess why most balloon-popping shots appear to be shot in a coal mine ...
 
fvdbergh said:
Correct. Take something like a Dremel tool and paint a white stripe radially across the front of a cutting disc. Spin the Dremel up, and take a photo of the front of the spinning disc (do not use flash). Instead of getting a motion-blurred straight white line, you will instead get a motion blurred curved line, just like the rolling shutter effect in (most) video cams.
Click to expand...
10,000rpm angle grinder…
1/8000th
RH1D4895.jpg
1/4000th
RH1D4887.jpg
 
bwana said:
10,000rpm angle grinder…
1/8000th
View attachment 149318
1/4000th
View attachment 149320
Click to expand...

Pretty cool. Seems like it is not really curving so much, but rather slanting (left occurrence of stripe).
It also appears to tear into two parts -- but it is cool to see how the section where the stripe is moving "horizontally" produces a more "classical" motion blur.
 
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