 Anita White |
The Scoop on Gray Cards What are 18% grey cards? Thanks.
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John Wright |
They are exactly what it says. It's a card (usually stiff cardboard) that is colored 18% grey. Why are they 18% grey? They are 18% grey because that is the value that all light meters see and meter for.
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- Gregory LaGrange Contact Gregory LaGrange Gregory LaGrange's Gallery |
Think of a scale that's black and white. At one end, you have totally black, and at the other end, you have totally white. In between, you have shades that gradually go to increasing lighter shades of gray till they go from black to white. Somewhere in there, there's a shade of grey that's 18%. It's kinda like room temperature - in that's it's considered normal. So to expose that shade of grey correctly would be correct exposure for anything that is under the same lighting.
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David Robinson |
The story goes that researchers into what the human eye can see divided black to white into 100 shades. They then discovered that somewhere past No. 36 most people could not differentiate between the darker shades. So 18% of the original 100 shades became the 50% mark of what people could see. ... So I'm led to believe.
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George Corbin |
And just to add some more clarity (a lot of new budding photographers don't know this about their cameras, and it has a profound impact upon your exposures): The camera, in effect, is "calibrated" to see everything as a medium gray. So, if you photograph a black cat, and your camera's auto settings say "This is the right exposure!", you'll be disappointed to find a much paler, overexposed, gray cat on your final picture. Why? Because the camera "assumes" the subject would be correctly exposed as a medium gray ... rather than black. Similarly, when you photograph white snow on auto settings, the snow in the resulting picture will be gray. The camera doesn't know that snow should be white, the cat should be black, etc. It assumes the average exposure of your photo should be gray. So metering off a gray card in each shooting situation can give you much more control over your final exposure.
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