exposure issue: light oscillation at DVinfo.net

Vince Denali · June 27th, 2003, 04:57 PM

Hi,

I was taping a Muay Thai (Thailand style) kickboxing event with my Sony vx-2000. The ring was illuminated by three, dome-shaped high output flourescent lights mounted on the ceiling. I'm not sure if they are HMI, but they were pretty quiet. Similar looking units at Home Depot buzz like killer bees. Anyway, in AES mode (shutter priority), I noticed a brightness oscillaton with a period of about four seconds. As I increased the shutter speed from 1/30 to 1/1000, it got worse. The period didn't increase, but the peaks and troughs of the brightness oscillation deepened. When I switched the camera to fully Automatic operation (manual focus, though) or AEA (aperture priority mode),
the oscillation largely went away. I'm not sure if the oscillation was caused by the mismatch of my video frame rate and the flourescent light frequency or that the three lights were not in sync with each other. What should I do in the future to avoid the brightness oscillationr? I want a faster shutter speed to reduce the blur of the fast motion.

Mike Rehmus · June 27th, 2003, 06:16 PM

I've had faster fluctuation when shooting in the UK with a NTSC camera. PAL camcorders have different default shutter speeds to avoid the problem induced by the 50 Hz power-line frequency.

A 4-second period seems to not fit into the concept of power-frequency vs video frequency hetrodyning in normal circumstances. But we've all seen it happen to the images of spoked wheels that are rotating. Could it be that the lights were run at a higher frequency than power line?

I hope someone knows the solution.

June 27th, 2003, 04:57 PM	#1
Vince Denali Regular Crew Join Date: Jun 2003 Location: Santa Clara, CA Posts: 33	exposure issue: light oscillation Hi, I was taping a Muay Thai (Thailand style) kickboxing event with my Sony vx-2000. The ring was illuminated by three, dome-shaped high output flourescent lights mounted on the ceiling. I'm not sure if they are HMI, but they were pretty quiet. Similar looking units at Home Depot buzz like killer bees. Anyway, in AES mode (shutter priority), I noticed a brightness oscillaton with a period of about four seconds. As I increased the shutter speed from 1/30 to 1/1000, it got worse. The period didn't increase, but the peaks and troughs of the brightness oscillation deepened. When I switched the camera to fully Automatic operation (manual focus, though) or AEA (aperture priority mode), the oscillation largely went away. I'm not sure if the oscillation was caused by the mismatch of my video frame rate and the flourescent light frequency or that the three lights were not in sync with each other. What should I do in the future to avoid the brightness oscillationr? I want a faster shutter speed to reduce the blur of the fast motion.

June 27th, 2003, 06:16 PM	#2
Mike Rehmus Wrangler Join Date: May 2002 Location: Vallejo, California Posts: 4,049	I've had faster fluctuation when shooting in the UK with a NTSC camera. PAL camcorders have different default shutter speeds to avoid the problem induced by the 50 Hz power-line frequency. A 4-second period seems to not fit into the concept of power-frequency vs video frequency hetrodyning in normal circumstances. But we've all seen it happen to the images of spoked wheels that are rotating. Could it be that the lights were run at a higher frequency than power line? I hope someone knows the solution. __________________ Mike Rehmus Hey, I can see the carrot at the end of the tunnel!

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