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.

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?

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

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.

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

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

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

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.