The Expo Disk is an attachment for setting the white balance. It has a prismatic lens in front of a translucent white disk. The disk is snapped on the front of the lens and white balance shot is taken, with the same light falling on the lens as on the subject . I have found that this accessory works very well for all but the tightest shots, where getting the light on the lens is difficult. While I have this, the gray card performs better in most situations. It does excel when setting the white balance when photographing the finished pieces.