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Photography Question 
Kirstie Goodman

Gray Card: How to Use It?

I own a Canon 20D and was wondering if anyone can explain how the gray card works. I basically know that it corrects wrongly exposed pictures - for instance, a snowy white landscape turning gray because of the camera. But how do you use it? Thanks in advance for any responses.

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10/11/2006 6:33:33 AM

Jon Close
BetterPhoto Member Since: 5/18/2000
  Exposure is determined by the amount of light falling on your subject. It is best measured with an incident light meter placed in the same light as your subject. The exposure meters in cameras cannot directly measure incident light falling on the subject, they measure through the lens the light reflected off the subject. This can introduce exposure error because white or light toned objects reflect more light than black or dark toned objects. The camera's meter is calibrated for light reflected off a middle tone, commonly referred to as 18% gray. This usually works fine for scenes with a mix of light and dark toned subjects since it will average to the mid tone. If the scene is predominantly light or white (snowscape, beach, bride's dress, etc.), the camera meter will think it is brighter than it actually is and tend to under expose (render the white dress as gray). If the scene is predominantly dark or black it'll tend to overexpose.
The 35-zone Evaluative metering of the 20D (as well as Nikon Matrix and other systems) uses artificial intelligence to try to detect such exposure errors and automatically correct for them, but it is far from perfect. Using an 18% gray card is an inexpensive alternative to using an incident light meter to determine exposure. The card is the same mid-tone that the meter is calibrated to. Set the gray card in the same light as your subject and meter off of it, using either filling the frame with the card (and any metering mode), or using Spot/Partial metering if the card covers only the center of the frame.
Per the Canon instruction manuals, the 18% gray card is also preferred for setting the Custom White Balance.

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10/11/2006 8:35:54 AM

Shawn Wilson
BetterPhoto Member Since: 4/25/2005
  Where to get them might be a question, so here ya go:

I personally have the white card and the color squares card. Is it better to use the grey card instead for white balance?

I have my own question on this subject though about a bride's dress.

I did a lot of prep on the last wedding I shot with a light meter and the white card to white balance but still ended up with blowing out the brides dress.

The pictures looked good overall, they didn't look blown out, but when I dropped the brightness down the details of the dress really popped out, but of course the rest of the picture faded to black.

I considered doing a composite of two pictures to get a great shot, basically just overlaying a darkened dress over each picture, but I need to be able to get it right the first time.

Any suggestions? Would a grey card white balance help this issue at all?

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10/11/2006 9:39:41 AM

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