This is the simplest, least expensive tool for reproducing accurate color in digital photography. I insert one of these 6-inch-long cards into a scene that I’m photographing as a reference with an absolute value. Back in front of my monitor, with the click of an eyedropper tool I’m able to indicate to Adobe Lightroom (my favorite imaging software) that the gray on this card is a neutral gray. In Lightroom, as with any worthy photo program, a “gray balance”; click on the card tells the software to identify this gray as neutral; the software then calculates the color temperature of the light hitting the card, adjusts the values accordingly, and the overall color of the scene falls into place.
In situations where there are multiple light sources with different color temperatures (say tungsten indoor lighting and daylight entering a window), I’d take two or more separate exposures with the QPcard positioned to catch each source.
I haven’t tried DataColor’s SpyderCube Calibration Tool, which performs a similar function, as it’s considerably more expensive, and offers much more than I need, which is really just a little touch of neutral gray. The SpyderCube does have two separate gray surfaces, but they’re at fixed angles relative to one another and won’t necessarily catch different light sources in a single exposure, anyway.
The minimal QPcard is inexpensive because it’s just a flimsy adhesive-backed piece of paper. I’ve been able to keep a card alive for a long time by sticking it onto a piece of sturdy cardboard and stowing it securely in my Domke bag.