Edited by Jon Rosenbloom, 02 June 2007 - 01:11 PM.
Posted 02 June 2007 - 01:10 PM
Posted 18 June 2007 - 01:05 PM
Posted 18 June 2007 - 01:15 PM
I understand a little desktop halogen won't run on a flicker-free ballast, but if I'm shooting 24fps, true progessive w/ no pull-down, why should it flicker?
Why would a Halogen lamp have a ballast? Are you sure it wasn't some sort of discharge lamp?
Posted 18 June 2007 - 02:10 PM
Posted 18 June 2007 - 02:38 PM
I suspect you shot on the lowest setting. I do not know for sure what those switches do (weather resistence or otherwise) but I suspect its a simple diode. When on full the light sees both phases of the AC cycle. When switched to low, it channels power through the diode, which naturaly cuts one phase off. The result is a light that is still at 60hz, but half the cycle the light is recieving no power at all (compared to full cycle where only a brief moment there is no power as the cycle crosses the zero border) I think that might have led to the flicker. Interesting though, I probably wouldnt have thought of it looking at a halogen light.
Posted 19 June 2007 - 10:02 AM
So I've been told.
I recall using both settings when I was using it as a source for a bounce, though no one was really keeping track. The main lesson of this job is to specify that the practicals be incandescent! (Lesson #2: When asking for set dressing for the walls, specify no glossy maps or photos!)
Posted 19 June 2007 - 10:07 AM
Posted 14 October 2007 - 02:02 AM
Many two-setting practical lamps do indeed flicker, and it's visible to the eye, not just at 24fps. I've got a couple at home like this. It's just the way that kind of dimmer works.
Most dimmers basically chop up the 60 cycle into variable square waves. Even though it is tungsten it can flicker as the dimmer is turned down and it is operating on a low cycle such 20 Hz. You can hear the lamp making noise as the filament is vibrating if you get close to the lamp.
You can dim tungsten lights without this artifact if you use a variac which is an oversized potentiometer. This device adds resistance and therefore drops the voltage without changing the frequency.
The cheap and easy thing to do is keep a bunch of different wattage lamps and some ND scraps with you.
Posted 15 October 2007 - 01:08 PM
The variable solid state dimmers work by turning off at every zero crossing of the 50 or 60 Hz. power, and then waiting to switch on again somewhere during each half cycle. That gives you 100 or 120 light pulses every second, not the 50 or 60 of the simple diode dimmer. The variac is actually a variable tap transformer, not a resistor. It only works on AC. There are no dimmers that alter the frequency of the power.