Set Exposure so it Never Clips Highlights? at DVinfo.net

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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 August 26th, 2008, 06:05 AM   #1
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Set Exposure so it Never Clips Highlights?

I've been using an HV-20 and manually adjusting the aperture to just before clipping of 100% zebras.

Is it possible to get the XH-A1 to automatically expose that way? I've read that the camera is incredibly tweakable, so I'm wondering if this is possible with it.

Thanks very much.
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Old August 27th, 2008, 05:54 AM   #2
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Spotlight mode might get you closer to you goal.
Depending on the lighting, avoiding all zebra may result in the loss of desired shadow detail.
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Old August 27th, 2008, 08:14 AM   #3
 
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Perhaps somewhat puzzling is that many pro level HDcams recorde highlights in the range of 100-110 IRE, as well as blacks below IRE 0. How is this possible? Given the range of broadcast color values 16-235, 0 IRE corresponds to RGB16 and 100 IRE corresponds to RGB235. Computer color values in an 8 bit system can display colors at RGB0 to RGB 255.

So, the editors task is to remap the recorded values back into the acceptable broadcast range without clipping. If you set your exposure to not exceed 100 IRE, you may be throwing away highlight detail in your image capture.
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Old August 27th, 2008, 08:37 AM   #4
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I noticed, when using my old camcorder, that setting zebra stripes to even 95% caused clipping. So I think the best thing is to test different zebra settings until you get one that doesn't clip. You can see by eye any glaring white spots and colour aberrations on peoples faces when you view the footage on your computer monitor.

Has anyone here tested optimum zebra stripe settings on the XH A1?

I will do some tests myself when I get my XH A1 and then post the results here if no one else posts them in the meantime.

Last edited by Stuart Graham; August 27th, 2008 at 08:38 AM. Reason: spelling
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Old August 29th, 2008, 07:01 AM   #5
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Dom, thanks ;) and yes, I have been using Spotlight Mode sometimes, but it allows only auto white balance.

What I'm wondering is if the XH series allows more control over how auto exposure exposes? And is there a way to set the auto exposure so it doesn't clip (or barely clips), regardless of what that does to the shadows? Thanks again very much.
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Old August 29th, 2008, 07:08 AM   #6
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Quote:
Originally Posted by Bill Ravens View Post
Perhaps somewhat puzzling is that many pro level HDcams recorde highlights in the range of 100-110 IRE, as well as blacks below IRE 0. How is this possible? Given the range of broadcast color values 16-235, 0 IRE corresponds to RGB16 and 100 IRE corresponds to RGB235. Computer color values in an 8 bit system can display colors at RGB0 to RGB 255.

So, the editors task is to remap the recorded values back into the acceptable broadcast range without clipping. If you set your exposure to not exceed 100 IRE, you may be throwing away highlight detail in your image capture.
Bill, I do realize this. But I don't feel particularly comfortable knowing when the super-white level has been exceeded. That's why I'm using 100% zebras and not 70%. I figure if I'm seeing zebras (or just before seeing zebras) at 100%, then I haven't blownout the highlights.

In some scenes, I have to decide which is more important the highlights or the shadows, but a lot of the times, I'm just clicking away on the tiny camera joystick to make the zebras disappear. I'm wondering if the XH-A1 can do this automatically for me.
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Old August 29th, 2008, 03:38 PM   #7
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There's another way if you want to let the camera adjust either the aperture. In the menu somewhere is an adjustment to tune the max brightness. Default is 0 but you can back that down.

I shoot motor races where the cars can fall into the shadows of the grandstand so I shoot in Tv mode until the sun is fully down. I have tweaked this adjustment down by .25 (as I recall) to scale back the maximum luminance Hal will let into the camera. It helps, although it doesn't always keep the signal legal but it gets the levels more where I want them in this instance. (Sorry I don't remember what this function is called or where it is in the menu but I don't have my camera handy.)

As an aside, I find capturing extralegal white and black values helpful in post when I need to adjust color and luminance either for look or to match another shot. If you darken the overall image you'll lose that headroom and footroom, not to mention running the risk losing the darker blacks completely.
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