This particular Expodisc is not a disc- it's a big 4x5.65" slab designed to slip into matteboxes- but it works the same as the classic. Basically, it's a specialized flat 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. This is a hard to find, discontinued variant of the Expodisc, in a rugged aluminum housing that's designed for professional use.