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Mechanical Shutter


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#1 Neil Duffy

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Posted 17 February 2009 - 11:59 PM

I noticed that EPIC continues with the rolling shutter. What are the advantages of a mechanical shutter over a rolling shutter? Is it worth the costs? :unsure:
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#2 Jim Jannard

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Posted 18 February 2009 - 01:10 AM

4 m/s vs. 5 m/s.

Jim
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#3 Keith Walters

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Posted 18 February 2009 - 07:07 AM

I noticed that EPIC continues with the rolling shutter. What are the advantages of a mechanical shutter over a rolling shutter? Is it worth the costs? :unsure:


"Is it worth the costs?"
Since it would probably at least double double the price of the camera, I would say definitely not.

Anyway rolling shutter and mechanical shutter refer to two different things.

The advantages of a mechanical shutter are various, depending on what sort of camera it is.
In a film camera you obviously have to have one to the stop the film from being exposed while it's moving.

With CCD cameras, the pattern of electric charges moves in a film-like manner during frame readout, and so either:

You have to either have a lot of extra electronics on the chip to prevent the charge image being contaminated during readout, (which reduces light sensitivity, since silicon real estate that could have been used to gather photons has to be used for "housekeeping")

Or you can have no anti-smear circuitry at all and simply depend on a film-type rotating mechanical shutter, which is what Dalsa do (did) with the Origin.

Another advantage of a mechanical shutter is that you can put a mirror on the back of it to reflect light onto a groundglass screen when it's blocking light to the film, which gives you a viewfinder which needs no power to operate, has extremely high resolution, and allows you to see outside the frame actually being captured.

Yet another advantage is that because the shutter blade is normally a long way from the film emulsion. its edges are out of focus, and so there is a less abrupt transition from one frame to the next. 24p or 25p from a video camera with a 180 degree electrionic shutter does not look like something shot on film, for this and other reasons.

CMOS sensors work on a different principle to CCD, and they don't suffer from readout smear, so a mechanical shutter is not strictly necessary

However Arri's D21 camera which uses a CMOS sensor does have a shutter, but that's mostly because the D21 is basically an Arri 435 film camera body that's been modified to take an electronic sensor. The mirror shutter was already there so they made use of it.

With a CCD camera sensor, the charge image (ie the pattern of electric charges corresponding to the light image) is moved off the sensor all at once, similar to a frame of film.

With a CMOS sensor, this is not possible; some parts of the frame are read out before others which can produce image skew on fast moving images, although I think the extent of problem has been somewhat exaggerated.

The latest versions of the RED operating system are supposed to address this problem, although I'm not exactly sure how they would go about it.

A mechanical shutter would eliminate the skew problem as well, but the extra cost would never justify the small improvement gained. Most people would probably prefer the money was spent on improving other aspects of the camera, or just not spent:-)
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#4 Keith Walters

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Posted 18 February 2009 - 07:09 AM

4 m/s vs. 5 m/s.

???
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#5 Matthew Rogers

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Posted 18 February 2009 - 10:14 AM

???


I think Jim means that the mechanical shutter runs at 4ms vs their rolling shutter at 5ms in the new cameras. Personally, I haven't shot anything yet where I have noticed the rolling shutter. But then again, I don't shoot the Bourne movies!

Matthew
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#6 Mike Thorn

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Posted 18 February 2009 - 10:31 AM

Thanks Keith, great post.
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#7 Keith Walters

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Posted 18 February 2009 - 02:45 PM

Thanks Keith, great post.

ActuallyI should clarify something I forgot about.

Most of the discussion about mechanical shutters has been about the reflex mirror type as used with Arri Film cameras. Because the image reflected off the mirror is used for primary focus, the whole assembly has to be made to extremely close tolerances and is thus very expensive to build.

Some video cameras made by Philips/BTS used a much simpler (and cheaper) non-reflecting "bow itie" shutter driven by a small motor. Of course this doesn't give the much-desired optical viewfinder, but it would completely remove the rolling shutter issue.
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#8 Phil Rhodes

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Posted 18 February 2009 - 05:08 PM

Some video cameras made by Philips/BTS used a much simpler (and cheaper) non-reflecting "bow itie" shutter driven by a small motor. Of course this doesn't give the much-desired optical viewfinder, but it would completely remove the rolling shutter issue.


Viper, too, I suspect.

P
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#9 Keith Walters

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Posted 18 February 2009 - 06:59 PM

Viper, too, I suspect.

P

Viper as well.
PV UK have an informative brochure on the Viper here
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#10 Igor Trajkovski

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Posted 18 February 2009 - 07:08 PM

From PANAVISION Australia ---> Viper Mechanical Shutter Info (PDF)

Best

Igor
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#11 Neil Duffy

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Posted 19 February 2009 - 12:48 AM

Thanks Jim, Keith and Igor for the info. Keith, that was the most concise description of the differences between rolling and mechanical shutters in digital cameras I have read. I never understood it before.

If you are mainly after skew, the Viper approach looks the most cost effective. But skew will probably be less on an issue with EPIC's as they advance.
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#12 Keith Walters

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Posted 19 February 2009 - 05:30 AM

If you are mainly after skew, the Viper approach looks the most cost effective. But skew will probably be less on an issue with EPIC's as they advance.

Like I said, I don't think it's an issue at all.
It's something you can find if you look for it, but the general public would never notice it.
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#13 Phil Rhodes

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Posted 19 February 2009 - 08:08 AM

I have seen Red footage in which the apparent wobble due to skew was quite severe. It's probably at least as much of a problem as judder in fast pans that affects all 24p imaging, although of course Red has that problem too. At least they can be solved in the same way.

Bear in mind that Viper doesn't have a mechanical shutter to avoid skew, it has a mechanical shutter because it needs to offload its sensors in the dark. It has frame-transfer CCDs, and if it didn't have a mechanical shutter, you'd get vertical streaking, Saving Private Ryan style. This is actually visible when you first turn the camera on, before the mechanical components have had a chance to get in sync with the electronics.

P
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#14 Mike Thorn

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Posted 19 February 2009 - 09:53 AM

I have seen Red footage in which the apparent wobble due to skew was quite severe. It's probably at least as much of a problem as judder in fast pans that affects all 24p imaging, although of course Red has that problem too. At least they can be solved in the same way.

Phil,

Just curious...what firmware build was being used at the time?
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#15 Keith Walters

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Posted 19 February 2009 - 05:31 PM

Bear in mind that Viper doesn't have a mechanical shutter to avoid skew, it has a mechanical shutter because it needs to offload its sensors in the dark.
P

Quite so. What I meant was, the same shutter mechanism applied to the RED etc would nail the skew problem for once and for all.
(Although it would probably be worth it to stop the fanboys fixating on that particular non-issue and redirect their talents back to solving the real problems the RED has :o)

Well maybe not. Too many of them have their hearts set on an optical viewfinder, which would send the price into a polar orbit.
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#16 Matthew Rogers

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Posted 19 February 2009 - 07:06 PM

Quite so. What I meant was, the same shutter mechanism applied to the RED etc would nail the skew problem for once and for all.
(Although it would probably be worth it to stop the fanboys fixating on that particular non-issue and redirect their talents back to solving the real problems the RED has :o)

Well maybe not. Too many of them have their hearts set on an optical viewfinder, which would send the price into a polar orbit.


What about putting a cheap mechanical shutter in the RED, but still doing the monitoring via electronics? No optical focusing, but lose the rolling shutter. Personally, my eyesight is not all that great for optical focusing, but the 1:1 zoom on the RED is fantastic for making sure you have nailed focus before you start shooting.

Of course, if RED did switch to a mechanical shutter, wouldn't all these new digital lenses (like the Agunx DP's) no longer work on the RED?

Matthew
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#17 John Sprung

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Posted 23 February 2009 - 06:41 PM

What about putting a cheap mechanical shutter in the RED, but still doing the monitoring via electronics? No optical focusing, but lose the rolling shutter.


The rolling shutter isn't so much a shutter as it's the timing of the readout from the CMOS chip. In order to use a mechanical shutter, they'd have to be able to read out much faster, which would give the desired result even without a physical shutter

The only reason for a mechanical shutter would be to use it for an Arri style mirror reflex viewing system. The number of machinists still alive who are capable of working to that degree of precision is probably in the dozens to low hundreds.




-- J.S.
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#18 Chris Pickle

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Posted 23 February 2009 - 06:51 PM

The rolling shutter isn't so much a shutter as it's the timing of the readout from the CMOS chip. In order to use a mechanical shutter, they'd have to be able to read out much faster, which would give the desired result even without a physical shutter

The only reason for a mechanical shutter would be to use it for an Arri style mirror reflex viewing system. The number of machinists still alive who are capable of working to that degree of precision is probably in the dozens to low hundreds.
-- J.S.


Exactly. Even if they had a mechanical shutter, the chip read speed still couldn't keep up with an "open" shutter.

I love the Red and it has given my latest feature film a great level of production value, but the skew shows up way more then I thought it would. Parts of my film are busy handheld, but not crazy cam. The skew shows up even in very calm pans.

Again, I love the Red.... but...

Chris
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#19 Matthew Rogers

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Posted 24 February 2009 - 09:07 AM

The rolling shutter isn't so much a shutter as it's the timing of the readout from the CMOS chip. In order to use a mechanical shutter, they'd have to be able to read out much faster, which would give the desired result even without a physical shutter

The only reason for a mechanical shutter would be to use it for an Arri style mirror reflex viewing system. The number of machinists still alive who are capable of working to that degree of precision is probably in the dozens to low hundreds.


That's what I thought, but I was thinking aloud. I think it won't really matter with the Scarlett and Epic because it has sounded like they are getting the chips up to a much faster refresh rate.

Matthew
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#20 Sam Wells

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Posted 24 February 2009 - 10:30 AM

Exactly. Even if they had a mechanical shutter, the chip read speed still couldn't keep up with an "open" shutter.

Chris


I'm not so sure - my Nikon w/ CMOS can do up to 9 fps Full Frame & 11 fps DX frame & I see no significant skewing... admittedly higher frame rates there would be mechanical issues with that type of shutter (but a focal plane disk could solve them) but I don't see why you'd have skewing / reset artifacts at say 24 if you don't have then at 10 or 11, just more frames

-Sam
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