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using waveform for exposure


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#1 stephen lamb

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Posted 07 November 2008 - 08:16 PM

Hey,

Well I think the title says it all. If I am shooting a gray card with an HD camera and have a monitor attached with it's waveform on, what percentage of exposure would be correct for the card? I've heard that 30% on a waveform correlates to correctly exposed gray card, any truth there? Thanks!
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#2 Bob Hayes

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Posted 09 November 2008 - 08:39 PM

I'll check tomorrow. It really depends on where the gamma is set on your camera. The cross over on a DSC chart tends to fall where you have the gamma set. So if your gamma is set a .45 I'd expect to set the cross over there.
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#3 Bruce Greene

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Posted 10 November 2008 - 01:00 AM

Hey,

Well I think the title says it all. If I am shooting a gray card with an HD camera and have a monitor attached with it's waveform on, what percentage of exposure would be correct for the card? I've heard that 30% on a waveform correlates to correctly exposed gray card, any truth there? Thanks!


Hi Steve,

I don't think there's any "truth" to be had here, but 30% for the grey card is in the ballpark, assuming a fully front-lit shot on a standard type broadcast camera set-up.

I think the challenge here is that digital capture often can record a much narrower range of light than film and we have to squeeze the scene into this narrow range. That usually means we have to do some of the post production color timing right in the original exposure and camera settings. So I guess what I'm saying is that I wouldn't use a gray card and waveform as an exposure meter. Though I would use the waveform to make sure your exposure fits within the range of the camera.

Edited by Bruce Greene, 10 November 2008 - 01:01 AM.

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#4 Joshua Jackson

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Posted 11 January 2009 - 03:16 AM

18% grey cards should fall at exactly 46%/46.2 IRE/328mV on your waveform monitor IF you have it set on a normal gamma (if you're not sure what gamma is, then assume it's normal). ALL video cameras (consumer, professional, HD, SD, RED, etc.) initially captures light linearly (i.e. an 18% grey card is captured as .18, with "white" normalized to 1). It's where the linearly captured light is mapped that makes them different. Video cameras without a "gamma" setting applied or those that don't map to a Log space, will set to a gamma of .45. 18% (.18) raised to the power of .45 gives us... 46.2%.

Now, shooting in something like PanaLog will map that grey card closer to 30%.
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