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How To Use A Gray Card

Gray cards are essential for more than just white balance

Ask any darkroom printer about the color gray and they’ll tell you all about its many subtleties and distinctive qualities. Middle gray, for instance, is a technical term that refers to the gray that’s perceived to be approximately halfway between black and white. It’s not 50 percent gray, however. It’s known as 18 percent gray because it reflects approximately 18 percent of the light. See? Gray is complex. And that’s just one of many grays at our disposal.

Back in high school, my photography teacher taught us to think about seven stops of gray that we could look for in our photographs. From light to dark, they are White, White with Detail, Light Gray, Middle Gray, Dark Gray, Black with Detail and Black. With these seven different grays in mind, photographers can learn to think differently about exposure. And a good gray card will aid in this.

For instance, if an element in a scene appears to the naked eye to be black with detail but in your photograph it appears pure black, you might want to consider upping the exposure to lighten up that black. Conversely, if you’re trying to take creative control and you deliberately want to make an element that’s in reality black with detail appear pure black, you’ll need to light and process your image files specifically to create that look. How you do it—whether decreasing exposure, lessening fill light or increasing contrast by adjusting the tone curve in post-processing—is where much of the creative magic happens.

Getting a handle on gray doesn’t have to be difficult. It can start with something as simple as photographing a gray card.

In the film era, Kodak practically had the gray card market cornered with its 4×5 and 8×10 gray cards. These heavy-duty cards were a staple in every serious photographer’s kit, and their scientifically accurate 18 percent gray surface was used for everything from establishing proper exposures to monitoring contrast during printing.

These days, though, there are lots of options when it comes to gray cards. Photographers who use color checker charts might find gray cards incorporated into them. My DataColor chart, for instance, has a traditional 24-chip color card on one side (see last week’s post for more information on using color checkers) and a large gray swatch along with six additional small chips on the other. These don’t just provide for accurate color balance, they also come in very handy when evaluating exposure, modifying contrast and adjusting the tone curve in post.

For instance, with a few different neutral gray steps at your disposal, you can see how adjusting the tone curve to modify, say, the dark end of the range also influences midtones and highlights. If you’re inadvertently turning light gray into pure white, you might want to reevaluate the contrast adjustment.

The simplest way to use a gray card, of course, is to photograph the neutral gray in the scene under the light source being used for the subject and then white balance accordingly. You can do this by filling the frame with the gray card and then setting a custom white balance in camera or by simply capturing a shot that contains the gray card and then making the white balance adjustment in post.

It’s critically important to photograph the gray card under the same lighting that the subject will be shot. That means if you’ll be using window light for your primary exposure, don’t photograph the gray card with the room lights on too. And if you’ll be photographing in open shade, don’t photograph the gray card in bright sun. Once you’ve captured a photo of the gray card, and assuming you’re capturing raw image files, of course, open the image in Lightroom, Camera Raw or any number of other RAW image processing applications and use the white balance tool, typically an eye dropper clicked on the gray card in the photo, to set a custom white balance that will be applied to all of the images captured under the same lighting conditions.

Why not just use a neutral white card for color balance? One problem with white balancing off of a white card is that under bright illumination the white card can appear pure white even if the light falling on it has a color cast. A middle gray tone, however, could be more than a stop overexposed and still read as gray, allowing the color cast to be captured and compensated for.

The 18 percent gray card also has an interesting history in photography in one other way that still comes in handy. In black-and-white film photography, photographers could use a spot meter (or an in-camera TTL meter) to measure the light reflected off of an 18 percent gray card in order to determine the correct exposure. In the auto-exposure era, camera meters measured the light in a scene and assumed that scene’s reflectance was 18 percent gray. Modern metering systems are more evolved and do a great job at accurately metering scenes that are both much lighter and much darker than middle gray. But fundamentally that 18 percent gray measurement is still the foundation of camera metering. This means if you’ve got an 18 percent gray card and you fill the frame with it, making a test exposure based on the camera’s auto-exposure modes will ensure that the indicated meter reading will translate into a perfect exposure. 

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