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Why No Rolling Shutter on Film Cameras?


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#1 Peter Ellner

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Posted 02 September 2011 - 10:38 PM

Out of frustration with the rolling shutter effects seen in my own footage, I looked into the cause and discovered that it's really just because the entire frame isn't exposed all at once but rather from top to bottom during the exposure period...this leads to the jello effect and other rolling shutter artifacts seen in DSLR and other CMOS sensor footage.

But here's my question: isn't the way that film is exposed by the rotating shutter in a motion picture camera similar to that of a CMOS sensor with a rolling shutter? So why, with the same shutter speed and sensor/film size, would a CMOS sensor camera have rolling shutter effects while a film camera recording the same thing (say a fan or helicopter blades) would not?

And while on the topic, I'm wondering what used to be required when film cameras shot scenes with TVs in the frame. Was it simply a matter of changing the shutter angle to equate with a shutter speed of 1/60? And even in that situation, since the frame rate of a film camera is 24 but the refresh rate of a CRT is 60, wouldn't some frames show an empty screen while others would show an image due to the fact that the frame rate doesn't match the refresh rate of the TV? I"m a little confused.

Thank you for the help!
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#2 Brian Drysdale

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Posted 03 September 2011 - 02:39 AM

The analogy used regarding film by digital camera manufacturers is that there is an extremely high refresh rate on film and by increasing this on their latest CMOS cameras RED and ARRI the jello cam effect is gone or reduced to levels that aren't noticeable. Most CMOS cameras don't have or with DSLR don't normally require these higher refresh rates to take stills. However, that's not to say you won't get part exposed areas within a frame when using a high speed strobe or flash light.

Shooting NTSC on a CRT television at 24fps, 144 degrees is the recommended shutter angle. I believe this isn't an issue with a LCD screen.
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#3 David Mullen ASC

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Posted 03 September 2011 - 07:03 AM

Out of frustration with the rolling shutter effects seen in my own footage, I looked into the cause and discovered that it's really just because the entire frame isn't exposed all at once but rather from top to bottom during the exposure period...this leads to the jello effect and other rolling shutter artifacts seen in DSLR and other CMOS sensor footage.

But here's my question: isn't the way that film is exposed by the rotating shutter in a motion picture camera similar to that of a CMOS sensor with a rolling shutter? So why, with the same shutter speed and sensor/film size, would a CMOS sensor camera have rolling shutter effects while a film camera recording the same thing (say a fan or helicopter blades) would not?

And while on the topic, I'm wondering what used to be required when film cameras shot scenes with TVs in the frame. Was it simply a matter of changing the shutter angle to equate with a shutter speed of 1/60? And even in that situation, since the frame rate of a film camera is 24 but the refresh rate of a CRT is 60, wouldn't some frames show an empty screen while others would show an image due to the fact that the frame rate doesn't match the refresh rate of the TV? I"m a little confused.

Thank you for the help!


The truth is that film shutters do cause some mild rolling shutter artifacts, but because the sweep of the shutter is so fast, it's pretty minimal. Rolling shutters in digital cameras can reduce the problem by increasing the read-out speed / shortening the read-out time of the CMOS sensor.

If you are talking about shooting LCD screens, the decay time of the pixels in the monitor is long enough, and the refresh rate high enough, that a 24 fps movie camera does not need to be synced to the monitor. It's a little like shooting a Kinoflo or something.

With Plasma TV's, there can be a bit of an AC pulse visible and you'd have to play with the shutter speed on the video camera to get rid of it. For a film camera, I'd try a 144 degree shutter angle for starters but the truth is that not all Plasma screens run at the same refresh rate of 60 Hz. But it's a mild pulse.

With CRT computer screens, it's very hard to shoot them with a 24 fps camera because they run at all sorts of frame rates. You'd have to get into the computer and change the refresh rate to something shootable. For movies avoided the problem by sticking CRT TV sets in the computer housing and running video playback through it, or sticking an LCD screen inside the CRT housing and not dealing with sync at all.

With CRT TV screens showing NTSC (59.94 Hz), which was the most common scenario for decades when shooting scenes at 24 fps, there were a number of steps involved in dealing with the roll bar.

First, if you set the shutter angle to 144 degrees (1/60th) instead of 180 degrees (1/48th) the SIZE of the roll bar shrunk from an 1"-ish tall band to something like a thin line.

If you changed the frame rate of the camera to 29.97 fps the roll bar would stop moving/rolling, and if you used a sync/phase box, you could phase/move the line out of the picture. Truth is that you could leave the shutter angle at 180 degrees in this case and phase the bar out of the picture.

If you used a 144 degree shutter angle (for a thinner roll bar) and changed the frame rate of the camera to 23.976 fps the thin roll bar would now stop moving/rolling... but you'd actually see two thin lines, and with the phase box, you could move them until either you had only one line visible in the center of the screen, or two lines, each at the top third of the screen. But you couldn't get rid of the lines unless you shot at 29.97 fps instead of 23.976 fps.

At that point, your option was to hire a 24fps video playback company. They would bring a 24fps TV set to the shoot -- 23.976 fps actually -- and a 23.976 fps betacam deck and any video footage you wanted for playback converted by them from 59.94i to 23.976 fps.

One problem with all of this was that not many 16mm cameras have adjustable shutter angles.
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#4 Stephen Perera

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Posted 04 December 2017 - 07:24 AM

....so the 'famous' Wes Anderson film camera moves will not give you the horrible skew effect you see in digital cameras on a film camera then right? Im wondering whether this will happen on my 180 degree shutter Aaton XTR 16mm


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#5 Brian Drysdale

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Posted 04 December 2017 - 08:05 AM

You may get a "strobe" effect from panning too fast, but the film cameras tend to have a softer wiping motion from the mechanical shutter, so any rolling shutter tends not to be noticeable in practice. I've done shots on an Aaton which would drive rolling shutter digital cameras crazy without having any noticeable skew on the Aaton.


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#6 Simon Wyss

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Posted 04 December 2017 - 08:24 AM

The only way to do away with any kind of shutter effect is to use a, as we call it, central or leaf (leaves) shutter. Unfortunately I can’t name a corresponding product that would be capable of opening and closing fast enough and sustained 24, 25 or 30 times a second. If we abandon the symmetrical layout of such a shutter, we could devise something of a revolving disc shutter intersecting the lens, say, the focal plane shutter moved up to the lens’ main plane or close to the diaphragm plane, but that would make necessary an additional camera feature that shuts the aperture lighttight. Worse still, each lens would need its own main plane shutter which is sort of uneconomical.

 

My advice: pam slower. Arrange the moving things in a steeper angle to the optical axis. Don’t panoram. Employ the widest shutter opening angle you can. Change the speed of the moving things. Use shorter focal lengths. Avoid backlight.


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