How does this work? (slow shutter speeds) at DVinfo.net

Bill Zens · July 24th, 2005, 08:39 PM

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...

Bill

Kyle Ringin · July 24th, 2005, 10:00 PM

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.

Norm Couture · July 25th, 2005, 08:10 AM

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.

Andre De Clercq · July 25th, 2005, 01:18 PM

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.

Scott Lovejoy · July 28th, 2005, 10:37 PM

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).

July 24th, 2005, 08:39 PM	#1
Bill Zens Regular Crew Join Date: Mar 2005 Location: Kirkland, WA Posts: 191	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... Bill

July 25th, 2005, 08:10 AM	#3
Norm Couture Regular Crew Join Date: Sep 2003 Location: Quebec, QC, Canada Posts: 123	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. __________________ Norm :)

July 24th, 2005, 10:00 PM	#2
Kyle Ringin Major Player Join Date: Oct 2004 Location: Melbourne, Australia Posts: 241	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.

July 25th, 2005, 01:18 PM	#4
Andre De Clercq Major Player Join Date: Sep 2002 Location: Belgium Posts: 804	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.

July 28th, 2005, 10:37 PM	#5
Scott Lovejoy Regular Crew Join Date: Jul 2005 Location: Boston, MA Posts: 73	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).