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Neil Duffy

Mechanical Shutter

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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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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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???

 

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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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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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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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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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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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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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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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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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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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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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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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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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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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

 

If we agree that there is skew and that it's caused by a slow chip read, then a mechanical shutter wouldn't fix the issue, because when the shutter is open, the weak link would still be the chip.

 

Now, maybe we don't agree that there is skew, but from my experience, there is major skew in the Red footage. As I said, I'm a fan, but I was surprised at how much there is. But that's my experience.

 

Chris

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Hi Chris, I guess you may be right.

 

maybe the Nikon D3 with (12 ?) taps on the chip is just doing a fast read ?

 

Wherever the reset "is", I haven't found it. I thought maybe the shutter was hiding it, or maybe it is an effective global shutter.

 

Curious....

 

-Sam

 

ps not denying the Red has skewing issues (wouldn't deny D90 in video mode either !!)

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Hi Chris, I guess you may be right.

 

maybe the Nikon D3 with (12 ?) taps on the chip is just doing a fast read ?

 

Wherever the reset "is", I haven't found it. I thought maybe the shutter was hiding it, or maybe it is an effective global shutter.

 

Curious....

 

-Sam

 

ps not denying the Red has skewing issues (wouldn't deny D90 in video mode either !!)

 

I think the solution will come via faster read chips and processors, verses a mechanical fix. I can't imagine Red wanting to go that route. Especially at their biz model / price point. There's speculation that their new models will already see an improvement.

 

I always admit to loving the Red, but when I see (not meaning you) folks bash the D90 or other cams as having horrible skew and making fun of it, I wonder if they are really being honest about the Red, and doing a bit of a white-wash because they are such fans or they are justifying buying the Red.

 

In a best case scenario we could be fans of the Red and yet speak truthfully about it's shortcomings. It's a first generation camera from essentially a ground-up design. There's bound to be short-comings and challenges.

 

Just my perspective.

 

Chris

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From what I have seen, there is skew with pans on the RED. It was discussed at the time of the Peter Jackson clip release. The cause of the skew was the way the rolling shutter captured the shot. It did not capture the whole shot at once. Rather it captured like a scan - from top to bottom. During a pan, this caused a sort of bending of the image - as the top part of the image was captured before the bottom part.

 

This scan is the cause of the skew. Not necessarily the speed of the frame capture. A slow full frame capture would give a blurry image, not skew.

 

Please correct me if I am wrong. But I have not seen this skew with the Canon cameras in photography mode (in video tap mode I have seen it). I believe the Canon and Nikon cameras, when they are in photography mode, are using mechanical shutter, and don't have skew. The image might be blurry if the capture is slow. But no skew.

 

I am wondering if there is an inexpensive solution to this. A mechanical shutter, ala the Viper, might work. But, the chip would still have to all "on" when the shutter is open. And then reset its scan when the shutter is closed. That would have to be a very fast chip. :D

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From what I have seen, there is skew with pans on the RED. It was discussed at the time of the Peter Jackson clip release. The cause of the skew was the way the rolling shutter captured the shot. It did not capture the whole shot at once. Rather it captured like a scan - from top to bottom. During a pan, this caused a sort of bending of the image - as the top part of the image was captured before the bottom part.

 

This scan is the cause of the skew. Not necessarily the speed of the frame capture. A slow full frame capture would give a blurry image, not skew.

 

Please correct me if I am wrong. But I have not seen this skew with the Canon cameras in photography mode (in video tap mode I have seen it). I believe the Canon and Nikon cameras, when they are in photography mode, are using mechanical shutter, and don't have skew. The image might be blurry if the capture is slow. But no skew.

 

I am wondering if there is an inexpensive solution to this. A mechanical shutter, ala the Viper, might work. But, the chip would still have to all "on" when the shutter is open. And then reset its scan when the shutter is closed. That would have to be a very fast chip. :D

 

You are half right. CMOS does "scan" the sensor instead of capture the scene at one instant. And there were skew issues with Peter Jackson footage shot on the 1st prototypes "Boris" and "Natasha" two years ago.

 

All CMOS are not the same. What matters is the read-reset time. The RED prototypes had a very slow (like most CMOS sensors) read-reset time (skew). We have been able to speed up that time dramatically since then. No one complains about the skew on the new firmware builds. They are now 3.5 times faster than they were. And the next generation, Scarlet and Epic, are even faster. About the same as a current film camera. Technically there is still skew in a film camera, just not enough to warrant a discussion. The N90 and 5D II have CMOS sensors that still have a very slow read-reset time... 3-4 times slower than a current RED ONE. That is why there is a LOT more skew when shooting motion with those two.

 

Read-reset times are independent of frame rates. Scanning a CMOS sensor is similar to a mechanical shutter "wiping" the film if the read-reset times are fast enough. Both of these have a "look" that is not the same as a global shutter. There is a "feel" to this wipe/scan time. Most people like it. Too slow read-reset time gives a bunch of skew... which is obviously not good.

 

The biggest problem with the discussion of CMOS sensors is that most people want to lump them all together. There are a million differences between them all, from pixel design, pixel size, A/D conversion, read-rest times, etc. Future discussions about CMOS will hopefully be more detailed, because the details matter. Sweeping generalizations don't work.

 

Jim

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