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Frame Rates and Stadium Lighting


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#1 Steve Denny

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Posted 02 September 2005 - 12:22 AM

Please bear with me. I shoot many events at sporting venues. Certain stadium lights cause a timing problem or strobing if I do not shoot at 30, 60, 120 fps. (are there other compatible frame rates?)

What kind of lights are these that cause this problem? What is the technical explanation for this situation? And, finally, is there a way for me to determine or recognize this kind of lighting system when I walk into a new stadium with which I am not familiar?

Thank you.
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#2 Alvin Pingol

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Posted 02 September 2005 - 03:11 AM

>>Certain stadium lights cause a timing problem or strobing if I do not shoot at
>>30, 60, 120 fps. (are there other compatible frame rates?)

It is shutter speed you should be concerned about, but if you are sticking with 180 degrees then framerate is obviously a consideration. Use the "twice-the-frequency" rule: Any shutterspeed (disregard the numerator) that evenly divides into *twice* the frequency of the source should work.

>>What kind of lights are these that cause this problem?

Mainly discharge lamps, like HID Mercury Vapor or whatever gas cocktail they're using now (I believe stadiums use a certain type of HID lamp that allows for greater color rendition, but I'm not sure what exactly it is. Metal Halide maybe?).

>>is there a way for me to determine or recognize this kind of lighting system
>>when I walk into a new stadium with which I am not familiar?

The way I check for non-flicker-free sources is by quickly shifting my eyes left to right, such as that the light source quickly pans accross my vision (it helps to momentary close your eyes immediately after doing so). If you see breaks in the streak of light, it is not flicker-free. [I hope I'm not the only one that does this... hah. You can practice on your CRT display at home].
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#3 Michael Nash

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Posted 02 September 2005 - 04:48 PM

It is shutter speed you should be concerned about, but if you are sticking with 180 degrees then framerate is obviously a consideration. Use the "twice-the-frequency" rule: Any shutterspeed (disregard the numerator) that evenly divides into *twice* the frequency of the source should work.

<{POST_SNAPBACK}>


Actually that's backwards. It's FRAME RATE that follows the "twice the frequency rule," and you adjust the shutter angle (and therefore shutterspeed) to compensate for non-safe frame rates. Any frame rate that divides evenly (produces a whole number) into twice the frequency of the power supply is safe at ANY shutter angle.

For example 48 (the denominator of 1/48, and the shutterspeed of 24fps with a 180 shutter) does not divide evenly into 120; it gives you 2.5. Yet 24 divides evenly into 120 (=5).

If you're shooting video then typically you cannot change the frame rate, just the shutterspeed. In which case you need to find a scan rate that matches or halves the power cycle. For example if shooting in Europe with an NTSC camera, you would need a shutterspeed of 1/100 to avoid flicker; similarly in the US when shooting PAL you need to set the shutter to 1/60, instead of the native 1/50.

Almost ALL industrial lighting these days is some form of gas discharge, which flickers at the frequency of the power cycle (60Hz. in the US; 50Hz. in Europe). You can tell the type of lights by looking at the color and type of fixture. Pretty much if it's not some kind of tungsten or Halogen, it's a discharge if in a stadium.

Stick to the "HMI safe" frame rates and you shouldn't have any problem. Following the "twice the frequency rule," they are:
1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, 24, 30, 40, 60, 120.
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#4 Alvin Pingol

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Posted 05 September 2005 - 01:16 AM

>>Actually that's backwards.

This is what I get for posting at 1:11 AM. Thanks for the correction!
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#5 Michael Nash

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Posted 05 September 2005 - 02:57 AM

>>Actually that's backwards.

This is what I get for posting at 1:11 AM.

<{POST_SNAPBACK}>


Don't worry, it happens to me too!
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