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ASA ratings for a video camera


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#1 Levent Cimkentli

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Posted 17 January 2007 - 11:49 AM

I hope this doesn?t sound like a dumb question.

But, I would like to establish an ISO rating on some of my video cameras (they are standard definition NTSC cameras with the gain set at 0 db) and I would prefer to use the method outlined by Harry Mathias in the ?American Cinematographer Video Manual?.

To test the cameras I have a well-lit chip chart, a Spectra Pro IV-A and a Macbook Pro laptop running a software (?Scopebox?) waveform monitor.

My question is what should I set my set my meter at?

Spectra Cine?s site suggests 25fps. I have seen other websites say 1/30 of a second (which seems close enough to the NTSC frame rate). My instincts tell me to set it at 1/60 of second because of the fact that it is an interlaced image and I think it might be prudent to err on the side of lower sensitivity.

What would you do?

Thanks,
Levent Cimkentli
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#2 Frank Barrera

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Posted 18 January 2007 - 12:44 AM

When evaluating the exposure index of an interlaced camera I would set my Spectra to 30 fps since the video will be recording at 29.97. If you don't have the ability to plug in a frame rate then I would just go with 1/60th of a second. It would not be precise but then again checking the "ASA" of a video camera is not a precise science. It is merely to get you in the ball park because a video camera does not respond to light the way film does. Besides I always trust the zebras anyway when it comes to video exposure. If someone tells me that my camera is "ASA" 320 and the moniter looks weird (too dark or too bright) I just go with the zebras. When I first started with HD I had me and the gaffer use our meters. But quickly we realized that it was like sticking car tires on the bottom of a boat. It just doesn't make too much sense.

I am a little confused as to why Spectra's site would recommend 25fps. That would only be the case if you were shooting PAL progressive.
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#3 Daniel Sheehy

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Posted 18 January 2007 - 01:20 AM

If your camera is NTSC (as you say it is) then work with 30fps or 1/60th of a second.

Some websites seem to give directions on this issue without specifying which format they are talking about.
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#4 Levent Cimkentli

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Posted 18 January 2007 - 12:08 PM

Thanks for the advice. I will probably use 1/60 of a second.

I mosted wanted to know the sensitivity of the cameras in order to help me determine how many extra lights I would need when scouting a location. At 1/60 of a second, I cannot make the mistake of over estimating the cameras abilities.

During the actual shooting I am more trusting of the Waveform and the Zebras, than the Spectra Pro.

Thanks,
Levent Cimkentli

P.S. The MAC OS X users might want to look at the free demo of the "Scopebox" software. It is a very nice Mac alternative to the Windows only "DV Rack" waveform/vectorscope monitor program.
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#5 Greg Gross

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Posted 23 January 2007 - 10:24 AM

I would suggest that you go by way of calibrated monitor,zebra. I shoot productions with a
a PD-170 and you should see the look I get in 4:3. I'm using monitor,zebra and of course
I can send it to post for the desired format. Now I agree the PD-170 is a toy compared to
what most dp's are using here. I believe at one time that I had my ASA figured out for the
PD-170 and I think it measured out to be ASA 400. Like everybody else here I left my Spe-
tra IVa in the bag and relied on the monitor,zebra.

Greg Gross
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#6 Walter Graff

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Posted 23 January 2007 - 12:18 PM

Just a technical note. There really is no such term as ASA when talking about video. Exposure index is prbably the closest correct term you could use but even that does not fit onto log equations for exposure in video. The only problem you may face is accuracy throught the range of exposure. While a meter stays relatively accurate within a stop or two fo video extreme aperatures create completely differnt measurable sensativity readings with video. A camera that might be rated by someone as 320 indoors could be 640 outdoors. The CCD design and the use of microlenses on CCDs allows for the discrepency. Mircolenses help you least when you need them most and magnify light more when you have too much light.

The single most accurate way of reading video levels is with awavefomr. If not that then zebras will do you justice. Setting your faces at between 75 and 85 will pretty much garuntee you non overexposure in that area. I have an article on zebras that I thought was on my website. I see it is not. I'll put it up in the next day or two.
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#7 Levent Cimkentli

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Posted 23 January 2007 - 12:52 PM

I have an article on zebras that I thought was on my website. I see it is not. I'll put it up in the next day or two.


Thanks Walter! I look forward to reading your article.

Best regards,

Levent Cimkentli
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#8 Greg Gross

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Posted 23 January 2007 - 10:13 PM

I enjoyed reading your post sir. Please post your article on zebas as time permits
you.

Greg Gross
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