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Old Jun 20, 2012, 4:52 PM   #1
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Default Shutter Speed to freeze motion

i have a D800, the sync speed is about 1/250s. i believe that that is the time it takes the first curtain to drop all the way down.
if i set the shutter speed to 1/8000s, then the first curtain will drop and very shortly thereafter, the rear curtain will drop creating a small "slit" to expose the sensor. if my thinking is correct, then, it will take about 1/250s for the "slit" to pass down the sensor. if this is so, then the top of the image will be exposed about 1/250s before the bottom of the image.

why then do we think that faster shutter speeds will freeze motion.

it seems like the fastest motion you are going to freeze is at the sync speed of the camera unless you use tricks with flash photography.

is this correct
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Old Jun 20, 2012, 5:36 PM   #2
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First, when using flash, the shutter speed doesn't freeze the motion, the duration of the flash does. During a large part of the duration of a shutter speed of 1/250, the entire sensor is exposed. That is, the first curtain has exposed the bottom of the sensor but the second curtain hasn't started covering the top. During that 1/250 second, the flash may actually be as short as 1/40,000 second if the flash can provide sufficient light during that time to produce a proper exposure. So, the flash is what freezes motion, not the shutter speed.

When the "slit" starts to appear is at shutter speeds faster than 1/250. That's when the second curtain starts covering the top of the sensor before the first curtain is finished uncovering the bottom. As the shutter speed gets faster and faster, the distancebetween the two curtains as they travel, gets smaller and smaller, but at 1/250, the entire sensor is exposed for a large portion of that time, which is when a very brief flash can illuminate everything that will appear in the frame. That is what determines the flash sync speed. (There is high speed sync, but that works differently. It makes several weak flashes as the slit moves over the sensor, instead of one strong flash when the entire sensor is exposed.)

When we're not using flash, the shutter speed does freeze motion. While it's true that, during a 1/8,000 second exposure time, each portion of the sensor is exposed for that length of time, but it takes longer than that to accomplish it. That slit is traveling down very quickly, but if, for instance, a vertical post is traveling very quickly across the frame, then the top of that post might appear at one side of the frame, and the bottom of the post might appear at the other side. In the resulting photograph, that vertical post would appear as a diagonal post.

But it would have to be moving very quickly, and that doesn't happen very often. When it does, we can see how all this works, but that's a rare occurance.
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Old Jun 22, 2012, 2:52 PM   #3
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does anyone know just how long it takes the front curtain to drop (D800).

i shoot birds and get a lot of (500mm) non-sharp images at pretty fast shutter speeds with more than enough depth of field, so i'm thinking maybe this top to bottom time lag may have something to do with it. if i knew the amount of time it takes for a given slit to pass from top to bottom, then maybe i would just use that as a shutter speed and live with it instead of fooling myself with faster shutter speeds and the resultant loss of depth of field and high ISO.
(still in denial about lack of ability)
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Old Jun 22, 2012, 3:53 PM   #4
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G'day Lindner

TC has given you a very accurate description of the workings of the focal plane shutter

Now you are asking about practical picture taking - good !!
For a BIF to succeed you will need a s/speed of well over 1/1000 with your 500mm lens
Have a chase up of previous submissions here for BIF pix and [presuming you have an exif viewer attached to your browser] - look at the exif data for those images

In the end you will find that good quality BIF images is a combination of good equipment PLUS lots of experience from good technique

Regards, Phil
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Old Jun 22, 2012, 4:58 PM   #5
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thanks for your time and answers
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Old Jun 22, 2012, 7:40 PM   #6
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Quote:
Originally Posted by lindner514 View Post
i shoot birds and get a lot of (500mm) non-sharp images at pretty fast shutter speeds with more than enough depth of field, so i'm thinking maybe this top to bottom time lag may have something to do with it.
If it were, you'd be seeing motion blur. That is, a directional smearing of an object or objects in the frame, as opposed to a general out-of-focus blur. Are you?

Motion blur is always directional. Motion blur due to camera shake is generally vertical, and motion blur due to subject movement is always in the direction of the movement. Are you getting motion blur, or just blur?

Or are you pixel-peeping a 36MP image, and discovering that, perhaps your lenses aren't as sharp as you first presumed.
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Old Jun 23, 2012, 5:40 AM   #7
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Still trying to get my head around the actual speed of the shutter. I'm thinking if the sync speed is 1/250 then it must take the curtain around 1/250 to traverse the sensor. My thinking is that if the total exposure is 1/250 it must take 1/250 for the first curtain to open then a further 1/250 for the rear curtain to close. So the first part of the sensor is exposed for the time it takes the shutter to open then blanked as soon as the rear curtain starts to close. The last part of the sensor to be exposed is open until the rear curtain has finished it's traverse. That is unless there is a pause between the first curtain fully opening and the rear curtain starting to close. The duration of the flash is very short so any pause would only need to be brief to allow the flash exposure with the sensor fully exposed.
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Old Jun 23, 2012, 6:43 AM   #8
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If it took 1/250 (0.004) second for a curtain to traverse the sensor, then the sensor would only be entirely uncovered for an instant before the second curtain started to cover it. The flash sync speed is the fastest shutter speed where the sensor is entirely uncovered for a significant amount of time so that a flash can illuminate everything that will appear in the image.

I think it's much more likely that it takes 1/1000 second for a curtain to travel across the sensor. If it took 0.001 second, then during a 1/250 (0.004) second exposure, when you fully depress the shutter button (0.000 second) the first curtain starts to uncover the sensor. After 0.001 second, the first curtain has finished its travel, and the entire image sensor is exposed. The exposure time is 0.004 seond, so the second curtain won't start to cover the sensor for another 0.003 seconds, which is plenty of time for the flash to provide sufficient light to produce a proper exposure. At the end of that 0.003 second, exactly 0.004 (1/250) second after the first curtain started uncovering hte sensor, the second curtain starts to cover the sensor. At 0.005 second, the second curtain has finished covering the sensor.

So it would take 0.005 second for the camera to expose the sensor for 0.004 second. The key factor to remember here is that the entire sensor must be exposed so the flash can illuminate the entire frame. The reason the flash sync speed isn't faster is because the second curtain starts covering the sensor before the first curtain has finished uncovering it.

I think a time of 0.001 (1/1000) second for the curtain to travel across the sensor is a reasonable guess, as that leaves a lot of time for the entire sensor to be exposed during a 1/250 second shutter speed (anything slower wouldn't), and if it were faster than 0.001 second, then the flash sync speed could be faster (a shutter speed of 1/500 would leave the entire sensor exposed long enough for the flash to be effective.)
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Old Jun 23, 2012, 7:57 AM   #9
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During a 1/250 second shutter speed, if the first curtain took 1/1000 second to uncover the sensor (and 1/1000 second for the second curtain to cover it again) then there would be 3/1000 second fo the flash to illuminate the entire frame. If the first curtain took longer than 1/1000 second, there would be less time. If the first curtain took 1/250, then when the first curtain finished, the second curtain would immediately have to start, so the sensor would never be fully exposed, so a flash would never be able to illuminate the entire frame.



Certainly, the time for a curtain to travel from one side of the sensor to the other, could be faster than 1/1000, but if it were a lot faster, then the flash sync speed could probably be faster too. But as this chart shows, it couldn't be much slower than 1/1000 and still allow a flash sync speed of 1/250.
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Old Jun 23, 2012, 10:48 AM   #10
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I'm sure you're right about there needing time for the flash to expose the image just not sure how long it is. I think modern flash tubes take about 1/1000 to discharge so it's unlikely that the whole frame has to be exposed for as long as 3/1000.

All academic anyway as we have to live with it and I've never noticed any distortion caused by movement during the exposure other than motion blur.
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