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Old March 26th, 2009, 07:22 PM   #1
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5D Mark II Shutter Exposed! (Part II)

Jon Fairhurst did the only scientific study I've seen on the 5D2 actual shutter speeds in video on 12/28/08 here:

~ The Murder of Dirk Snowglobe - Article: 5D Mark II Shutter Exposed! ~

He referred to it here in this forum: http://www.dvinfo.net/conf/981873-post1.html

I've done some more work in this area. I was interested more in speeds I am using so I studied 1/30, 1/40, and 1/50. He studied that and much higher. I think Jon's conclusions were at least partially wrong.

His study had two problems:

1) He used a relatively slow turntable which gave small blurs.
2) He compared the blurs to still shots of given speed settings.

First let's look at problem 1. I improved the test by using a 1200 rpm fan instead of his 33 rpm turntable.

Here is my setup. I apologize for the pic quality but my backup camera wasn't available and I used my cellphone. The left shot is the hub of the fan with white duct tape on it and a black dot made with a magic marker. It is offset from the center by about a half inch.

The right photo shows my 5D2, a 35mm 1.4 lens, and a 20mm extension tube. The lens was an inch or so from the hub (a bit scary).



The 1200 rpm fan gives us 20 rotations/second. So a 1/20 shutter gave us a full circle, 1/40 a half circle, etc. I was able to get high resolution this way.

Problem 2 was unexpected. The blurs from the still shots showed a different blur pattern than that of the video. The reason is that at the slow speeds I was using, the still shot had three phases. First a curtain was opening, then the whole sensor was exposed, then a curtain was closing. You can see all three phases in a still shot. You can actually see the ramp up, full dot, and the ramp down. This is 1/40 sec with ISO 400 (all pics are locked at 5.6).

1 is curtain opening, 2 is exposed sensor, and 3 is curtain closing. It is a little more than 180 degrees because you really need to take the center of sections 1 and 3 to see true exposure.



I took the stills at 1/20 to 1/50 and all I really did was verify that the fan was exactly 1200 rpm. Note that now I am calibrated on the fan pictures, I can measure any speed without comparing anything to stills.

--------- results ---------

I locked the aperture at 5.6 and played with speeds and ISOs. The video frames had a normal fade-in and fade-out instead of the weird tails the still pictures had. It was easier to measure.

I sometimes went smoothly through medium ISO with a fixed speed and sometimes I played with the edges at 3200 ISO and the speed would hop around on each exposure lock. This was an attempt to find cases where the video disagreed with the readings between the locked readings and the real video.

Here is a list of the measurements I took overall. The speed on the left is the display when locking the exposure and the right is the measured speed. The multiple numbers on the right show that different frames from the same video showed different speeds! And the different frames were intermixed, not long stretches of one or the other. I just stopped the video at random times so I didn't look for patterns.

1\30 3200 ISO 30-40
1\30 3200 ISO 30-40
1\40 400 ISO 30-40
1\40 800 ISO 30-40
1\40 3200 ISO 30-40
1\50 800 ISO 30-40-50
1\50 1600 ISO 30-40-50
1\50 3200 ISO 30-40-50

Here are sample pics from 1/30, 1/40, & 1/50 ...



------------- conclusion -----------

It is easy to see why Jon thought he was getting intermediate speeds. In reality the camera seems to shoot at three different speeds intermixed (at low speeds). I'm measured these at 1/30, 1/40, 1/50, but they could be a little bit different due to my measurement error.

One might think these different exposure times would cause some kind of flicker, but that is not the case if it is really 30fps and it is only the exposure time changing.

I'm guessing the the buffer that takes the data is sometimes ready for more data and sometimes not. Then it would vary these speeds to get the overall light average it wants for the current ISO.

------------- future -----------

I could use premiere to step through frame by frame and look for a pattern.

I could use a 3600 rpm fan setting to see what happens up into the 1/60, 1/80, 1/125 ranges, but I don't care much about those speeds.
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Old March 26th, 2009, 10:31 PM   #2
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may be bullsh**

I didn't take into account the rolling shutter. It could make the resulting photos look wrong. I need to analyze this possibility. The blur are perfect arcs which makes them seem real, but they may not be.

This only affects the "results" section and below (a minor point <grin>).
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Old March 27th, 2009, 10:30 AM   #3
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I'm betting the rolling shutter is the source of the apparently variable shutter speeds. If the camera was really changing things from frame to frame it should be visible in the final image.

The rolling shutter is presumably constant though, so if you can find frames where the dot starts at the same place in each frame you should be able to make valid comparisons.

Also, isn't there a live-view still shooting mode which uses only the electronic shutter? I thought it was one of the modes designed for nearly silent shooting... if so you may be able to more accurately compare the electronic shutter signature to stills using that mode.

And thanks for doing these tests - I like this kind of stuff but don't have the time to do it myself!
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Old March 27th, 2009, 12:00 PM   #4
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Hi Mark,

This is excellent work. My shutter analysis only covered a small number of cases, so more analysis and more cases can help us to better understand what we can and cannot do.

My tests were all with Movie & Stills mode, highlight tone priority off and 100 ISO. We used long persistence Fresnel lights, so we didn't get light flicker. I checked many frames of video, and didn't see any measurable variation from frame to frame.

If you do more tests, it would be great if you go up to at least 1/80. For many of us, 1/60 is the holy grail (at 30 fps.) It's nice to bracket your work, so measuring 1/80 (rather than stopping the 1/60 target) is a good idea.

Keep us informed. This is fun stuff!
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Old March 27th, 2009, 03:59 PM   #5
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I am going to continue working on this when I get the time.

It is frustrating because it seems to me the only thing the rolling shutter could do is shorten the blur, which would make the longest blurs the valid ones. But this would make every test come out at 1/30 which is absurd.

I like the idea of looking at only ones whose starting point is at one place, like the top. I can try that quickly with the video I already have. I did look at the sequence of shutter speeds per frame on my video and it makes absolutely no sense with no discernible pattern.

My holy grail is 1/48. I want it perfect on film and on video the wrong rate will make it look like video, which it is. I can think of absolutely no reason why 180 degree shutter is special, unless you are running 24fps and want that exact film look.
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Old March 27th, 2009, 04:28 PM   #6
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My understanding is that in Live View there is no curtain. The CMOS returns information from rows of the chip, going across (the cause of "Rolling Shutter") That means that while the difference in time between "row 1" in frame 1 and frame 2 should be the same as the frame rate, if something was in row 1 in frame 1, but is in row 50 in frame 2, then the point in time at which it was in row 50 would actually be slightly longer than the frame rate.

I don't know how - or if - this would effect your specific test, but it could have some impact...

Also, can you be certain the fan is running at the speed indicated?

Last edited by Michael Murie; March 27th, 2009 at 05:23 PM. Reason: Changed 'columns' to 'rows'
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Old March 27th, 2009, 04:39 PM   #7
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Originally Posted by Mark Hahn View Post
My holy grail is 1/48. I want it perfect on film and on video the wrong rate will make it look like video, which it is. I can think of absolutely no reason why 180 degree shutter is special, unless you are running 24fps and want that exact film look.
On film, they use 180 degrees or 1/48, so that's our given baseline. My guess is that this isn't super sensitive - 1/46 or 1/50 (+/- 4%) is almost certainly fine. I'm not sure where we start noticing the difference.

With the 5D MkII at 30 fps and 1/48 (if we could even get that), we'd have the same open time as film, but a much shorter closed time (1/80). (IMHO, this looks too smooth, compared to film.)

With the 5D MkII at 30 fps and 1/80, we get a much shorter open time, but the same closed time as film. (Personally, I think this looks more like film. It's got a bit less motion blur, but we have more fps. We also don't have the additional time error introduced by 3:2 syncopation on 60i TVs.)

For reference...
1/30 = 33.3ms
1/45 = 22.2ms (my guestimate for the 5D MkII, though it's probably closer to 1/42 or 1/43)
1/48 = 20.8ms
1/80 = 12.5ms

Also...
1/45 @ 30 fps = 67% or 240 degrees (60 degree error)
1/80 @ 30 fps = 37.5% or 135 degrees (45 degree error)

Of course, 1/45 gives you another stop of light, which can be significant.

Still, a lot of 5D MkII low light stuff looks too blurry to me. For that reason alone, I prefer 1/80. Aside from personal taste, it makes sense. The 30 fps adds smoothness. The 1/80 adds judder. Combine them and you approximate film. On the other hand, 240 degrees adds smoothness on top of 30 fps, risking a too-smooth, soap opera look.

Hopefully a new firmware release will soon give us 24/25p and manual control, making these gyrations moot. :)
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Old March 27th, 2009, 05:02 PM   #8
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I only care about the amount of blur to make "film-look" aficionados happy. They won't be happy unless it is 24 fps and 180 shutter anyway and therefore 1/48 (1/50) is perfect for them.

Don't worry though, when I get my problems solved I will shoot some higher speeds also, I will also look at the shorter blurs for you (or I will let you look at them <grin>). I have not been doing any shooting near 100 because I was using 800 to 3200 to get my slow speeds. It is ironic that i need the high ISO to trigger the slow speed. I put my hand over the lens to do this.

The reason I am doing this is just to understand the behavior of my camera. My real holy grail is to understand it so well that I can control it so each scene looks like every other scene.

Later this year we begin shooting an indie that will be distributed by a firm that only sells cheap films overseas. I'm sure the distributor won't give a damn about any film-look. The only reason we the makers care is that we will be trying to make it good enough to be appreciated here (Hollywood) and maybe get us more work. For that the film-look might matter.
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Old March 27th, 2009, 06:24 PM   #9
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On We also don't have the additional time error introduced by 3:2 syncopation on 60i TVs.)
Syncopation - ha ha I love it!
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Old March 27th, 2009, 06:31 PM   #10
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Syncopation - ha ha I love it!
From Wikipedia:
"In music, syncopation includes a variety of rhythms which are in some way unexpected in that they deviate from the strict succession of regularly spaced strong and weak beats in a meter (pulse). These include a stress on a normally unstressed beat or a rest where one would normally be stressed."

Syncopation - Wikipedia, the free encyclopedia

That pretty much describes 3:2 pull down, doesn't it? :)
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Old March 27th, 2009, 06:57 PM   #11
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From Wikipedia:
"In music, syncopation includes a variety of rhythms which are in some way unexpected in that they deviate from the strict succession of regularly spaced strong and weak beats in a meter (pulse). These include a stress on a normally unstressed beat or a rest where one would normally be stressed."

Syncopation - Wikipedia, the free encyclopedia

That pretty much describes 3:2 pull down, doesn't it? :)
Since we have digressed, I'll add my 2 cents ...

You are aware that progressive 24 fps (i.e. film) frame-converted to 30fps interlaced can be converted back to the perfect original progressive 24 fps with 3/2 pull-down. Right? 3/2 pull-down is awesome thanks to the nice coincidence of timing.

Also, 60i doesn't sound right to me. 30i seems more accurate. Neither is in common usage.
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Old March 28th, 2009, 12:41 AM   #12
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Also, 60i doesn't sound right to me. 30i seems more accurate. Neither is in common usage.
60 sounds better to me, since I started my video career in the analog days. NTSC is "60" (okay, 59.94) and PAL is "50." Nobody ever talked about 30 and 25 back then.

That's probably because we had to deal with vertical sync and retrace 50/60 times a second. That one frame was odd and the other even was just a detail, as far as most equipment (aside from cameras, monitors and DVEs) was concerned.

Is it technically right? I'm not sure. When we wanted to be precise, we'd say "25 frames per second" vs "50 fields per second." We never said "25i" or "50i", since interlaced was a given.

BTW, today I was asked if I qualified for a senior discount. :(

I responded, "I hope not." I'm only 50 - does that make me 25? :)
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Old March 28th, 2009, 01:04 AM   #13
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60 sounds better to me, since I started my video career in the analog days. NTSC is "60" (okay, 59.94) and PAL is "50." Nobody ever talked about 30 and 25 back then.

That's probably because we had to deal with vertical sync and retrace 50/60 times a second. That one frame was odd and the other even was just a detail, as far as most equipment (aside from cameras, monitors and DVEs) was concerned.

Is it technically right? I'm not sure. When we wanted to be precise, we'd say "25 frames per second" vs "50 fields per second." We never said "25i" or "50i", since interlaced was a given.

BTW, today I was asked if I qualified for a senior discount. :(

I responded, "I hope not." I'm only 50 - does that make me 25? :)
I've qualified for senior discounts for years. And we only called things that appeared 60 times per second "fields" and two fields made a "frame". And yes, everything was interlaced.

I was at some standards committee meetings when HD was being developed and the young people were absolutely livid that interlacing was even being considered. They were right of course. To have interlacing you have to vertically filter to half the precision which destroys any bandwidth advantage of the interlacing in the first place.

I think interlacing originally happened because the TVs couldn't handle the scanning rate required. If not for that the standard we might now be 30hz progressive for everything, which would be awesome.
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Old March 28th, 2009, 02:05 AM   #14
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I was at some standards committee meetings when HD was being developed and the young people were absolutely livid that interlacing was even being considered.
We're clearly both young at heart - and honorary members of the Interlaced TV Stinks Society. :)

Quote:
I think interlacing originally happened because the TVs couldn't handle the scanning rate required. If not for that the standard we might now be 30hz progressive for everything, which would be awesome.
I like to think of interlace as the second form of video compression. (The first was Black and White. The third was vestigial sideband color.) Interestingly, each method had a 2:1 compression ratio - and none was lossless.

BTW, do you know why NTSC has setup? (For you whippersnappers, setup elevates black by 7.5 IRE above "back porch", or the reference level after the sync pulse.)

Setup saved one tube from the design of a television receiver. Yep. One stinking vacuum tube.

So instead of being able to "clamp" on the backporch, and know exactly where black is, we had to add an adjustment for almost every piece of studio gear that had to approximate where black was.

The design of NTSC, for all its shortcomings, was brilliant in many ways. Even interlace was pretty smart, considering the available technology. But setup was really, really stupid...
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Old March 28th, 2009, 04:21 AM   #15
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OK, enough digression.

I have some questions about how the rolling shutter does it's exposure.

It cannot just expose each row one at a time and then read that line out at that time. If it did this all horizontal motion would have an extreme case of "jello" bend. A frame of a person walking would have their feet much in front of their head (anyone remember Zap comix?).

It cannot do the entire exposure and then read the lines out or there would be no jello at all. I think this is how CCD sensors work.

If it always let all sensors collect light and went row by row reading and resetting the row, then 30fps would always be 1/30 exposure.

Does it take one pass emptying the rows one at a time and then a second pass reading them out one row at a time? That would work almost exactly like normal curtains in terms of exposure behavior. The exposure time would simply be the difference in time between the two passes. The amount of jello bend would be a function of how fast it can do a scan of either type.

I didn't realize how little I knew about cmos rolling shutter operation until just now. Can someone fill me in? I'll also head off to google-land. I can't understand my experiments unless I can understand this.
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