Technical Q on shutter speed with video

My apologies if this is a stupidly simple question, but it's been bugging me. In both this forum and the Nikon forum I've been hearing about adjusting the shutter speed in video mode. In my limited understanding, not only is there no actual shutter action, but the frames per second IS the equivalent of shutter speed. In 5DM2's case, that would be 30th/sec. If you could increase or decrease the shutter speed it would be the same as changing the fps, wouldn't it? It would also eliminate things like the jello effect. Or am I missing something?

Thanks,

RobS555 said:

My apologies if this is a stupidly simple question, but it's been
bugging me. In both this forum and the Nikon forum I've been hearing
about adjusting the shutter speed in video mode. In my limited
understanding, not only is there no actual shutter action, but the
frames per second IS the equivalent of shutter speed. In 5DM2's case,
that would be 30th/sec. If you could increase or decrease the shutter
speed it would be the same as changing the fps, wouldn't it? It
would also eliminate things like the jello effect. Or am I missing
something?

Thanks,

--

Click to expand...

The problem is that "frames per second IS NOT the equivalent of shutter speed"
The 30fps means that the camera records 30 images in a second(fps).

The shutter speed is the time that each of those images is exposed. So you can have 30fps with a shutter speed of 1/8000th.

About the jello effect, i think that higher shutter speeds will help to eliminate the problem, but video with high speed shutter doesn't look very smooth

You can't have 1/8000th shutter in video mode because the mechanical shutter is always open and the electronic shutter on the sensor cannot go that fast. Thats why all the 5D mkII videos show motion blur even with people just walking.

--
Dave Peters

Dave Peters said:

You can't have 1/8000th shutter in video mode because the mechanical
shutter is always open and the electronic shutter on the sensor
cannot go that fast. Thats why all the 5D mkII videos show motion
blur even with people just walking.

--
Dave Peters

Click to expand...

--

I was trying to explain that the number of frames per seconds is not directly related to the exposure of each frame...........
I wasn't talking about the 5dii or any camera in particular.

Hi JonhyHu,

That was my first assumption, too, but I don't believe it's correct. If shutter speed controlled exposure time, then there would be no jello, ever. If your shutter speed was 1/3000th, then camera would capture one frame in a 100th of a second. It would then wait 1/30th of a second, and capture the next image, yes? Jello occurs because it takes a 30th of a second for the sensor to scan from top to bottom, allow for motion to affect the image. If the camera could capture at a faster rate, you wouldn't see vertical lines bend (| -> \), they'd theoretically jump (| -> |) . In this scenario, jello would only occur if our EYES scanned top to bottom while watching-- but they don't. I say theoretically above, because the eye isn't supposed to be able to detect individual images faster than 24 fps, so unless the footage is slowed down, movement should look smooth. At least that's what I was taught.

My guess is that when you adjust the shutter speed, your actually adjusting the Aperature, which is in turn adjusting the ISO.

But please, someone correct my theory.

JonhyHu said:

The problem is that "frames per second IS NOT the equivalent of
shutter speed"
The 30fps means that the camera records 30 images in a second(fps).
The shutter speed is the time that each of those images is exposed.
So you can have 30fps with a shutter speed of 1/8000th.

About the jello effect, i think that higher shutter speeds will help
to eliminate the problem, but video with high speed shutter doesn't
look very smooth

--

Click to expand...

The shutter speed is implemented electronically on a line by line basis. Each line starts recording every 1/30 of a second. It stops recording after the exposure interval 1/30 to 1/125 after it starts. The start time of each line is different with the top line starting first progressing down the sensor taking about 1/30 of a second for all of the lines to start. This staggered start is what causes the jello effect.

Thanks for that explanation. What you're suggestiong-- and correct me if I've got it wrong-- is that each line takes 1/30th of a second to read. Is it interlaced like a TV so that a line is actually all the odd or even lines? You don't mean a line across, do you?

If a line always takes 1/30th of a second, I'm not sure what effect a 1/125th shutter setting would have then.

Thanks,

DSPographer said:

The shutter speed is implemented electronically on a line by line
basis. Each line starts recording every 1/30 of a second. It stops
recording after the exposure interval 1/30 to 1/125 after it starts.
The start time of each line is different with the top line starting
first progressing down the sensor taking about 1/30 of a second for
all of the lines to start. This staggered start is what causes the
jello effect.

Click to expand...

RobS555 said:

Thanks for that explanation. What you're suggestiong-- and correct
me if I've got it wrong-- is that each line takes 1/30th of a second
to read.

Click to expand...

There are 1080 lines, each line takes about 1/30 * 1/1080 to read so reading all the lines one after the other from top to bottom takes 1/30 of a second.

RobS555 said:

Is it interlaced like a TV so that a line is actually all
the odd or even lines? You don't mean a line across, do you?

Click to expand...

It is progressive not interlaced. I mean one horizontal line of 1920 pixels.

RobS555 said:

If a line always takes 1/30th of a second, I'm not sure what effect a
1/125th shutter setting would have then.

Click to expand...

Very similar to what happens for the still shutter with really fast motion: it takes perhaps 1/400 of a second for the mechanical curtains to move across the sensor but the second curtain can be just 1/4000 of a second behind the first curtain. The blur for the motion depends on the very short shutter duration but vertical lines can end up skewed to a diagonal for a horizontal pan. The pan speed needed to cause skew is an order of magnitude less for video than it is for still.

It's not the most technically elaborate explanation, but it does cover the salient points:

http://www.hv20.com/showpost.php?p=28512&postcount=6