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Old October 10th, 2002, 01:16 PM   #1
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Gray card. Do you use them/how?

Does anyone use the 18% gray card? How do you use them?
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Old October 10th, 2002, 03:58 PM   #2
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Well, you don't really in video production. In still photography the meters built into the cameras and hand held meters are calibrated to 18% reflectance. The assumption is that the "average" scene reflects 18% of the light that hits it. In many cases the 18% reflectance is correct (after all that's what meters are calibrated to). But, we can all think of scenes that reflect more or less than 18%. Black cat in a coal bin or polar bear on snow field.

The meter well determine the correct aperture and shutter speed for the film used (ISO/ASA). If the camera is in program, it will select both settings. Aperture priority and shutter priority give the user control of the settings (aperture in AP etc.) and the camera then selects the correct corresponding shutter or aperture setting. All of this assumes 18% reflectance.

So what happens with the black cat in a coal bin? Easy, the meter (camera) always assumes 18% reflectance (18% gray). The camera will set the aperture and shutter to over expose the picture and make the black objects 18% gray. Exactly what it is calibrated to, 18% gray. How about the white polar bear? You guessed it. The polar bear and snow will be under exposed and turn out 18% grey.

The common response from most of my students at this point is, I know how to fix that, I'll set my camera to manual. Wrong, it does no good. In whatever mode you're using the camera is still using the internal meter and still assuming the scene reflects 18% of the light that hits it. Manual only means you have more work to do, set both aperture and shutter speed.

Enter the 18% gray card. Hold the card so that the card is in the same quantity and quality of light as your cat or polar bear. If the card is at a slight angle it may alter your meter reading so be careful. Point your camera towards the card so that the image fills the entire view finder. Look at your reading and manually set your aperture and shutter speed accordingly. Now recompose your image and shoot that cover shot for National Geographic.

I've been shooting pictures for years and have not had any problems with my metering. If what you say is true why haven't I noticed this? There are several reasons that contribute to your success in picture taking. One is that most people use color negative film and not slide film. Slide film has an exposure latitude of 1 stop. The film can be over exposed about 1/3 of a stop and under exposed about 2/3 of a stop and you'll still have an acceptable image. However, consumer print film has a latitude of about 5 stops and still getting acceptable prints (3 stops under exposed and 2 stops over exposed). The tremendous latitude of modern color negative film covers your butt.

Exposure meters in cameras are now a little smarter than they used to be. Nikon's Matrix Metering and Canon's Evaluative Metering can and do help in these type of situations. However, they still miss in the examples I've given. Very dark scenes and very light scenes will still confuse the metering and exposure systems.

Jeff
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Old October 10th, 2002, 04:46 PM   #3
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Jeff,
You are good!

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Old October 10th, 2002, 07:56 PM   #4
 
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I use an 18% gray card to set my white balance. It's true that a white card is recommended, however, a white card could potentially max out the light meter. I've gotten very good results with the 18% gray. Just a matter of personal preference. I;m an old still 35mm photographer. Old habits die hard.
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Old October 11th, 2002, 10:45 AM   #5
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Is there a reason to use a gray card to white balance?

I know that several people here are using gray cards to white balance. Is there a reason to do this? When is it a bad idea?
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Old October 11th, 2002, 11:40 AM   #6
 
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The use of a white card tends to saturate the luma detection in the light metering system. An 18% gray will avoid this. Use it with the same care that you would a white card....nice even lighting, no shadows or reflected light.
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