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Old May 1st, 2006, 10:52 PM   #1
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how to use an 18% grey card for video

Hi,

I'm wondering if I need to purchase an 18% grey card for proper exposure for using mini dv, i've heard some people use it for video. How would I go about using it, and how will it benefit me, i know the reverse side has white on it for white balancing. i have a Panasonic GS400.

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Old May 2nd, 2006, 12:06 AM   #2
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You use it much like the white balance card. Make sure it fills the frame (zoom in as needed) and the light falling on the scene falls on the card. I'm not familiar with your camera, but generally you'd set the aperture where you want it and let the camera select the shutter speed (Av mode) or vice-versa (Tv mode) and then lock the exposure.
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Old May 2nd, 2006, 08:03 AM   #3
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Besides white balance, the big thing it's used for is matching color between shots & shoots in post.

Ideally, the card has white, black and 18% gray.

Then, after shooting with multiple cameras or several days of shooting with a single camera, you can use something like a curves filter to match the RGB values in the cards across all your shots.
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Old May 2nd, 2006, 01:55 PM   #4
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I have used a gray card for exposing video in the past (on very controlled shoots, not run-and-gun stuff), and it is highly accurate, but it's also a pain in the butt to work with. It can be difficult to get the card to fill the frame, even at full zoom, depending on the camera position. If you have to move the camera to make it happen, then you lose the accuracy perhaps. Also, if you are using a wide iris (F2 for example) and your camera stops down when zooming in to fill the frame (usually to F2.8), then again your reading will be inaccurate.

In any case, I am planning on upcoming shoots to at least roll tape on each shot with the gray card properly positioned and lit, in frame, but not filling the frame. That will give me a good little reference in post. If I need to adjust exposure or white balance slightly, I'll have the gray reference in every shot. This could also be done with a white/gray/black card, or some more complex card, but if you just have the gray card, it's worth using.
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Old May 2nd, 2006, 06:42 PM   #5
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So it's main function is to just match shots in post, can't this be done by eye? Or is pretty manditory to use a grey card, I'm wondering if it's really worth the cash. Since i'm using a GS400 I have just been using zebra lines for exposure, what is youe exact set up procedure for exposing?
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Old May 4th, 2006, 09:48 AM   #6
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Luke, you can certainly eyeball it. However, 18% gray is supposed to be 50 IRE (I know, IRE doesn't exist in digital world), or around 128,128,128 RGB value in video. So, if you at least have a gray reference at the head of your shots, you can tell a few things. One, is the gray really gray, or does it skew to a different color (140,128,128) being overly red. Two, being are you over or under exposed (100,100,100) being underexposed. So, even if in the end you eyeball your color corrections, at least the gray reference will point you in the right direction. Like I said, even better to shoot a more comprehensive test chart of some point at the head of each shot, but 18% gray still tells you a lot.

You also have to know how to use the card, and they mostly come with instructions. It needs to be positioned where the object you are exposing for is (say, the lead actor's face), and you don't just point it straight at the camera, you angle it between the angle of the camera and key light, sort of the like the curve of a face catching a highlight.

Honestly, I've only used a gray card on a few productions. I want to use it more as a reference only in future productions, to ease my color correction. Generally, I work without it. I let the camera auto-expose, then perhaps tweak a little and flip to MANUAL to lock the exposure, so it doesn't change during a shot, which would look very unprofessional (same goes for focus, white balance, shutter, etc.).
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