Low Light EX1 Overcrank

Barry_Green said:
Yes, but with a MASSIVE difference -- the mechanical shutter exposes the entire frame for a large part of the exposure time. The CMOS rolling shutter doesn't, it only exposes a line at a time. Huge difference. A film camera will have global exposure for almost the entire shutter time, and thus will have the signature motion blur we come to expect from a film camera. And the entire frame is exposed all at once for most of the exposure time. Even the top and bottom of the frame will be exposed simultaneously on a rolling mechanical shutter for some amount of time.
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I certainly don't know the details of what specific cameras do - and what you say certainly seems true of some of the more extreme cases we're seeing. However, the clearest explanation I've seen ( http://www.isgchips.com/pdf/Shutter_Operations_Kodak_App_Note.pdf ) contradicts this statement - the time between the 2 sweeps determines the shutter speed, and the "time" (measured in lines) may be as much as the full frame.

In your scenario, exposure of, say, a 1080p24 image would be (1/24)/1080 - that is, about 1/24000 - at best. So I think it makes sense that with an electronic rolling shutter, too, the much of the frame is exposed simultaneously. For 1/48, half the frame would be exposed at all times, etc.

Having never laid eyes on the shutter mechanism of a film camera, I'm also surprised when you say "the entire frame is exposed all at once for most of the exposure time"... I thought the shutter angle was proportional to the amount of exposure time, with 360 meaning no exposure, and 0 meaning no occlusion - this would indicate that, for non-0 angles, the entire frame would never be exposed at once -and at common settings of around 180 degrees, only about half the frame would ever be exposed at once...?
 
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Barry_Green said:
By cranking up the frame rate, that inherently speeds up the "sweep" rate. I don't know how Red is trying to address the issue. I guess it's possible Sony might do something similar, who knows... maybe they employ double-speed sweeping up to 30fps (i.e., internally running at 48fps when recording 24p, and discarding every other frame) but once they hit 30fps or more, dropping to no double-sampling? Or maybe internally the thing's running at massive speeds, 120fps or 240fps, and thus using only one out of every two or four frames, but using that cranked up frame rate to get a faster sweep rate?

If so, though, you'd expect a limit on exposure time based on the frame rate... if they were running 120fps internally to generate 60fps, you should theoretically not be able to set a shutter speed slower than 1/120th. At 120fps the "sweep rate" must be completed in 1/120th of a second, so you couldn't go exposing 60p at 1/60th in that scenario. So, question for EX users: can you set a shutter speed of 1/60th when shooting 720/60p? If so, that would seem to imply that they're not using a doubled (or more) internal frame rate.

Wouldn't it also seem to mean that the rolling shutter lean/wobble would be a lot more noticeable at slower frame rates... If you're running 2 fps at 1/2 second, the sweep would have to be no faster than 1/2 second, yes? Therefore massive leaning should happen, should it not? Of course, much of the lean would be hidden under the blur of a 1/2-second exposure too.
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I think they might mean that they are indeed speeding up the sweep rate, and allowing the voltage to accumulate over multiple sweeps - that is, if sweep time for a frame is S, they start the "read" sweep more than S after the reset sweep. This would be equivalent to having the entire frame exposed for some percentage of the exposure time. This isn't exactly "discarding" frames, though, if that's precisely what they meant...

Actually, maybe they are "discarding"... if they are for some reason able to run a reset sweep when they want, but the system requires the "read" sweeps to happen like clockwork once per frame - they can speed up the framerate, but not do the reset every frame. This would allow the voltage to accumulate over multiple read cycles - so, say, at 120 fps, with resets every 1/60th, discarding the result of every other read sweep, you'd have the exact same thing as 60fps, but with a significant amount of full-frame exposure...

blah blah
 
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ullanta said:
I certainly don't know the details of what specific cameras do - and what you say certainly seems true of some of the more extreme cases we're seeing. However, the clearest explanation I've seen ( http://www.isgchips.com/pdf/Shutter_Operations_Kodak_App_Note.pdf ) contradicts this statement - the time between the 2 sweeps determines the shutter speed, and the "time" (measured in lines) may be as much as the full frame.
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Excellent document, thanks for pointing it out, and yes anyone interested in the subject should be reading stuff like that.

I'm basing my statements on observation of what happens on existing rolling shutter cameras. You have to keep in mind that the Kodak document was talking about still camera/DSLR use, not high-frame-rate film use. It's possible to get full-frame exposure depending on how much readout time is available, and DSLRs are slow frame rate, thus they can delay readout until the entire frame has been exposed. In video cameras the readout process is happening simultaneously with the exposure process; lines are being exposed and then shut off and read out while the next set of lines are being exposed.

How much of a "buffer time" can be allocated is probably dependent on price, I would guess. Lower-cost rolling shutter systems are likely to have less buffer time and thus more wobble. Maybe?

Having never laid eyes on the shutter mechanism of a film camera, I'm also surprised when you say "the entire frame is exposed all at once for most of the exposure time"... I thought the shutter angle was proportional to the amount of exposure time, with 360 meaning no exposure, and 0 meaning no occlusion - this would indicate that, for non-0 angles, the entire frame would never be exposed at once -and at common settings of around 180 degrees, only about half the frame would ever be exposed at once...?
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360 degrees would mean no occlusion. A 10 degree shutter angle means that 350 degrees is shrouded, with a 10-degree opening. A 350-degree shutter would operate, sweep-style, like a rolling shutter CMOS video cameras that I've explored.

With a typical film camera shutter you're looking at a half-moon shape on a shutter circle that's maybe 2.5 inches in diameter, which unveils a little square of film that's 10mm wide by 7mm tall. That shutter is going to sweep the frame in 1/48th of a second, and unveil the top left corner (which is actually the bottom right, but who cares) first, and sweep diagonally right down the frame at a decreasing angle until the midpoint of the frame at which point it'll be level, and then the angle will reverse as the rotation continues until the whole frame is exposed. And the entire frame will continue to be exposed simultaneously until the gap is past and the opaque surface begins to obscure the frame in the same pattern.

If you're using a tiny gap like 22 degrees, then no probably the entire frame wouldn't be exposed simultaneously. But if you're up around, say, 45 degrees or more, then yes the entire frame will be exposed simultaneously, and at 180 degrees the entire frame is exposed simultaneously for a goodly amount of time.
 
To sum up in practical terms my experience with the issue is that it's basically a non-issue---bumping the camera and walking handheld, even running and fast panning doesn't exhibit rolling shutter artifacts anywhere close to the level of the HV20.

Yeah, it can slightly wobble and skew, but it really has to be coaxed out with ridiculously abusive shooting and then micro-analyzing stills from a moving medium.

But, oh yeah, where were we? Ahh...it's damn good in low light.
 
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Elton said:
But, oh yeah, where were we? Ahh...it's damn good in low light.
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Shooting 60p and at 1/120 at 0dB, I have to say the outcome of the footage is amazing. I would like to see the footage and compression outcome if the same shoot was at 24p and various shutter speeds, say 1/48, through to 1/120 and then 0db through to 18dB.

The existing footage has a little room to move with grading, however, soon the compression artifacts are revealed if you heavily brighten the image, as I did using a Curves filter. This is not unusual when grading this type of low light shoot. Perhaps shooting 24p and perhaps a combo of slower shutter speed and gaining up, will provide more detail in the darkest areas, and allowing better manipulation in post. Also, a comparison may reveal how robust the new 35mbps VBR codec is. Oh, and then there is all those other settings in the menu, including Cine 1-4 and individual settings, for those with a studio monitor with HD-SDI handy and a DIT approach to setup. Seems like a lot of testing to follow. Thanks Elton.
 
Barry_Green said:
Shutter speed has nothing to do with rolling shutter. A faster frame rate would help assist in preventing a rolling shutter effect (as in, rolling shutter is more prevalent at slower frame rates than at faster frame rates) but the shutter speed is irrelevant. You'll get as much rolling shutter at 1/2000th as you will at 1/24th.
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I found that shutter speed does affect rolling shutter, at least for partial exposure. I ran some tests with an HV10 and a camera flash unit and I found I got more partial exposure at higher shutter speeds than at lower shutter speeds. Very high shutter speeds would only have a bar across the frame showing the flash. At an extremely slow shutter speed, say 1/8th, the entire frame was lit and there was no partial exposure at all. Also, at lower shutter speeds, partial exposure doesn't necessarily happen all the time. I popped the flash 10 times at 1/30 shutter speed and half the time the entire frame was exposed, the other times it only exposed part of the frame. At normal playback speed, you could tell there was a partial exposure, but only briefly. It only occurs over 1, maybe 2 frames. Again, keep in mind this is the HV10 I'm talking about, not the EX1.
 
nice piece

nice piece

My head hurts encompassing 360 degrees. Nice article and thanks for educating us on the rolling shutter.

Jim Kinsey
 
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