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Some general questions on digital shutters


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#1 Rob McGreevy

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Posted 09 November 2015 - 05:39 PM

How do rolling shutters on digital cameras work? Or more specifically, how is it that it's measured in degrees? With film cameras that made sense, it was a disc with a leaf that could  be adjusted mechanically to created an actual physical angle measured in degrees, but my understanding of how a rolling shutter works is it's basically a scan line that moves down the sensor of the camera...is the expression in degrees a measured equivalent of the amount of sensor that would be exposed if it were an actual mechanical shutter? Or is there some other mechanism in play entirely that I'm unaware of?

 

Also, i've read that rolling shutters can created serious artifact issues with CMOS sensors...but so many digital cameras still utilize both rolling shutter and CMOS sensors. There must be some advantage they offer over a global shutter, right? What's the advantage of one type of shutter vs. the other?


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#2 Brian Drysdale

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Posted 09 November 2015 - 06:59 PM

You're confusing the mechanical rotary shutter used on film cameras (and some digtial cameras) with what is termed rolling shutter.

 

https://en.wikipedia...Rolling_shutter

 

A sensor without the rolling shutter artifact is termed global shutter

 

A comparison of these is made here:

 

http://www.red.com/l...rolling-shutter


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#3 Phil Rhodes

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Posted 09 November 2015 - 07:06 PM

With most rolling shutter cameras, at least those intended for film and TV work as opposed to webcams and cellphone cameras, there is a period of time during which all of the photosites on the sensor are sensitive to light which will become part of the final exposure. There is then a generally much shorter period in which the rows of photosites are read out sequentially.

 

Yes, it's an equivalence to how a 35mm film camera would work. There is no angled disc involved, but come cameras refer to shutter angle because it's a familiar terminology.180 degrees implies a 50% duty cycle, that is, 1/48s exposure at 24fps, or 90 degrees implies a 25% duty cycle, or 1/96s at 24fps. The digital camera is using sensor timing to create those exposures rather than mechanical parts (except in a small number of cases), but the result is similar or identical.

 

This simple equivalence is (albeit very slightly) inaccurate in the case of rolling shutter cameras where some pixels will be active for longer, or during a different period, to others. Other than in very specific circumstances this will not make any practical difference. I'm not sure if there's a convention as to whether the per-frame exposure time of a digital cinematography camera includes the readout period, or perhaps some part of it, or none of it at all. Either way, in many cases the readout period will be thousandths of a second whereas the entire frame's exposure will be many tenths, so the difference it makes is tiny.

 

The use of shutter angle terminology does not relate in any way to the potential for apparently skewed objects as viewed through a rolling shutter camera during horizontal motion. That's an entirely unconnected phenomenon.

 

CMOS sensors are used because they make it easier to build electronics other than just the photosite array onto the sensor itself. Modern sensors require multiple "taps," that is, data pipelines to get the picture information off the sensor quickly enough to work properly. This means that various areas of the sensor will be individually served by output amplifiers, digital to analogue conversion and other electronics. It is not possible (or at least not effective) to try to build this with the sort of manufacturing processes used for CCD sensors.

 

CMOS sensors can have rolling or global shutters. All things being equal, the global shutter is preferable because it creates fewer problems (flash banding, skew, and so on). The Blackmagic 4K cameras have a globally-shuttered 4K sensor. The problem is that to create a globally-shuttered CMOS sensor, each photosite on the device must have (at least) one extra transistor built into its design to control whether or not it is sensitive to light. This is a disadvantage because we want the sensitive face of the device to be filled as much as possible with light-sensitive area, not electronics, to maximise sensitivity and dynamic range, and minimise noise. It also makes the sensor harder to manufacture. Future technologies based on layering electronics and light-sensitive components together may make it possible to produce globally-shuttered sensors without these compromises, and we're all looking forward to that.

 

The simple solution is to use a mechanical shutter just like a conventional film camera, which is what the F65 and certain Alexas do, but of course the mechanical engineering is expensive.

 

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#4 Tyler Purcell

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Posted 09 November 2015 - 08:20 PM

As a side note, the mechanical shutter really doesn't remove the rolling shutter effect entirely. The sequential read out of the imager, speed of the pipeline and processor, all play a critical role. The bigger the imager, the more photocell's need to be read, the harder it is to create an image without some rolling shutter effect.

The only real reason why all cameras don't use global shutter systems is due to sensitivity, data pipeline speed and processing power. It's hard to get an ultra sensitive imager with the all the processing power into a small package. Today's technology is better then it has been in the past, but still, only a few of the mid to high end cameras have global shutters. Even RED cinema is rolling shutter, with an "add-on" for their Epic and Scarlet cameras to make them global shutter. It's basically an LCD panel in front of the main imager that counteracts the issues.
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#5 Robin R Probyn

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Posted 10 November 2015 - 12:01 AM

A mechanical shutter is also a rolling shutter.. in film or video..   the read out of "higher" end camera,s is so fast now that its very rarely a problem..  look at all the stuff shot with Alexa..   even an F5 its not a problem expect flashes going off..


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#6 Tyler Purcell

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Posted 10 November 2015 - 02:31 AM

Yes, a mechanical shutter rolls from one side to the other side on film cameras. However, the motion blur that comes from this is more natural looking since it's left and right vs up and down.
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#7 Phil Rhodes

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Posted 10 November 2015 - 03:41 AM

The principal difference is that the blade of a mechanical shutter on a motion picture film camera is extremely out-of-focus, being placed some distance from the film to allow room for the mirror and viewfinder optics. The result is that the exposure broadly fades in and out almost uniformly. Well, it isn't always that uniform, depending on the camera design: if you look at Aliens and notice the use of xenon strobes to reinforce gunfire effects, you'll observe that the flickering of the strobe light appears brightest at times in the top left of frame, and brightest at other times in the bottom right of frame. It's subtle and represents a gradient across the frame, but it's there. It wouldn't be noticeable apart from the fact that in some shots, where there's a long burst of the strobe, the bright area moves from top left to bottom right as the phase relationship between the strobe and the shutter drifts.

 

Film cameras (and, presumably, mechanically-shuttered digital cameras) can and do exhibit rolling shutter artefacts, but since it's so soft, it's very subtle and only visible in extremely specific circumstances. It depends on the design of the camera, of course. One would expect different behaviour from an F65, where the shutter is solely a shutter and sits parallel to the plane of the sensor, than from an optical-viewfinder Alexa, which has a film-style mirror shutter at 45 degrees to the sensor with film-style viewfinder optics.

 

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#8 Robin R Probyn

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Posted 10 November 2015 - 05:22 AM

Phil ..  thanks for the detailed description .. I just wanted to make the point to the OP.. that any gnashing of teeth over rolling shutter is 99% of the time,  a waste of effort.. as all films for the last 100yrs have had rolling shutter..   even flash banding now has pretty much been accepted by audiences .. we see a scene with photographers and we some some flashes on the screen.. thats all people need to see..  it was really only a problem with DSLR and Go,pro,s... From the F5 up.. (dont know about Fs7) its a non problem with fast read sensors.. you can wiggle those camera,s around and they are totally fine..   unless your work is specifically enhanced with a global shutter.. or some DP,s what to use an optical VF on a video camera.. because thats what they are used to..

 

Thanks


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