pulsating in low light at DVinfo.net

Terry Lyons · November 15th, 2010, 09:33 AM

Hi all, I am shooting in lower light, all manual, and the image looks as if it is pulsating. like the iris is maby moving ever so slightlly open to close. I have noticed this under different light sources also.

Don Palomaki · November 16th, 2010, 12:11 PM

This can happen if you have discharge type lighting, an interaction with frame/field rate and shutter speed. What are the shooting and venue details?

Terry Lyons · November 16th, 2010, 05:38 PM

two places I noteced it. One in a race car trailer that had flouresent lights, and the other was in a parking lot that was lit by the tall poll lights that were there and a generator powered construction light on a trailer

Don Palomaki · November 17th, 2010, 05:27 PM

You have discharge type lighting. In discharge lighting the light output and color tends to not be uniform over the power cycle but has peaks and valleys.

What shutter speed are you using?

At least for NTSC in USA, line frequency is 60, field rate is 59.97, and generator frequency is usually about 60, but may depart significantly. At certain shutter speeds (usually faster than 1/60) you can get a bit of a beat effect that can cause the color balance and/or brightness to drift back and forth (up and down?) slowly, cycling over several seconds.

Terry Lyons · November 18th, 2010, 01:43 PM

Thank you Don, I was shooting at 1/48th of a second, both times. Does the florescent do the same?

Chris Soucy · November 18th, 2010, 03:41 PM

Yes and no. Some of the new fluoro's have electronic ballasts that run significatly faster than 120 fires a second, (twice for every single mains cycle).

It is that 120 fires a second that means that your shutter speed should be an even divisor of 120 - 1/60 or 1/30. This ensures you get 2 or 4 complete fire cycles per frame no matter what drift there might be in either the mains frequency or camera clock.

A high frequency gas discharge system can be several Khz from memory, so may not be a problem, or may need some other divisor.

CS

Terry Lyons · November 18th, 2010, 06:17 PM

Awesome Chris, I will definitely vary my shutter next time. Hopefully I see it in the viewfinder and not on the computer after the fact. Thanks Don and Chris for the education. TAG

Don Palomaki · November 19th, 2010, 05:17 AM

Chris hit the nail on the head. The variation you see would probably be worse if you shot at, say 1/250 where you see only a fraction of a power half cycle.

November 15th, 2010, 09:33 AM	#1
Terry Lyons Regular Crew Join Date: Feb 2004 Location: Alta Sierra Posts: 182	pulsating in low light Hi all, I am shooting in lower light, all manual, and the image looks as if it is pulsating. like the iris is maby moving ever so slightlly open to close. I have noticed this under different light sources also. __________________ TAG

November 16th, 2010, 12:11 PM	#2
Don Palomaki Wrangler Join Date: Sep 2001 Location: Northern VA Posts: 4,487	This can happen if you have discharge type lighting, an interaction with frame/field rate and shutter speed. What are the shooting and venue details? __________________ dpalomaki@dspalomaki.com

November 16th, 2010, 05:38 PM	#3
Terry Lyons Regular Crew Join Date: Feb 2004 Location: Alta Sierra Posts: 182	two places I noteced it. One in a race car trailer that had flouresent lights, and the other was in a parking lot that was lit by the tall poll lights that were there and a generator powered construction light on a trailer __________________ TAG

November 17th, 2010, 05:27 PM	#4
Don Palomaki Wrangler Join Date: Sep 2001 Location: Northern VA Posts: 4,487	You have discharge type lighting. In discharge lighting the light output and color tends to not be uniform over the power cycle but has peaks and valleys. What shutter speed are you using? At least for NTSC in USA, line frequency is 60, field rate is 59.97, and generator frequency is usually about 60, but may depart significantly. At certain shutter speeds (usually faster than 1/60) you can get a bit of a beat effect that can cause the color balance and/or brightness to drift back and forth (up and down?) slowly, cycling over several seconds. __________________ dpalomaki@dspalomaki.com

November 18th, 2010, 01:43 PM	#5
Terry Lyons Regular Crew Join Date: Feb 2004 Location: Alta Sierra Posts: 182	Thank you Don, I was shooting at 1/48th of a second, both times. Does the florescent do the same? __________________ TAG

November 18th, 2010, 03:41 PM	#6
Chris Soucy Inner Circle Join Date: Feb 2007 Location: Fairfield, Dunedin, New Zealand Posts: 3,682 Images: 18	Hi, Terry............ Yes and no. Some of the new fluoro's have electronic ballasts that run significatly faster than 120 fires a second, (twice for every single mains cycle). It is that 120 fires a second that means that your shutter speed should be an even divisor of 120 - 1/60 or 1/30. This ensures you get 2 or 4 complete fire cycles per frame no matter what drift there might be in either the mains frequency or camera clock. A high frequency gas discharge system can be several Khz from memory, so may not be a problem, or may need some other divisor. CS

November 18th, 2010, 06:17 PM	#7
Terry Lyons Regular Crew Join Date: Feb 2004 Location: Alta Sierra Posts: 182	Awesome Chris, I will definitely vary my shutter next time. Hopefully I see it in the viewfinder and not on the computer after the fact. Thanks Don and Chris for the education. TAG __________________ TAG

November 19th, 2010, 05:17 AM	#8
Don Palomaki Wrangler Join Date: Sep 2001 Location: Northern VA Posts: 4,487	Chris hit the nail on the head. The variation you see would probably be worse if you shot at, say 1/250 where you see only a fraction of a power half cycle. __________________ dpalomaki@dspalomaki.com