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Old July 24th, 2005, 09:39 PM   #1
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How does this work? (slow shutter speeds)

OK, so I just got off another X-country flight and am a little bleary eyed, but, this thought sort of popped into my consciousness (sp???) somewhere over Arkansas...

How is it possible to shoot a shutter speed slower than your fps?

In other words, how is it possible to shoot at a shutter speed of say 1/8th of a second, with a frame rate of 15 frames per second. How can the shutter be open longer than the frame is in position?

Not that this is all that important, or that it affects anything...I think I'll just have a glass of red and forget...


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Old July 24th, 2005, 11:00 PM   #2
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The video camera pretty much just leaves the shutter open for the set shutter speed - 1/8th sec or whatever then just repeats that frame until the next frame is 'ready'.

eg at 16fps ('cause it's easier on the noggin):

Displayed Frame|Captured frame
1 | 1
2 | 1
3 | 2
4 | 2
5 | 3
6 | 3
7 | 4
8 | 4
...etc

That's why video shot at a slow shutter is blurry (motion blur from the slow shutter) *and* 'stuttery' because effectively you aren't watching the full frame rate anymore.
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Old July 25th, 2005, 09:10 AM   #3
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Slow shutter on a DV cam

Normally your NTSC cam records a 30 fps stream of 2 interlaced 1/60th fields.
When you select Slow Shutter speed of 1/30, the cam will expose the first field (the odd one) for 1/30 but, since it doesn't have the time to shoot the second field (the even one), it will replace it with a duplicate of the odd one. So you'll have a 30 fps stream of 1/30th deinterlaced frames with half the vertical resolution, each line appearing twice. At this point, no stuttering shows because you still have 30 different frames per second. Some blurring may appear in moving subjects.

When you select a slower shutter speed, the cam will expose the odd field for 1/15th, 1/8th or 1/4th sec., according to the setting, and duplicate the entire deinterlaced frame enough times to make the required 30 fps stream. At 1/15th for instance, You'll get 15 different frames, each showing twice. Now the movement gets stuttering and blurr is more important.

Some high-priced camcorders may "interpolate" to create the even field rather than simply duplicate the odd one.
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Old July 25th, 2005, 02:18 PM   #4
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Some cams which have progressive mode shooting can alternate the 2 fields in these low shutter speed modes. No line doubling, no interpollation, I think DVX100 has this possibility. Besides motion blurr and strobing when there is motion, also still pics show resolution reduction due to pix leakage while charging, and also fixed pattern noise becomes more visible.
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Old July 28th, 2005, 11:37 PM   #5
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I always think of it like an actual film camera, since DV cameras try to imitate that (at least sometimes).

So the film moves fast the shutter at 24 FPS but the shutter stays open for however long it needs to (in this case say 1/8th of a second). Now this isn't how it works with DV cameras but just picturing a normal camera works for me (specifically for figuring out my settings).
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