The ExpoDisc is a clip-on (or hold-in-front-of) diffuser designed to facilitate setting custom white balance. Rather than hauling around a gray card, or trying to find a plain white surface, or hoping that your camera guesses correctly, you can toss this in front of your lens and lock in an accurate custom white balance in seconds. Bonus: if you actually use this on a lens with a 77mm filter thread, it can replace your lenscap. Yeah, it’s just a fancy diffuser, but it gets results, dammit.