Found the link: http://www.8112studios.com/nyc.mov

The thread: http://www.dvxuser.com/V6/showthread.php?t=121427

Look forward to further comments, now that the full clip is available to assess. I will remove my frame grab, in due course.

I won't change my comment except to say that looking at that shot, it might be an even harsher flickering source that is creating the multiple exposure - perhaps old fluorescents from a store.

Also, it is clear from the footage that the shutter is indeed turned off in the camera, because the blur from one frame connects with the blur on the next.

And so the 120Hz/5 images per frame flicker explanation makes sense.

Eric, this is very interesting information and discussion. Thank you.

That's what i was talking about - a flickering light source. NYC (and most every city) is lit with fabulous sodium vapors which flicker 120 times per second.

I would guess that that clip was shot at 24p with shutter off - hence five cycles of the light per frame. Notice that the cab's taillight, at 12VDC does not exhibit the same stepping (because it does not flicker).

This would happen on any video camera.

Eric, the following Cloverfield article affirms your position that sodium-vapour lighting is the cause, and I refer you to the penultimate paragraph of the article:
http://www.videography.com/articles/article_15469.shtml

"When you do a fast pan with the Viper or the F23 under sodium-vapor lighting, you get a triple image," the cinematographer notes. "You get the image itself and then two trails."

Indeed, Viper and F23 suffer similarly. With the hand held approach in that movie, I can see a lot frames suffering with this issue. Cloverfield 'team roto' post nightmare for VFX crew.

Well there you go.

The reason they get three images (really 2.5) instead of 5 is that they were using 1/48 second shutters, as any feature filmmaker would.

So, armed with this knowledge, one could use 1/125 shutter speed and eliminate any multiple images.

Hmm. 1/125 sounds dicey. It's worth a test, and would depend on the duration of the light's "on" cycle vs. "off" cycle

If there are any incancesdents at all in the shot, I would stick to 1/48 to get the natural blur from those and let the flickering lights to their steppy thing.

Eric I think you are right, it is pretty much impossible to predict the exact flicker rate of a light source (especially in a place with so many different types) so there is no arbitrary safe speed, but if you're in a room with a single source you may be able to phase it out using the ECS function. Also, I would go beyond saying that it would happen with any video camera, it will happen with film, both motion picture and still.

-Sean

Are any of you guys musicians? If you are, then you must understand harmonics. Harmonic principles in physics are what causes resonance and can generate amplification.

The same happens with any frequencies when they beat against each other. Two matching frequencies provide the most noticeable results(harmony)...as in the frequency of the fluorescent matching the frequency of the scanner. If you can get sufficiently above the nyquist frequency, which is 2x the base frequency,the beating goes away....go figure. In this case, the base frequency is 120Hz. Nyquist is then 240 Hz. I think 1/250 will do quite nicely. Of course, then you start experiencing other effects like wheels appearing to rotate backwards.

Bill,

I think you are confusing shutter speed with frame rate. Since frame rate is constant, frequency (of the camera) is constant.

If a wave metaphor applies at all to a camera, changing the shutter speed changes the shape of the wave, not its frequency. Like changing the duty cycle of a square wave perhaps?

1/250 would certainly eliminate the multiple images, but it would also render the exposure unpredictable because it could be photographing the lights on or the lights off, depending on how the camera is phased to the light source. But as Sean said, you could dial it in, just like when you shoot a CRT television.

Edit: But all of this goes to hell with the rolling shutter as Bob mentions below.

Sean,

Of course you are correct. This would happen on any camera, conditions being equal.

I'm with Eric on this issue, 1/250 of a second with any shutter (rolling or global) would cause problems. With a rolling shutter, as it "moves down" the lights blink ON-OFF-ON-OFF, thus we'd have 2 bright bars and 2 dark bars.

With a global shutter, the overall brightness of the image would rise and fall as the camera's shutter opens and closes in and out of sync with the times the lights are bright.

A time of 1/120 of a second would be best. That way, any part of the image would be exposed for exactly 1 cycle.

IF I want to sample audio, 2x minimum would be the rule, but anything higher would always be better. However, in this case, we are not generating a waveform from our samples, so Nyquist does not apply here.

So, it's best to match the exposure time to the time of one cycle, 120 Hz or 1/120 of a second.


Sincerely,

Bob Diaz

Well the rolling shutter finally reared its ugly head. Had a shoot last night of a live band, big crowd and with it lot's of camera's. I hate to say it but there are quite a few shot's that would have otherwise looked pretty decent that are instead "wtf" moments if anyone else were to look at em. Like the police car, the whole line across the frame with have of it being bright is a lot more painfully obvious than I had hoped it would be just reading about it and not experiencing it myself. From what I'm reading there really aint a whole lot I can do about this is there? I shoot bands quite often so a wee bit concerned.

- Ray

This certainly sounds like the camera's weak point.
I'm interested to hear more details about your footage.

Did it screw up your whole shoot?
Was it just still photographers or di the bands lighting also set off problems.

Please let us know if you find any "quasi fix" in post, maybe just whiting out the whole image? Do the frames with opposite half exposures match up so you could blend them? (I imagine they wouldn't be perfect though.)

I was watching the news the other night and they were showing some paparazzi footage in the upper corner. i can not say for sure if it was from an EX1 but they kept looping a clip of some poor celebrity and the whole thing was showing over exposed bands as the flashes went off around the scene.

Makes me glad I will be shooting most of my work outside in broad daylight.

Nah, didn't screw up the whole shoot (if it did I need to find a new job!). The stage lights had no issue what-so-ever but what it did mess up a fair amount of lot of shots from the stage that were pointed toward the crowd and several that didn't have the camera/crowd at all but someone took a pic. Gotta love camera phones. Only fix I'm aware of is go to another shot. As one can expect from this cam so much of the footage is simply stunning but it's disheartening to have this one painful caveat.

Will post some screen grabs as soon as I'm able.

- Ray

Raymond,

I'm heading out to Austin to shoot some SXSW parties and shows in March, so I'd love to hear your experience with shooting live bands in the coming weeks.

I'm also trying to figure out what to do with the bands/labels/production companies who don't need an edit, but just want raw footage when I'm done...but I guess that's for a different thread!

I know it's a pain, but have you tried fixing this in post?
Obviously, this would be crazy if there were non stop flashes.

I guess you'll have to minimize your shots directly into the audience.

I use a 80-90% transparency white frame over the top to fix the flash problem. Works perfectly every time in Avid Liquid - should work just as well with other editors. I know that it would be a lot of work with constant flashes - but this is pretty rare - even at weddings.

Graeme,

Is that 80-90% transparency or 80-90% opacity?

Is is mostly white or mostly transparent? I would think you would want something mostly white to mask the partial exposure nastiness.

Graeme, Could you post before and afters and explain for the dummies about post (me) exactly what your talking about.

Sorry Eric,

Yes that should read opacity - writing in too much of a hurry.

Leonard,

I just paste a 80-90% opacity white frame over the offending frames, and it looks on playback like a normal flash from a still camera. I have found that a photographers flash is almost always 2 frames long, so I have set up a short cut in liquid which is a 2 frame 80% opacity white frame, which I simply pull straight over the top of the offending footage. You can even set it up so that it matches the half a frame (in CMOS cameras such as my Sony A1 and the EX1) if you wish by setting it up cropped to match the offending frame (i.e. the white opaque bit only crops over the part of the frame that is not flashed - but this sometimes requires a bit more fiddling to get the intensities of the 2 halves well matched). In Liquid this takes all of 10 seconds to accomplish. I have been using it for 2 years to cover the frame crash associated with HDV when it encounters a photographers flash (an ugly square pixellation effect) - and have yet to have a single negative comment about how it looks on any of my finished work. Liquid is a native MPEG2 editor, and actually fixes these HDV artifacts if you make it re-render them with a partially transparent clip over the top. I often use a 100% transparent white frame to repair HDV codec crashed associated with extreme motion. It does cost a tiny bit of resolution, but fixes the HDV codec crash artifacts extremely well.

If you just look at the still frames, you can often see that all of the frame is not a uniform shade of white from the flash, but at normal playback speed it is entirely imperceptible (and that is what matters), as clients almost never watch a video frame by frame!

Hope this helps.

I've just been reading a CML thread about the RED's rolling shutter. It was pointed out that some CMOS cameras like the Dalsa, Phantom V and Arri D20 don't have this effect. It describes how the Dalsa has two layers, the first of which collects the light, then the light information is dumped down to the second layer (blind to light), where it's removed in packets in a rolling shutter style. The net result is that the Dalsa CMOS sensor acts like a global shutter.

It would seem that these global shutter CMOS sensors are more complex to produce, so the rolling shutter found on most CMOS cameras is a cost trade off.

I've just been reading a CML thread about the RED's rolling shutter. It was pointed out that some CMOS cameras like the Dalsa, Phantom V and Arri D20 don't have this effect.

The DALSA cameras actually don't use a CMOS censor. It's a CCD with a spinning mirror shutter to complement the optical viewfinder.

I.

Illya Friedman
DALSA
Digital Cinema Division
www.dalsa.com/dc

sorry to resurrect this old thread, but i was playing around with a EX1 at Abel today and am buying one very soon, but while i was there i tried zooming in and whipping the camera back and forth while zoomed in. i thought we had concluded earlier in this thread that the cmos pixel readout rate is constant no matter the format (1080/24p 720/60p etc), but in my quick test i noticed that there was a lot of skew present when shooting at 1080/24p HQ, but skew effects were almost unnoticeable when shooting at 720/24p overcranked to 60p. i was viewing this on a panasonic (17" i believe) HD monitor through SDI, and at 720/24p overcranked, it appeared to be outputting 720/60p over HDSDI. at 1080/24p skew was very noticeable, more so than i remember it being when testing the ex1 previously, a huge contrast to 720/60 where the image had very smooth 60fps motion and what appeared to be much much less significant skewing. were the tests wrong? any other ideas would be very much appreciated. could it have been old firmware?

i didnt get to test it that extensively, but if someone who has a camera readily available could do further testing, it would be very interesting to see. if not, i may go back and do some more careful testing and make sure i check the firmware version as well as all the other settings.

From the user tests I've read, setting the camera to any of it's normal rates 1080/720 24/25/05/60 all write the normal 35mbps to card. However real overcranking actually writes closer to 70mbps to the card and may burst higher.

Obviously, this affects record times, so that must be taken into account, but it was a very nice bit of information to get.

im not sure how that relates. were you responding to me? im not talking about the cards or compression at all, this was realtime live HDSDI feed to an HD monitor. if anyone can do further testing or verify that they noticed more skew at 24p than 60p i would be very interested in it.