This Altman Donut is a 6.25" diameter metal Donut with 2.5" opening which is essentially a snoot for the Altman Baby Zoom Ellipsoidal and other Altman spotlights.
Use of a snoot or donut will result in a small circle of light when it is attached to the front of your light source. The size of the circle of light will be determined by the opening of the snoot, its length, and the distance of the subject. Consider them as a painter would a brush or a sculptor would a knife or chisel.