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 Post subject: question about video
PostPosted: Sun Apr 22, 2012 9:57 am 
Hi,

I'm new to using dslr's. I don't even have the proper words to ask the question!

I have a Canon eos 600d, and I noticed a strange artifact in the video. Best describes as something between motion blur and double exposure. In searching the net I found your review and downloaded the video to see for myself. Sure enough your video has it too.

It's the first video ( vimeo.com/21252521 ). It's quite noticeable near the end. Notice the white flag pole on the dock, and the white railing. At frame 886 they're in focus. Then they blur. Then back in focus at 894. The apparent angular motion of the camera doesn't seem to change, and the distance from camera to pole is certainly constant. So what causes the blur?

I have noticed the same thing in my own videos, even to the point of seeing two distinct images.

Thanks for you help.
Jeff


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 Post subject: Re: question about video
PostPosted: Mon Apr 23, 2012 3:43 am 
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Joined: Wed Apr 08, 2009 2:29 am
Posts: 711
Sounds like "judder" which is caused by the camera being moved (usually panned, but can be a result of any movement) too quickly relative to the focal length used. (It occurs with all motion picture cameras, but the ASC (American Society of Cinematographers) has a table that lists maximum recommended pan speeds for for film cameras, and 35mm film cameras are/were larger and heavier than today's DSLRs which made the movie cameras more difficult to move quickly.)

The effect can be compounded by frame rate conversion. (Note that computer monitors (vimeo) often have to convert video to their refresh rate. i.e. 25 fps to 72 Hz) And it's particularly noticeable with high contrast edges that are perpendicular to the direction of movement. (i.e. vertical edges--e.g. a white flag pole--if panning horizontally.)

Unfortunately, the only way to truly eliminate judder is to move the camera more slowly. But thinning out your DoF so that just your stationary (relative to the camera) subject is in focus, while the judder is blurred, and there are fewer hard edges in the background, will make it less distracting.

HTH - Mark


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 Post subject: Re: question about video
PostPosted: Mon Apr 23, 2012 6:30 am 
Thanks for your reply Mark.

The effect is not the result of frame rate conversion. I down loaded it so that I could look at it frame by frame.
It is possible, but doesn't seem likely, that the panning speed had an effect. It seems that the panning speed is nearly constant.

At any rate, motion blur due to panning speed does not account for the "double exposure" I found in my own video. In one particular frame, two distinct images could be seen.

I'm looking for a more technical answer. I'm an electronic engineer, and even did work on a digital ( not dslr ) camera once ( about 15 years ago ).


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 Post subject: Re: question about video
PostPosted: Mon Apr 23, 2012 7:38 am 
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Joined: Mon Jan 07, 2008 6:50 am
Posts: 425
Location: Sydney Australia
Keep in mind that video on a DSLR is not like a Video camera - Video cameras shoot at 50fps and some higher end cameras can be changed to 25fps.

In HD on DSLR cameras, the only choice you have is 25fps

This is called film Judder - you can replicate this using actual movie film - its only seen DURING PANNING a camera. DSLR cameras replicate film at 25fps because the "shutter" is the pixels sending information captures at 25fps... Judder occours optically when one frame captures an image, then the second frame captures the movement, and the 3rd captures a still frame and the 4th captures movement. This Movement Im talking about is a frame that is slightly smudged, or blurry and is the result of certain objects moving faster than the given shutter speed. At 25fps, we can notice this movement as it is slow enough for our eyes to see.

This DOUBLE imaging is basically our eyes noticing objects that move across the screen faster than other objects where the blured frame follows a sharp frame and it looks like its juddering behind the object. It doesnt happen all the time, only on certain objects like a striaght vertical line it MIGHT as the image mixes with the objects around it.

In old movies, judder was hidden by the fact that directors would only PAN the camera where there are no vertical objects and always at distances.

Here is a link about the differences and advantages of interlaced video (video cameras) and progressive (dslrs and higher end video cameras) http://www.hometheaterhifi.com/volume_7 ... -2000.html

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