What's up with...Gray Cards

A gray card is exactly what it says it is: a piece of poster board, usually 8x10 inches or thereabouts, coated on one side with a medium matte-gray finish. The other side is often matte white, occasionally matte black.

The gray card is designed to provide an exact midtone for an exposure meter to read. There is an ongoing controversy over whether the 18-percent light reflectance used by most gray card makers, including Kodak, is really the exact midtone—but I am going nowhere near this argument.

The gray card has two uses. One is to make exposure readings. You put the card in the same light as your subject and take a narrow-angle reading of just the card. (If you’re making a portrait, you can have your subject hold the card.) Since the meter provides exposure settings to produce a midtone, and the gray card is a midtone, your exposure should be on the money—or at least very close.

The other use for the gray card is as a color reference. When using color negative film, for example, you can shoot one frame of the gray card in the lighting you’re shooting in. After processing, this frame can be used to fine-tune filtration for printing: The lab makes prints to match the color and density of the card.

Now that the photography world’s gone digital, a gray card is a much better choice for setting white balance than a white card. “White” comes in a zillion different shades (just look in a paint store) and, if you’re setting white balance under high-power studio strobes, white is way too reflective and will overwhelm the camera’s metering system.

You can make gray cards as big as you want: Take a small gray card to any paint or hardware store that custom-matches paint, and have a can of matte latex made up. The Pop Photo lab is painted entirely midtone gray. Yes, it’s a terrible decorating idea, but it’s much better for calibrating color monitors.