What is a gray card used for in photography, and how is it used to set custom white balance?

• A. It's a decorative prop
• B. Provides a neutral reference (18% gray) for exposure and white balance; capture a shot of the card under the scene's lighting and use it to set custom white balance or calibrate exposure in post
• C. Measures color temperature only
• D. Used to measure light intensity only

Answer: (B)

A gray card provides a neutral reference under the scene’s light so you can render colors accurately. It’s typically 18% gray, a standard midtone that cameras use for metering and white balance. By capturing a shot of the card in the same lighting as your subject, you create a baseline image you can use to set custom white balance or to calibrate exposure in post-processing. When you set custom white balance, the camera reads the gray card’s neutral color and adjusts its color temperatures so whites and neutrals read true, reducing color casts across your photos. You can also use that reference in post to fine-tune exposure and ensure consistent tonality across a sequence of shots.

A gray card isn’t merely a decorative prop, nor is it limited to measuring color temperature or light intensity alone. Its value lies in providing a stable, known reference that helps both white balance and exposure stay consistent under varying lighting.