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Sony XDCAM EX Pro Handhelds
Sony PMW-300, PXW-X200, PXW-X180 (back to EX3 & EX1) recording to SxS flash memory.


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Old December 5th, 2007, 09:48 AM   #1
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half-image flash effect

I just took a close look at some EX1 footage I shot this weekend and am seeing the effect I've heard about for other CMOS cameras where only half the image (top or bottom) is lit for two successive frames when a photographer's flash goes off. Given that this is footage shot in 1080i mode, what exactly does that represent in terms of the technology used by the camera, and is there any way to avoid this effect?
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Old December 5th, 2007, 10:13 AM   #2
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Not really. On a CMOS with a rolling shutter, one row of pixels is read at a time during the exposure (instead of the whole chip at once with a global shutter). Depending on your shutter speed, when the flash goes off, you are going to have a couple of frames of overexposure. But you are invariably going to have one frame where the flash was on during the first pixel-row readings, but off during the remaining pixel-row readings. Thus, you get half a frame over-exposed and the other half is normal. I don't think there as any way around this. I have not seen it during playback yet, so I don't know how severe it looks, but I doubt it is as bad as looking at a still picture.
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Old December 5th, 2007, 10:31 AM   #3
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I saw a post elsewhere from someone who said none of his wedding video customers have commented on this for footage from a Sony V1U, so I suppose how you feel about it depends on how picky you are. I didn't notice at first until I went looking for this, but now I see it clearly. By the way, it didn't happen for all flashes so I guess it relates to the duration and timing of the flash relative to the frame timing and shutter speed.
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Old December 5th, 2007, 11:13 AM   #4
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When played back does the flashgun-rolling-shutter combination look weird? Or would people not notice?
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Old December 6th, 2007, 05:03 AM   #5
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Kevin,

I put a 70-90% transparency white filter over the frame in post and all is cured. It just looks like a normal flash. I have been doing this for all sorts of frame crashes (CCD's and CMOS) from compression artifacts for some time with great success. I am using Avid Liquid as my editor, which re-renders the mpeg long GOP automatically and eliminites the problem. YMMV with other editors, but I am sure that it will help.

All the best,
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Old December 6th, 2007, 07:32 AM   #6
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I did this project for a client a few months ago in which one of the scenes employed a number of photographer flashes. Of course, I was very concerned with the rented V1 I was using, and, unfortunately, the camera didn't dissappoint. Rolling Shutter distortion was evident whenever the flashes went off, which was often. Of course, the client comment about it saying that "It looked weird." I told him camcorders never handled photographer flashes well, and high definition just pronounces the problem. Nothing else was said. So, it was noticed by a fairly knowledgeable client, who was the producer, but it wasn't that big a deal. Thus, I don't think Joe or Jane public will really notice it. Of course, that doesn't ease my dissatisfaction with Sony for not employing a global shutter in the EX-1.
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Old December 6th, 2007, 08:24 AM   #7
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So, it was noticed by a fairly knowledgeable client, who was the producer, but it wasn't that big a deal. Thus, I don't think Joe or Jane public will really notice it.
Speaking from a position of some ignorance, it does occur to me that what was noticed by you and your producer was a rolling shutter effect from *a V1*. My expectation is that from *an EX1*, the effect should be lessened, if still present. I don't think you can tar all rolling shutters with the same brush as used for lower end cameras.
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Old December 6th, 2007, 11:29 AM   #8
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Originally Posted by Bill Spence View Post
Not really. On a CMOS with a rolling shutter, one row of pixels is read at a time during the exposure (instead of the whole chip at once with a global shutter). Depending on your shutter speed, when the flash goes off, you are going to have a couple of frames of overexposure. But you are invariably going to have one frame where the flash was on during the first pixel-row readings, but off during the remaining pixel-row readings. Thus, you get half a frame over-exposed and the other half is normal. I don't think there as any way around this. I have not seen it during playback yet, so I don't know how severe it looks, but I doubt it is as bad as looking at a still picture.
Yep, this is a function of the CMOS chips...I shoot weddings with my V1s and see this every time.
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Old December 6th, 2007, 10:03 PM   #9
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Originally Posted by David Heath View Post
Speaking from a position of some ignorance, it does occur to me that what was noticed by you and your producer was a rolling shutter effect from *a V1*. My expectation is that from *an EX1*, the effect should be lessened, if still present. I don't think you can tar all rolling shutters with the same brush as used for lower end cameras.
The effect might be lessened but will still show up. The duration of a camera flash is SO short its not reasonable to think reading the CMOS imager is going to get that fast in the near term.

Be prepared to cover the flashes with some post production magic. Its dead simple to do and your clients won't know the difference.
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Old December 7th, 2007, 08:13 AM   #10
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To me wedding flashes would not be that big of a deal, but what if you are taping an event where they put a strobe effect on a stage?

This seems like a nightmare situation for a rolling shutter?
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Old December 7th, 2007, 08:18 AM   #11
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Horses for courses... I would definitely avoid using a camera with a rolling-shutter for stage shows with lighting effects.
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Old December 7th, 2007, 09:05 AM   #12
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Hmm.
I'll be the one that can answer that question.
I'm going to grab two of my moving heads (MSD lamps).
They have the full gamut, gradient color changes, strobe.

I will post my results.
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Old December 10th, 2007, 09:50 PM   #13
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Quote:
Originally Posted by Kevin Shaw View Post
I just took a close look at some EX1 footage I shot this weekend and am seeing the effect I've heard about for other CMOS cameras where only half the image (top or bottom) is lit for two successive frames when a photographer's flash goes off. Given that this is footage shot in 1080i mode, what exactly does that represent in terms of the technology used by the camera, and is there any way to avoid this effect?
Kevin,
How bad is this in Slo-mo?

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Old December 10th, 2007, 10:22 PM   #14
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How bad is this in Slo-mo?
It's very obvious in slow-mo, but I understand you can mitigate this by throwing a white clip with some transparency over the affected frame(s).
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Old December 10th, 2007, 11:29 PM   #15
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I cover fashion events quite often. What will happen if I used a camera with CMOS rolling shutter when there's 20-30 camera flashes firing at the catwalk models at the about the same time? Will this be very disastrous?? I owned a FX1 at the moment was thinking of switching to tapeless next year and EX1 is on my top list.
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