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    Article: CMOS, CCD, Rolling Shutters, and more
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    Admin Jason Ramsey's Avatar
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    Last edited by Jason Ramsey; 09-27-2007 at 08:30 PM.

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    Admin Jason Ramsey's Avatar
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    Thanks, Barry.

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    Admin Jason Ramsey's Avatar
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    Ok... I'm curious about two things.

    #1. How come there are no Global shutter CMOS cameras? Meaning, is there some benefit to using a rolling shutter with CMOS that I didn't catch in the article or otherwise?

    #2. What causes CMOS to be immune to vertical smear? If the CMOS sensor utilized a global shutter, would this still be the case?

    Seems a global shutter, CMOS sensor would be the best of both worlds, so I'm wondering what complication there is, or what is cheaper/easier, or whatever about using a rolling shutter instead of a global one in CMOS sensors, that has prevented this from happening.

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    Jason

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    #4
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    #1 is better answered by the manufacturers themselves as to why they keep choosing rolling vs. global.

    As for #2, the type of shutter is irrelevant to the smear. In a CCD an entire line of pixels is read out at once, and if one of the pixel wells blooms over it affects the entire line. In a CMOS chip each well is read individually, so what happens in one well can't affect any other.


     

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    Director of Photography TimurCivan's Avatar
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    Hmmm..... if you shoot with a very narrow shutter angle on a Film camera, isnt that like a rolling shutter?

    PS, i shot my first 35mm Today!!!! Arri BL4, 1000ft Mags... Zeiss primes the works.. I love it...
    Director of Photography
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    Quote Originally Posted by TimurCivan View Post
    Hmmm..... if you shoot with a very narrow shutter angle on a Film camera, isnt that like a rolling shutter?
    You'd have to use an extraordinarily narrow angle, like maybe 10 degrees, to simulate the rolling shutter effect on a film camera.

    In most circumstances you're using a 180-degree shutter, and so a film camera will exhibit properties more like a global shutter, with a very little hint of rolling shutter-ness in it. But it's quite different.

    With a film camera, the entire frame is exposed at once for a good amount of time. So any motion of the frame will result in blur. With a rolling shutter, only a certain small portion of the frame is exposed, so the amount of skew and the amount of blur is very different than a film camera.

    Under normal shutter circumstances I'd say it's around 85% like a global shutter, maybe 15% like a rolling shutter. The narrower you make your shutter angle, the more like a rolling shutter it'll become.


     

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    DVXuser Sponsor Dennis Wood's Avatar
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    Barry, like your previous article on CA, this one will be referenced by us often. Thanks for making the effort to compile it.


     

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    Steadi-Guru mikkowilson's Avatar
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    ... and then you throw interlacing into the mix, and you can get som *really* interesting unwanted effects!

    - Mikko
    Mikko Wilson
    Steadicam Owner / Operator - Juneau, Alaska, USA
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    #9
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    Basically the two problems from Jason are the same. CMOS sensor works in a row fashion. All commands, like reseting the sensor, enabling the integration (ie, open the shutter) and sampling, are performed row by row. This makes it easy to control. Furthermore, it matches the way the memory works. Since you can only read/write one row of the memory at one time, sampling the whole image at the same is useless because you have no place to save it.

    There exist some ways to 'hold' the image inside the CMOS sensor array. In this way the global shutter can be implemented. However it inevitably reduces the light-sensitive area of the sensor, so most companies do not like it.

    BTW I'm Chia-Kai Liang who mentioned in the article...this project travels far beyond my imagination. We are still working on that problem and having some new results. I will try to present them when it is mature


     

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    Hey, thanks for stopping by and commenting! Your work was very interesting, and we look forward to hearing what you've come up with next.


     

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