HV20 Warping Space and Time?

marko5000

marko5000

Active member
Been playing around with shutter speeds today in 25p mode.

Now I understand 1/50 means the shutter is open for 1 50th of a second or half the length of the frame and 1/2000 is 1 two thousandth of a frame or 1 80th of the frame but what is happening at 1/6 shutter?

That's slightly over four times the length of the frame isn't it? Is this actually slowing the frame rate whilst recording then somehow converting this to 25p?

Or am I just being really thick?
 
Frames per second is always the same. Shutter speed simply refers to how long the camera lets light shine on the sensor (actually it's an electronic shutter but that's another issue...)
So yes, the light shines over four plus frames if your shutter is 1/6th. That's why you get motion blur so easily. But it's also why in low light situations the camera tries to set a longer shutter speed - to get more light on the sensor.
 
When the shutter is open longer than the frame rate (i.e., 1/12 shutter at 24p) it starts duplicating frames. The frame is held and buffered for the full duration of the shutter. So when the tape needs to write a new frame and the currently-imaging frame isn't done yet, it just duplicates the prior frame.

So at 1/12, you will only see 12 distinct separate frames per second. And at 1/6 you will see 6 frames per second. There will still be 24fps recorded, so each frame will be written to tape 4 times.
 
Barry_Green said:
So at 1/12, you will only see 12 distinct separate frames per second. And at 1/6 you will see 6 frames per second. There will still be 24fps recorded, so each frame will be written to tape 4 times.
Click to expand...

Hmmm... need some help with that Barry. Wouldn't the image appear jerky then instead of blurred? If one frame is repeated 4 times (in the 1/6th example) then only the unique subject motion of the first frame would be captured and repeated. Then when the next four frames are written, the subject would be much further ahead the natural path of motion causing the image to appear to jump.

I'm sure I'm missing something so please explain if you don't mind.
 
So whilst it's recording 24/25 frames a second it's only really captuing 6 frames with the shutter, effectivly, kept open the whole time.

I suppose I can see an use for it.
 
It's not jerky because the frame is accumulating blur by blending that 1/6th of a second of motion all together. It's not just grabbing one frame quickly and then ignoring the next five frames' worth, it's blurring them all together in one long-exposure frame.

Once the shutter closes and commits that to tape the shutter immediately opens again and starts processing the next frame, picking up where the last one left off.
 
i always thought it would be a great effect if somebody could build a sensor that would make the frames be 24 separate 1/6th of a second long frames that where all individual and overlapping each other by 3/4 of the time they are open
 
Barry_Green said:
It's not jerky because the frame is accumulating blur by blending that 1/6th of a second of motion all together. It's not just grabbing one frame quickly and then ignoring the next five frames' worth, it's blurring them all together in one long-exposure frame.

Once the shutter closes and commits that to tape the shutter immediately opens again and starts processing the next frame, picking up where the last one left off.
Click to expand...

Well that's basically what I said in my response. But then you said "The frame is held and buffered for the full duration of the shutter. So when the tape needs to write a new frame... it just duplicates the prior frame."

The camera captures 24 individual - not duplicative - frames per second. The image blurs because the shutter isn't closing quick enough to capture a distinct image (assuming some kind of subject/camera movement - if the subject/camera is stationary there's nothing to blur.)
 
No, the camera captures six individual frames per second. It writes 24 frames to tape, but it only images six. Each of those six "imaging periods" is governed by the shutter; the shutter is held open for 1/6 of a second. So in the course of one second it only creates six different frames.

But the tape still demands 24 frames. So each frame gets written out as a duplicated copy until the new frame is ready. The previous frame will get written four times until the new frame is ready.
 
OK I just sho a few second of rain at 1/6 shutter.

In FCP stepping hrough individual frames they are indeed in groups of four identical frames. each frame shows a lot of motion blur and the next set of four starts right where the last left of. No real jerkiness and no motion lost.

It does look a little strange played back. Jerky simply because of how long each picture is held.
 
If you shoot in 60i/50i with 30th/25th of a second shutter speed, will it be capturing progressive 25p/30p or really bad interlaced 25p/30p.

People used to shoot really blurry 25fps with the HV10.
 
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