half-image flash effect at DVinfo.net

Kevin Shaw · December 5th, 2007, 09:48 AM

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?

Bill Spence · December 5th, 2007, 10:13 AM

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.

Kevin Shaw · December 5th, 2007, 10:31 AM

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.

Alex Leith · December 5th, 2007, 11:13 AM

When played back does the flashgun-rolling-shutter combination look weird? Or would people not notice?

Graeme Fullick · December 6th, 2007, 05:03 AM

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,

John Bosco Jr. · December 6th, 2007, 07:32 AM

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.

David Heath · December 6th, 2007, 08:24 AM

Quote:

Originally Posted by John Bosco Jr.

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.

Daniel Boswell · December 6th, 2007, 11:29 AM

Quote:

Originally Posted by Bill Spence

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.

Chris Medico · December 6th, 2007, 10:03 PM

Quote:

Originally Posted by David Heath

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.

Tim Polster · December 7th, 2007, 08:13 AM

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?

Alex Leith · December 7th, 2007, 08:18 AM

Horses for courses... I would definitely avoid using a camera with a rolling-shutter for stage shows with lighting effects.

Steven Thomas · December 7th, 2007, 09:05 AM

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.

Rusty Rogers · December 10th, 2007, 09:50 PM

Quote:

Originally Posted by Kevin Shaw

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?

Rusty

Kevin Shaw · December 10th, 2007, 10:22 PM

Quote:

Originally Posted by Rusty Rogers

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).

John Woo · December 10th, 2007, 11:29 PM

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.

December 5th, 2007, 09:48 AM	#1
Kevin Shaw Inner Circle Join Date: Sep 2004 Location: Sacramento, CA Posts: 2,488	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?

December 5th, 2007, 11:13 AM	#4
Alex Leith Major Player Join Date: Mar 2005 Location: Northampton, England Posts: 500	When played back does the flashgun-rolling-shutter combination look weird? Or would people not notice? __________________ Alex

December 6th, 2007, 05:03 AM	#5
Graeme Fullick Major Player Join Date: Mar 2005 Location: Newcastle, Australia Posts: 337	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, __________________ Graeme

December 7th, 2007, 08:18 AM	#11
Alex Leith Major Player Join Date: Mar 2005 Location: Northampton, England Posts: 500	Horses for courses... I would definitely avoid using a camera with a rolling-shutter for stage shows with lighting effects. __________________ Alex

December 5th, 2007, 10:13 AM	#2
Bill Spence Regular Crew Join Date: Nov 2007 Location: Indianapolis, IN Posts: 29	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.

December 5th, 2007, 10:31 AM	#3
Kevin Shaw Inner Circle Join Date: Sep 2004 Location: Sacramento, CA Posts: 2,488	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.

December 6th, 2007, 07:32 AM	#6
John Bosco Jr. Major Player Join Date: Jan 2007 Location: Pinellas Park Posts: 232	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.

December 7th, 2007, 08:13 AM	#10
Tim Polster Inner Circle Join Date: Dec 2004 Location: Arlington, TX Posts: 2,231	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?

December 7th, 2007, 09:05 AM	#12
Steven Thomas Trustee Join Date: Sep 2005 Location: Gilbert, AZ Posts: 1,896	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.

December 10th, 2007, 11:29 PM	#15
John Woo Regular Crew Join Date: Apr 2007 Location: Singapore Posts: 160	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.