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Canon XH Series HDV Camcorders
Canon XH G1S / G1 (with SDI), Canon XH A1S / A1 (without SDI).


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Old May 2nd, 2008, 11:59 AM   #1
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Film Speed / ASA / and the lack thereof

Preparing to film a short this summer, Iíve been shooting several test setups each week on my XH G1. As I refine my lighting setups I keep operating from my film head and wanting to apply an ASA rating to the camera film speed setup options of low, medium & high. Is there a relative comparison between the two?

Iíve read on other forums that you can forget ASA and should either use the zebra pattern or a 4:1 light ratio or both. I can get to my lighting destination but there is too much trial and error. I know this sounds weird, but it almost seems like the camera is automatically self correcting the exposure. I am only shooting in Manual Mode. Perhaps this is just a familiarity issue and in a month Iíll know the parameters of the video light sensitivity better.

Any thoughts?

Thanks
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Old May 2nd, 2008, 12:49 PM   #2
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Search there threads here for film speed - and in the XL1 forum too.

The video camera (camcorder) is a high priced light meter that gives real-time full image output. Connect a good monitor to it and you can judge exposure precisely, add a waveform monitor and you really have something precise. Use the zebra as a guide if these are missing. Note that the camcorder response to light (gamma, latitude) is not the same as film, so ASA/ISO will not have the same meaning with respect to highlight and shadow details. (Film is metered for exposure because you do not know the results until the film is processed.)

Seriously, about the only reason for using a light meter with video is to check set lighting before the camcorder arrives.

But if you need an ASA/ISO value, you can run some simple tests to select what works for you.
1. Create a set that is typical of what you intend to shoot.
2. Light it as you plan to light the set
3. Setup the camcorder (shutter, aperture, gain, filters, and custom presets) for what you think is the the best image.
4. Adjust lighting if necessary, and repeat auntil satisfied with the image
5. Meter the set, and adjusting the meter ASA/ISO to match the camcorder shutter and aperture.
That corresponds to the equivalent film speed for that set, camcorder gain, camcorder custom presets, and lighting package. Folks that have done this typically reported around ASA/ISO 320 give or take a stop or two.

If you use AGC turned on the camcorder will adjust gain to provide what it thinks is a properly exposed image, even if the aperture and shutter are set manuallly.

A 6 dB change in the gain setting is equal to one stop of exposure.

The L/M/H Gain switch settings on the camcorder change the gain settings, and the steps are user adjustable (page 60 of the manual). Adding gain is similar to push processing film.
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Old May 2nd, 2008, 02:38 PM   #3
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I completely agree with Don
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Old May 2nd, 2008, 02:51 PM   #4
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Thanks Don.

Yes, my AGC was turned on, thus the self correcting nature of my exposure.
Thanks as well for your concise breakdown of the XH as an HD lightmeter / video capture.

Thanks to the greater board for the wealth of quality information shared here.

Best,

Thom
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Old May 2nd, 2008, 04:37 PM   #5
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In addition to the auto gain, you want to make sure the auto iris, auto shutter, auto white balance are all off.
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Old December 8th, 2009, 10:11 AM   #6
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I can see a use for an incident light meter in lighting various camera angle shots of the same scene for ensuring continuity of key side exposure and fill side exposure.
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Old December 8th, 2009, 12:00 PM   #7
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Film speed, ASA

Nothing will fill the bill like an external or field monitor. I just won't rely on any other method to establish what I want as far as video quality is concerned. The AGC in my opinion should not be used at all. A gain of 6db will introduce a slight bit of grain in your video. And it just doesn't compare with film exposure.
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Old December 9th, 2009, 02:36 PM   #8
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Definitely don't use AGC and I wouldn't go above 6db, even then I reckon the picture starts to look ugly.
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