Posted 28 May 2011 - 03:00 AM
Posted 28 May 2011 - 03:12 AM
This is a post by Richard Andrewski on http://reduser.net/
what is the connection between shutter angle current flow and lights ???
I hope it helps. Tungsten lighting you can do any angles/any rates, HMI you need to do the maths:
There is a formula for figuring out all this with 50 or 60hz low frequency lighting and is applicable to both HMI and fluorescent (magnetic ballasts usually but there are actually electronic ballasts that operate at this lower frequency too):
Safe camera speeds with 60hz lighting :
Camera speed in fps = shutter angle / 3 / LPEP
Where LPEP = number of light peaks that you want to capture per exposure
period (typically 2 per cycle, or hz).
Safe shutter angles with 60hz lighting :
Shutter angle = speed in fps * 3 * LPEP
For 50hz power replace 3 with 3.6 in both formulas.
What all this means is that if you don't capture at least two cycles of light output during your exposure then you start to get flicker.
So for example in 50hz situations and assuming you want 30fps it would be as follows to figure out shutter adjustment:
30*3.6*2 = 216
Here's a bit of explanation. Each shutter moment is a moment where we're capturing light. If you're light is flickering that means that its not refreshing at a very high rate. 50 to 60 times per second is not so much! You are practically guaranteed to see flickering if you're shuttering is open at a moment the light is refreshing and not fully lit. The faster your shutter the more likely you are to be open at such a moment. Many feel that you need at least two refreshes of your light per shutter moment to be safe and not see flicker. Of course electronic ballasts like those in a fluorescent ballast refresh at 40,000 times per second and 75 to 150 times per second is common for metal halide / HMI types.
Posted 28 May 2011 - 11:44 AM
So that means at a 72 degree shutter angle, at 24 fps under 60Hz, you'd capture only one pulse per frame. So closed down even further, like a 45 degree shutter angle, you are only capturing part of the sine wave of the pulse, and depending on when you trigger the camera, you may be in sync with the peak or the valley of the wave and if the second, you won't be getting full exposure from the light. But it might not necessarily flicker on camera.