Lens cap gray card

Getting the white balance (color cast) correct when shooting with a digital camera is sometimes a challenge. Here's a quick project which can solve this problem.

Buy a cheap gray card from any photo shop (online, I've had good luck from Adorama and B&H). Cheap cards actually work better than expensive ones, because they're usually printed on thinner cardstock. Cut out circles to fit inside your lens caps. Stick to the lens cap with double-stick tape.

Do a quick check to make sure the card isn't touching the lens when the cap is on; some lens caps work better for this than others. You can also buy replacement lens caps which may work better than the one that came with your lenses.

Now you're never without a gray card, and it's right at hand when you need it. You can either zoom in on the gray card and use the "custom white balance" feature of your camera, or correct the images to match later in your photo editing software. The latter works better if you shoot RAW (which lets you change white balance after the fact), or if you set your camera to a white balance which is "pretty close" (for example, set it to daylight or cloudy outdoors). For this trick to work, it's important not to use in-camera auto white balance; you want the white balance to be the same in your keeper shots as it is in the throwaway shot which shows the lens cap.

Here's an example of where auto white balance can go horribly wrong, and the result with the gray card.

The first shot is straight out of the camera (Canon 30D), using in-camera auto white balance. Not bad, but the image has a blue cast. Still, it's much better that what Photoshop did with it (middle image). The last image is using click-neutral on the lens cap; this is the color the bouncy house actually is.